Antibody-Mediated Suppression Regulates the Humoral Immune Response to Influenza Vaccination in Humans

BACKGROUND

Preexisting immunity, including memory B cells and preexisting antibodies, can modulate antibody responses to influenza in vivo to antigenically related antigens. We investigated whether preexisting hemagglutination inhibition (HAI) antibodies targeting the K163 epitope on the hemagglutinin (K163 antibodies) could affect antibody responses following vaccination with A/California/07/2009-like A(H1N1)pdm09 influenza viruses in humans.

METHODS

Pre- and postvaccination sera collected from 300 adults (birth years, 1961-1998) in 6 seasons (2010-2016) were analyzed by HAI assays with 2 reverse genetics viruses and A(H1N1) viruses circulated from 1977 to 2018. Antibody adsorption assays were used to verify the preexisting K163 antibody-mediated suppression effect.

RESULTS

Preexisting K163 antibody titers ≥80 affected HAI antibody responses following influenza vaccination containing A/California/07/2009-like antigens. At high K163 antibody concentrations (HAI antibody titers ≥160), all HAI antibody responses were suppressed. However, at moderate K163 antibody concentrations (HAI antibody titer, 80), only K163 epitope-specific antibody responses were suppressed, and novel HAI antibody responses targeting the non-K163 epitopes were induced by vaccination. Novel antibodies targeting non-K163 epitopes cross-reacted with newly emerging A(H1N1)pdm09 strains with a K163Q mutation rather than historic 1977-2007 A(H1N1) viruses.

CONCLUSIONS

K163 antibody-mediated suppression shapes antibody responses to A(H1N1)pdm09 vaccination. Understanding how preexisting antibodies suppress and redirect vaccine-induced antibody responses is of great importance to improve vaccine effectiveness.

Redirecting antibody responses from egg-adapted epitopes following repeat vaccination with recombinant or cell culture-based versus egg-based influenza vaccines

Repeat vaccination with egg-based influenza vaccines could preferentially boost antibodies targeting the egg-adapted epitopes and reduce immunogenicity to circulating viruses. In this randomized trial (Clinicaltrials.gov: NCT03722589), sera pre- and post-vaccination with quadrivalent inactivated egg-based (IIV4), cell culture-based (ccIIV4), and recombinant (RIV4) influenza vaccines were collected from healthcare personnel (18-64 years) in 2018-19 (N = 723) and 2019-20 (N = 684) influenza seasons. We performed an exploratory analysis. Vaccine egg-adapted changes had the most impact on A(H3N2) immunogenicity. In year 1, RIV4 induced higher neutralizing and total HA head binding antibodies to cell- A(H3N2) virus than ccIIV4 and IIV4. In year 2, among the 7 repeat vaccination arms (IIV4-IIV4, IIV4-ccIIV4, IIV4-RIV4, RIV4-ccIIV4, RIV4-RIV4, ccIIV4-ccIIV4 and ccIIV4-RIV4), repeat vaccination with either RIV4 or ccIIV4 further improved antibody responses to circulating viruses with decreased neutralizing antibody egg/cell ratio. RIV4 also had higher post-vaccination A(H1N1)pdm09 and A(H3N2) HA stalk antibodies in year 1, but there was no significant difference in HA stalk antibody fold rise among vaccine groups in either year 1 or year 2. Multiple seasons of non-egg-based vaccination may be needed to redirect antibody responses from immune memory to egg-adapted epitopes and re-focus the immune responses towards epitopes on the circulating viruses to improve vaccine effectiveness.

Interpretation of molecular detection of avian influenza A virus in respiratory specimens collected from live bird market workers in Dhaka, Bangladesh: infection or contamination?

OBJECTIVES

Interpreting real-time reverse transcription-polymerase chain reaction (rRT-PCR) results for human avian influenza A virus (AIV) detection in contaminated settings like live bird markets (LBMs) without serology or viral culture poses a challenge.

METHODS

During February-March 2012 and November 2012-February 2013, we screened workers at nine LBMs in Dhaka, Bangladesh, to confirm molecular detections of AIV RNA in respiratory specimens with serology. We tested nasopharyngeal (NP) and throat swabs from workers with influenza-like illness (ILI) and NP, throat, and arm swabs from asymptomatic workers for influenza virus by rRT-PCR and sera for seroconversion and antibodies against HPAI A(H5N1) and A(H9N2) viruses.

RESULTS

Among 1273 ILI cases, 34 (2.6%) had A(H5), 56 (4%) had A(H9), and six (0.4%) had both A(H5) and A(H9) detected by rRT-PCR. Of 192 asymptomatic workers, A(H5) was detected in eight (4%) NP and 38 (20%) arm swabs. Of 28 ILI cases with A(H5) or A(H9) detected, none had evidence of seroconversion, but one (3.5%) and 12 (43%) were seropositive for A(H5) and A(H9), respectively.

CONCLUSION

Detection of AIV RNA in respiratory specimens from symptomatic and asymptomatic LBM workers without evidence of seroconversion or virus isolation suggests environmental contamination, emphasizing caution in interpreting rRT-PCR results in high viral load settings.

Fatal Human Rabies Infection With Suspected Host-Mediated Failure of Post-Exposure Prophylaxis Following a Recognized Zoonotic Exposure-Minnesota, 2021

BACKGROUND

No human rabies post-exposure prophylaxis (PEP) failure has been documented in the United States using modern cell culture-based vaccines. In January 2021, an 84-year-old male died from rabies 6 months after being bitten by a rabid bat despite receiving timely rabies PEP. We investigated the cause of breakthrough infection.

METHODS

We reviewed medical records, laboratory results, and autopsy findings and performed whole-genome sequencing (WGS) to compare patient and bat virus sequences. Storage, administration, and integrity of PEP biologics administered to the patient were assessed; samples from leftover rabies immunoglobulin were evaluated for potency. We conducted risk assessments for persons potentially exposed to the bat and for close patient contacts.

RESULTS

Rabies virus antibodies present in serum and cerebrospinal fluid were nonneutralizing. Antemortem blood testing revealed that the patient had unrecognized monoclonal gammopathy of unknown significance. Autopsy findings showed rabies meningoencephalitis and metastatic prostatic adenocarcinoma. Rabies virus sequences from the patient and the offending bat were identical by WGS. No deviations were identified in potency, quality control, administration, or storage of administered PEP. Of 332 persons assessed for potential rabies exposure to the case patient, 3 (0.9%) warranted PEP.

CONCLUSIONS

This is the first reported failure of rabies PEP in the Western Hemisphere using a cell culture-based vaccine. Host-mediated primary vaccine failure attributed to previously unrecognized impaired immunity is the most likely explanation for this breakthrough infection. Clinicians should consider measuring rabies neutralizing antibody titers after completion of PEP if there is any suspicion for immunocompromise.

Healthcare personnel in 2016-2019 prospective cohort infrequently got vaccinated, worked while ill, and frequently used antibiotics rather than antivirals against viral influenza illnesses

Background

Uncertainty about risk of illness and the value of influenza vaccines negatively affects vaccine uptake among persons targeted for influenza vaccination.

Methods

During 2016-2019, we followed a cohort of healthcare personnel (HCP) targeted for free-of-charge influenza vaccination in five Lima hospitals to quantify risk of influenza, workplace presenteeism (coming to work despite illness), and absenteeism (taking time off from work because of illness). The HCP who developed acute respiratory illnesses (ARI) (≥1 of acute cough, runny nose, body aches, or feverishness) were tested for influenza using reverse-transcription polymerase chain reaction (rt-PCR).

Findings

The cohort (2968 HCP) contributed 950,888 person-days. Only 36 (6%) of 605 HCP who participated every year were vaccinated. The HCP had 5750 ARI and 147 rt-PCR-confirmed influenza illnesses. The weighted incidence of laboratory-confirmed influenza was 10.0/100 person-years; 37% used antibiotics, and 0.7% used antivirals to treat these illnesses. The HCP with laboratory-confirmed influenza were present at work while ill for a cumulative 1187 hours.

Interpretation

HCP were frequently ill and often worked rather than stayed at home while ill. Our findings suggest the need for continuing medical education about the risk of influenza and benefits of vaccination and stay-at-home-while-ill policies.

Use of Biolayer Interferometry to Identify Dominant Binding Epitopes of Influenza Hemagglutinin Protein of A(H1N1)pdm09 in the Antibody Response to 2010-2011 Influenza Seasonal Vaccine

The globular head domain of influenza virus surface protein hemagglutinin (HA1) is the major target of neutralizing antibodies elicited by vaccines. As little as one amino acid substitution in the HA1 can result in an antigenic drift of influenza viruses, indicating the dominance of some epitopes in the binding of HA to polyclonal serum antibodies. Therefore, identifying dominant binding epitopes of HA is critical for selecting seasonal influenza vaccine viruses. In this study, we have developed a biolayer interferometry (BLI)-based assay to determine dominant binding epitopes of the HA1 in antibody response to influenza vaccines using a panel of recombinant HA1 proteins of A(H1N1)pdm09 virus with each carrying a single amino acid substitution. Sera from individuals vaccinated with the 2010-2011 influenza trivalent vaccines were analyzed for their binding to the HA1 panel and hemagglutination inhibition (HI) activity against influenza viruses with cognate mutations. Results revealed an over 50% reduction in the BLI binding of several mutated HA1 compared to the wild type and a strong correlation between dominant residues identified by the BLI and HI assays. Our study demonstrates a method to systemically analyze antibody immunodominance in the humoral response to influenza vaccines.

A Randomized Controlled Trial to Compare Immunogenicity to Cell-Based Versus Live-Attenuated Influenza Vaccines in Children

BACKGROUND

Few studies have focused on the immune response to more recent influenza vaccine formulations such as cell-cultured inactivated influenza vaccine (ccIIV4) or live-attenuated influenza vaccine (LAIV4) in older children and young adults, or differences in immunoglobulin response using newer antibody landscape technology.

METHODS

Participants ages 4-21 were randomized to receive ccIIV4 (n = 112) or LAIV4 (n = 118). A novel high-throughput multiplex influenza antibody detection assay was used to provide detailed IgG, IgA, and IgM antibody isotypes, along with hemagglutination inhibition levels (HAI), measured pre- and 28 days post-vaccination.

RESULTS

The HAI and immunoglobulin isotype response to ccIIV4 was greater than LAIV4, with significant increases in IgG but not IgA or IgM. The youngest participants had the highest LAIV4 response. Prior LAIV4 vaccination was associated with a higher response to current season ccIIV4. Cross-reactive A/Delaware/55/2019(H1N1)pdm09 antibodies were present pre-vaccination and increased in response to ccIIV4, but not LAIV4. Immunoglobulin assays strongly correlated with and confirmed the findings of HAI titers to measure immune response.

CONCLUSIONS

Age and prior season vaccination may play a role in the immune response in children and young adults to ccIIV4 and LAIV4. While immunoglobulin isotypes provide high-level antigen-specific information, HAI titers alone can provide a meaningful representation of day 28 post-vaccination response.

CLINICAL TRIALS NO

NCT03982069.

Immunogenicity of High-Dose Egg-Based, Recombinant, and Cell Culture-Based Influenza Vaccines Compared With Standard-Dose Egg-Based Influenza Vaccine Among Health Care Personnel Aged 18-65 Years in 2019-2020

Background

Emerging data suggest that second-generation influenza vaccines with higher hemagglutinin (HA) antigen content and/or different production methods may induce stronger antibody responses to HA than standard-dose egg-based influenza vaccines in adults. We compared antibody responses to high-dose egg-based inactivated (HD-IIV3), recombinant (RIV4), and cell culture-based (ccIIV4) vs standard-dose egg-based inactivated influenza vaccine (SD-IIV4) among health care personnel (HCP) aged 18-65 years in 2 influenza seasons (2018-2019, 2019-2020).

Methods

In the second trial season, newly and re-enrolled HCPs who received SD-IIV4 in season 1 were randomized to receive RIV4, ccIIV4, or SD-IIV4 or were enrolled in an off-label, nonrandomized arm to receive HD-IIV3. Prevaccination and 1-month-postvaccination sera were tested by hemagglutination inhibition (HI) assay against 4 cell culture propagated vaccine reference viruses. Primary outcomes, adjusted for study site and baseline HI titer, were seroconversion rate (SCR), geometric mean titers (GMTs), mean fold rise (MFR), and GMT ratios that compared vaccine groups to SD-IIV4.

Results

Among 390 HCP in the per-protocol population, 79 received HD-IIV3, 103 RIV4, 106 ccIIV4, and 102 SD-IIV4. HD-IIV3 recipients had similar postvaccination antibody titers compared with SD-IIV4 recipients, whereas RIV4 recipients had significantly higher 1-month-postvaccination antibody titers against vaccine reference viruses for all outcomes.

Conclusions

HD-IIV3 did not induce higher antibody responses than SD-IIV4, but, consistent with previous studies, RIV4 was associated with higher postvaccination antibody titers. These findings suggest that recombinant vaccines rather than vaccines with higher egg-based antigen doses may provide improved antibody responses in highly vaccinated populations.

Risk for Infection in Humans after Exposure to Birds Infected with Highly Pathogenic Avian Influenza A(H5N1) Virus, United States, 2022

During February 7─September 3, 2022, a total of 39 states experienced outbreaks of highly pathogenic avian influenza A(H5N1) virus in birds from commercial poultry farms and backyard flocks. Among persons exposed to infected birds, highly pathogenic avian influenza A(H5) viral RNA was detected in 1 respiratory specimen from 1 person.

Mild and asymptomatic influenza B virus infection among unvaccinated pregnant persons: Implication for effectiveness of non-pharmaceutical intervention and vaccination to prevent influenza

BACKGROUND

We estimated symptomatic and asymptomatic influenza infection frequency in community-dwelling unvaccinated pregnant persons to inform risk communication.

METHODS

We collected residue sera from multiple antenatal-care blood draws during October 2016-April 2017. We determined influenza infection as seroconversion with ≥ 4-fold rise in antibody titers between any two serum samples by improved hemagglutinin-inhibition assay including ether-treated B antigens. The serology data were linked to the results of nuclei acid testing (rRT-PCR) based on acute respiratory illness (ARI) surveillance.

RESULTS

Among all participants, 43 %(602/1384) demonstrated serology and/or rRT-PCR evidenced infection, and 44 %(265/602) of all infections were asymptomatic. ARI-associated rRT-PCR testing identified only 10 %(61/602) of total infections. Only 1 %(5/420) of the B Victoria cases reported ARI and had a rRT-PCR positive result, compared with 33 %(54/165) of the H3N2 cases. Among influenza ARI cases with multiple serum samples, 19 %(11/58) had seroconversion to a different subtype prior to the illness.

CONCLUSIONS

The incidence of influenza B infection in unvaccinated pregnant persons is under-estimated substantially. Non-pharmaceutical intervention may have suboptimal effectiveness in preventing influenza B transmission due to the less clinical manifestation compared to influenza A. The findings support maternal influenza vaccination to protect pregnant persons and reduce consequent household transmission.

Factors associated with humoral immune response in older adults who received egg-free influenza vaccine

BACKGROUND

Immune responses to influenza vaccination tend to be lower among older, frequently vaccinated adults. Use of egg-free influenza vaccines is increasing, but limited data exist on factors associated with their immunogenicity in older adults.

METHODS

Community-dwelling older adults ≥ 56 years of age were enrolled in a prospective, observational study of immunogenicity of 2018-2019 influenza vaccine. Hemagglutination inhibition (HAI) antibody titers were measured pre-vaccination (Day 0) and four weeks after vaccination (Day 28) to calculate geometric mean titers, seropositivity (HAI titers ≥ 1:40), seroconversion (fourfold rise in HAI titer with post-vaccination titer ≥ 1:40) and geometric mean fold rise (GMFR). Linear regression models assessed the association of predictors of GMFR for each vaccine antigen.

RESULTS

Among 91 participants who received egg-free influenza vaccines, 84 (92.3 %) received quadrivalent recombinant influenza vaccine (RIV4, Flublok, Sanofi Pasteur), and 7 (7.7 %) received quadrivalent cell culture-based influenza vaccine (ccIIV4, Flucelvax, Seqirus). Pre-vaccination seropositivity was 52.8 % for A(H1N1), 94.5 % for A(H3N2), 61.5 % for B/Colorado and 48.4 % for B/Phuket. Seroconversion by antigen ranged from 16.5 % for A(H1N1) and B/Colorado to 37.4 % for A(H3N2); 40 participants failed to seroconvert to any antigen. Factors independently associated with higher GMFR in multivariable models included lower pre-vaccination HAI antibody titer for A(H1N1), B/Colorado and B/Phuket, and younger age for A(H1N1).

CONCLUSION

Overall pre-vaccination seropositivity was high and just over half of the cohort seroconverted to ≥ 1 vaccine antigen. Antibody responses were highest among participants with lower pre-vaccination titers. Among older adults with high pre-existing antibody titers, approaches to improve immune responses are needed.

An external quality assessment feasibility study; cross laboratory comparison of haemagglutination inhibition assay and microneutralization assay performance for seasonal influenza serology testing: A FLUCOP study

Introduction

External Quality Assessment (EQA) schemes are designed to provide a snapshot of laboratory proficiency, identifying issues and providing feedback to improve laboratory performance and inter-laboratory agreement in testing. Currently there are no international EQA schemes for seasonal influenza serology testing. Here we present a feasibility study for conducting an EQA scheme for influenza serology methods.

Methods

We invited participant laboratories from industry, contract research organizations (CROs), academia and public health institutions who regularly conduct hemagglutination inhibition (HAI) and microneutralization (MN) assays and have an interest in serology standardization. In total 16 laboratories returned data including 19 data sets for HAI assays and 9 data sets for MN assays.

Results

Within run analysis demonstrated good laboratory performance for HAI, with intrinsically higher levels of intra-assay variation for MN assays. Between run analysis showed laboratory and strain specific issues, particularly with B strains for HAI, whilst MN testing was consistently good across labs and strains. Inter-laboratory variability was higher for MN assays than HAI, however both assays showed a significant reduction in inter-laboratory variation when a human sera pool is used as a standard for normalization.

Discussion

This study has received positive feedback from participants, highlighting the benefit such an EQA scheme would have on improving laboratory performance, reducing inter laboratory variation and raising awareness of both harmonized protocol use and the benefit of biological standards for seasonal influenza serology testing.

Influenza A(H7N9) Pandemic Preparedness: Assessment of the Breadth of Heterologous Antibody Responses to Emerging Viruses from Multiple Pre-Pandemic Vaccines and Population Immunity

Influenza A(H7N9) viruses remain as a high pandemic threat. The continued evolution of the A(H7N9) viruses poses major challenges in pandemic preparedness strategies through vaccination. We assessed the breadth of the heterologous neutralizing antibody responses against the 3rd and 5th wave A(H7N9) viruses using the 1st wave vaccine sera from 4 vaccine groups: 1. inactivated vaccine with 2.8 μg hemagglutinin (HA)/dose + AS03A; 2. inactivated vaccine with 5.75 μg HA/dose + AS03A; 3. inactivated vaccine with 11.5 μg HA/dose + MF59; and 4. recombinant virus like particle (VLP) vaccine with 15 μg HA/dose + ISCOMATRIX™. Vaccine group 1 had the highest antibody responses to the vaccine virus and the 3rd/5th wave drifted viruses. Notably, the relative levels of cross-reactivity to the drifted viruses as measured by the antibody GMT ratios to the 5th wave viruses were similar across all 4 vaccine groups. The 1st wave vaccines induced robust responses to the 3rd and Pearl River Delta lineage 5th wave viruses but lower cross-reactivity to the highly pathogenic 5th wave A(H7N9) virus. The population in the United States was largely immunologically naive to the A(H7N9) HA. Seasonal vaccination induced cross-reactive neuraminidase inhibition and binding antibodies to N9, but minimal cross-reactive antibody-dependent cell-mediated cytotoxicity (ADCC) antibodies to A(H7N9).

Low quality antibody responses in critically ill patients hospitalized with pandemic influenza A(H1N1)pdm09 virus infection

Although some adults infected with influenza 2009 A(H1N1)pdm09 viruses mounted high hemagglutination inhibition (HAI) antibody response, they still suffered from severe disease, or even death. Here, we analyzed antibody profiles in patients (n = 31, 17-65 years) admitted to intensive care units (ICUs) with lung failure and invasive mechanical ventilation use due to infection with A(H1N1)pdm09 viruses during 2009-2011. We performed a comprehensive analysis of the quality and quantity of antibody responses using HAI, virus neutralization, biolayer interferometry, enzyme-linked-lectin and enzyme-linked immunosorbent assays. At time of the ICU admission, 45% (14/31) of the patients had HAI antibody titers ≥ 80 in the first serum (S1), most (13/14) exhibited narrowly-focused HAI and/or anti-HA-head binding antibodies targeting single epitopes in or around the receptor binding site. In contrast, 42% (13/31) of the patients with HAI titers ≤ 10 in S1 had non-neutralizing anti-HA-stem antibodies against A(H1N1)pdm09 viruses. Only 19% (6/31) of the patients showed HA-specific IgG1-dominant antibody responses. Three of 5 fatal patients possessed highly focused cross-type HAI antibodies targeting the (K130 + Q223)-epitopes with extremely low avidity. Our findings suggest that narrowly-focused low-quality antibody responses targeting specific HA-epitopes may have contributed to severe infection of the lower respiratory tract.

Effect of Repeat Vaccination on Immunogenicity of Quadrivalent Cell-Culture and Recombinant Influenza Vaccines Among Healthcare Personnel Aged 18-64 Years: A Randomized, Open-Label Trial

BACKGROUND

Antibody responses to non-egg-based standard-dose cell-culture influenza vaccine (containing 15 µg hemagglutinin [HA]/component) and recombinant vaccine (containing 45 µg HA/component) during consecutive seasons have not been studied in the United States.

METHODS

In a randomized trial of immunogenicity of quadrivalent influenza vaccines among healthcare personnel (HCP) aged 18-64 years over 2 consecutive seasons, HCP who received recombinant-HA influenza vaccine (RIV) or cell culture-based inactivated influenza vaccine (ccIIV) during the first season (year 1) were re-randomized the second season of 2019-2020 (year 2 [Y2]) to receive ccIIV or RIV, resulting in 4 ccIIV/RIV combinations. In Y2, hemagglutination inhibition antibody titers against reference cell-grown vaccine viruses were compared in each ccIIV/RIV group with titers among HCP randomized both seasons to receive egg-based, standard-dose inactivated influenza vaccine (IIV) using geometric mean titer (GMT) ratios of Y2 post-vaccination titers.

RESULTS

Y2 data from 414 HCP were analyzed per protocol. Compared with 60 IIV/IIV recipients, 74 RIV/RIV and 106 ccIIV/RIV recipients showed significantly elevated GMT ratios (Bonferroni corrected P < .007) against all components except A(H3N2). Post-vaccination GMT ratios for ccIIV/ccIIV and RIV/ccIIV were not significantly elevated compared with IIV/IIV except for RIV/ccIIV against A(H1N1)pdm09.

CONCLUSIONS

In adult HCP, receipt of RIV in 2 consecutive seasons or the second season was more immunogenic than consecutive egg-based IIV for 3 of the 4 components of quadrivalent vaccine. Immunogenicity of ccIIV/ccIIV was similar to that of IIV/IIV. Differences in HA antigen content may play a role in immunogenicity of influenza vaccination in consecutive seasons.

CLINICAL TRIALS REGISTRATION

NCT03722589.

Multiplex Detection of Antibody Landscapes to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)/Influenza/Common Human Coronaviruses Following Vaccination or Infection With SARS-CoV-2 and Influenza

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza viruses continue to co-circulate, representing 2 major public health threats from respiratory infections with similar clinical presentations. SARS-CoV-2 and influenza vaccines can also now be co-administered. However, data on antibody responses to SARS-CoV-2 and influenza coinfection and vaccine co-administration remain limited.

METHODS

We developed a 41-plex antibody immunity assay that can simultaneously characterize antibody landscapes to SARS-CoV-2/influenza/common human coronaviruses. We analyzed sera from 840 individuals (11-93 years), including sera from reverse transcription-polymerase chain reaction (RT-PCR)-confirmed SARS-CoV-2-positive (n = 218) and -negative (n = 120) cases, paired sera from SARS-CoV-2 vaccination (n = 29) and infection (n = 11), and paired sera from influenza vaccination (n = 56) and RT-PCR-confirmed influenza infection (n = 158) cases. Last, we analyzed sera collected from 377 individuals who exhibited acute respiratory illness (ARI) in 2020.

RESULTS

This 41-plex assay has high sensitivity and specificity in detecting SARS-CoV-2 infections. It differentiated SARS-CoV-2 vaccination (antibody responses only to spike protein) from infection (antibody responses to both spike and nucleoprotein). No cross-reactive antibodies were induced to SARS-CoV-2 from influenza vaccination and infection, and vice versa, suggesting no interaction between SARS-CoV-2 and influenza antibody responses. However, cross-reactive antibodies were detected between spike proteins of SARS-CoV-2 and common human coronaviruses that were removed by serum adsorption. Among 377 individuals who exhibited ARI in 2020, 129 were influenza positive; none had serological evidence of SARS-CoV-2/influenza coinfections.

CONCLUSIONS

Multiplex detection of antibody landscapes can provide in-depth analysis of the antibody protective immunity to SARS-CoV-2 in the context of other respiratory viruses, including influenza.

A randomized controlled trial of antibody response to 2019-20 cell-based inactivated and egg-based live attenuated influenza vaccines in children and young adults

BACKGROUND

Hemagglutination inhibition (HAI) titers to the live-attenuated influenza vaccine (LAIV4) are typically lower than its counterpart egg-based inactivated influenza vaccines (IIV). Similar comparisons have not been made between LAIV4 and the 4-strain, cell-culture inactivated influenza vaccine (ccIIV4). We compared healthy children's and young adults' HAI titers against the 2019-2020 LAIV4 and ccIIV4.

METHODS

Participants aged 4-21 years were randomized 1:1 to receive ccIIV4 (n = 100) or LAIV4 (n = 98). Blood was drawn prevaccination and on day 28 (21-35) post vaccination. HAI assays against egg-grown A/H1N1, A/H3N2, both vaccine B strains and cell-grown A/H3N2 antigens were conducted. Primary outcomes were geometric mean titers (GMT) and geometric mean fold rise (GMFR) in titers.

RESULTS

GMTs to A/H1N1, A/H3N2 and B/Victoria increased following both ccIIV and LAIV and to B/Yamagata following ccIIV (p < 0.05). The GMFR range was 2.4-3.0 times higher for ccIIV4 than for LAIV4 (p < 0.001). Within vaccine types, egg-grown A/H3N2 GMTs were higher (p < 0.05) than cell-grown GMTs [ccIIV4 day 28: egg = 205 (95% CI: 178-237); cell = 136 (95% CI:113-165); LAIV4 day 28: egg = 96 (95% CI: 83-112); cell = 63 (95% CI: 58-74)]. The GMFR to A/H3N2 cell-grown and egg-grown antigens were similar. Pre-vaccination titers inversely predicted GMFR.

CONCLUSION

The HAI response to ccIIV4 was greater than LAIV4 in this study of mostly older children, and day 0 HAI titers inversely predicted GMFR for both vaccines. Lower prevaccination titers were associated with greater GMFR in both vaccine groups.

Serologic response to sequential vaccination with enhanced influenza vaccines: Open label randomized trial among adults aged 65-74 years

BACKGROUND

The effects of sequential vaccination with enhanced influenza vaccines are poorly understood. We conducted an exploratory open-label study to assess serologic response to sequential vaccination in older adults.

METHODS

160 adults aged 65 through 74 years were randomized (1:1:1) to receive trivalent inactivated standard dose (SD), high-dose (HD), or MF59-adjuvanted (AD) vaccine in 2016/17. In 2017/18, HD and AD recipients received the same vaccine; SD recipients were re-randomized to HD or AD. Hemagglutination inhibition assays were performed using turkey erythrocytes against A/California/7/2009(H1N1)-like and B/Brisbane/60/2008(B/Victoria)-like in both seasons, and A/Michigan/45/2015(H1N1)-like in season 2. Microneutralization assays were performed against cell-propagated A/Hong Kong/4801/2014(H3N2)-like using MDCK-SIAT1 cells. Postvaccination geometric mean titer (GMT), percent with titer ≥ 40, and mean fold rise (MFR, ratio of postvaccination versus prevaccination titer) in season 2 were compared across groups, and ratio of MFR in season 2 versus season 1 was assessed for each strain.

RESULTS

Analysis included 152 participants (55 HD → HD, 58 AD → AD, 19 SD → HD, and 20 SD → AD). Season 2 postvaccination GMTs and percent with titer ≥ 40 did not differ between HD → HD and AD → AD recipients for vaccine strains examined. However, a higher percent of HD → HD and AD → AD recipients had postvaccination titer ≥ 40 than SD → AD recipients for A/H1N1 (86%-89% versus 60%) and SD → AD and SD → HD recipients for A/H3N2 (83%-87% versus 40%-53%). GMTs were higher in AD → AD versus SD → AD recipients for A/H1N1 (p = .01) and A/H3N2 (p = .002). MFRs in season 2 were low in all groups for A/H3N2 (1.5-2.2) and B/Victoria (1.7-2.3). MFR was lower in season 2 versus 1 for HD → HD and AD → AD recipients for all vaccine strains (1.6-3.7 versus 2.6-6.2).

CONCLUSIONS

Sequential vaccination with enhanced vaccines did not reduce immunogenicity in adults aged 65 through 74 years. Serologic response to cell-propagated A/H3N2 was suboptimal for all vaccines. ClinicalTrials.gov identifier. NCT02872311.

Comparison of the Immunogenicity of Cell Culture-Based and Recombinant Quadrivalent Influenza Vaccines to Conventional Egg-Based Quadrivalent Influenza Vaccines among Healthcare Personnel Aged 18-64 Years: A Randomized Open-Label Trial

Background

RIV4 and cell-culture based inactivated influenza vaccine (ccIIV4) have not been compared to egg-based IIV4 in healthcare personnel, a population with frequent influenza vaccination that may blunt vaccine immune responses over time. We conducted a randomized trial among healthcare personnel (HCP) aged 18–64 years to compare humoral immune responses to ccIIV4 and RIV4 to IIV4.

Methods

During the 2018–2019 season, participants were randomized to receive ccIIV4, RIV4, or IIV4 and had serum samples collected prevaccination, 1 and 6 months postvaccination. Serum samples were tested by hemagglutination inhibition (HI) for influenza A/H1N1, B/Yamagata, and B/Victoria and microneutralization (MN) for A/H3N2 against cell-grown vaccine reference viruses. Primary outcomes at 1 month were seroconversion rate (SCR), geometric mean titers (GMT), GMT ratio, and mean fold rise (MFR) in the intention-to-treat population.

Results

In total, 727 participants were included (283 ccIIV4, 202 RIV4, and 242 IIV4). At 1 month, responses to ccIIV4 were similar to IIV4 by SCR, GMT, GMT ratio, and MFR. RIV4 induced higher SCRs, GMTs, and MFRs than IIV4 against A/H1N1, A/H3N2, and B/Yamagata. The GMT ratio of RIV4 to egg-based vaccines was 1.5 (95% confidence interval [CI] 1.2–1.9) for A/H1N1, 3.0 (95% CI: 2.4–3.7) for A/H3N2, 1.1 (95% CI: .9–1.4) for B/Yamagata, and 1.1 (95% CI: .9–1.3) for B/Victoria. At 6 months, ccIIV4 recipients had similar GMTs to IIV4, whereas RIV4 recipients had higher GMTs against A/H3N2 and B/Yamagata.

Conclusions

RIV4 resulted in improved antibody responses by HI and MN compared to egg-based vaccines against 3 of 4 cell-grown vaccine strains 1 month postvaccination, suggesting a possible additional benefit from RIV4.

Heterologous Antibody Responses Conferred by A(H3N2) Variant and Seasonal Influenza Vaccination Against Newly Emerged 2016-2018 A(H3N2) Variant Viruses in Healthy Persons

BACKGROUND

Swine origin A(H3N2) variant [A(H3N2)v] viruses continue to evolve and remain a public health threat. Recent outbreaks in humans in 2016-2018 were caused by a newly emerged A(H3N2)v cluster 2010.1, which are genetically and antigenically distinct from the previously predominant cluster IV. To address the public health risk, we evaluated the levels of heterologous cross-reactive antibodies to A(H3N2)v cluster 2010.1 viruses induced from an existing cluster IV A(H3N2)v vaccine and several seasonal inactivated influenza vaccines (IIVs) in adults, elderly individuals, and children.

METHODS

Human vaccine sera and ferret antisera were analyzed by hemagglutination inhibition (HI) and neutralization assays against representative A(H3N2)v viruses from clusters IV and 2010.1 and seasonal A(H3N2) viruses.

RESULTS

Ferret antisera detected no or little cross-reactivity between the 2 A(H3N2)v clusters or between A(H3N2)v and seasonal A(H3N2) viruses. In humans, cluster IV A(H3N2)v vaccine induced antibodies cross-reactive to cluster 2010.1 viruses in approximately one-third of the 89 adult and elderly vaccinees. Seasonal IIVs did not induce seroprotective antibodies (≥40) to A(H3N2)v viruses in young children, but induced higher antibodies to A(H3N2)v viruses in cluster 2010.1 than those in cluster IV in adults.

CONCLUSIONS

Cluster IV A(H3N2)v vaccine did not provide sufficient heterologous antibody responses against the new 2010.1 cluster A(H3N2)v viruses. Seasonal IIV could not induce seroprotective antibodies to 2010.1 cluster A(H3N2)v viruses in young children, suggesting that young children are still at high risk to the newly emerged A(H3N2)v viruses. Continued surveillance on A(H3N2)v viruses is critical for risk assessment and pandemic preparedness.

Comparative Immunogenicity of Several Enhanced Influenza Vaccine Options for Older Adults: A Randomized, Controlled Trial

BACKGROUND

Enhanced influenza vaccines may improve protection for older adults, but comparative immunogenicity data are limited. Our objective was to examine immune responses to enhanced influenza vaccines, compared to standard-dose vaccines, in community-dwelling older adults.

METHODS

Community-dwelling older adults aged 65-82 years in Hong Kong were randomly allocated (October 2017-January 2018) to receive 2017-2018 Northern hemisphere formulations of a standard-dose quadrivalent vaccine, MF59-adjuvanted trivalent vaccine, high-dose trivalent vaccine, or recombinant-hemagglutinin (rHA) quadrivalent vaccine. Sera collected from 200 recipients of each vaccine before and at 30-days postvaccination were assessed for antibodies to egg-propagated vaccine strains by hemagglutination inhibition (HAI) and to cell-propagated A/Hong Kong/4801/2014(H3N2) virus by microneutralization (MN). Influenza-specific CD4+ and CD8+ T cell responses were assessed in 20 participants per group.

RESULTS

Mean fold rises (MFR) in HAI titers to egg-propagated A(H1N1) and A(H3N2) and the MFR in MN to cell-propagated A(H3N2) were statistically significantly higher in the enhanced vaccine groups, compared to the standard-dose vaccine. The MFR in MN to cell-propagated A(H3N2) was highest among rHA recipients (4.7), followed by high-dose (3.4) and MF59-adjuvanted (2.9) recipients, compared to standard-dose recipients (2.3). Similarly, the ratio of postvaccination MN titers among rHA recipients to cell-propagated A(H3N2) recipients was 2.57-fold higher than the standard-dose vaccine, which was statistically higher than the high-dose (1.33-fold) and MF59-adjuvanted (1.43-fold) recipient ratios. Enhanced vaccines also resulted in the boosting of T-cell responses.

CONCLUSIONS

In this head-to-head comparison, older adults receiving enhanced vaccines showed improved humoral and cell-mediated immune responses, compared to standard-dose vaccine recipients.

CLINICAL TRIALS REGISTRATION

NCT03330132.

Age-specific effects of vaccine egg adaptation and immune priming on A(H3N2) antibody responses following influenza vaccination

A(H3N2) influenza vaccine effectiveness (VE) was low during the 2016-19 seasons and varied by age. We analyzed neutralizing antibody responses to egg- and cell-propagated A(H3N2) vaccine and circulating viruses following vaccination in 375 individuals (aged 7 months to 82 years) across all vaccine-eligible age groups in 3 influenza seasons. Antibody responses to cell- versus egg-propagated vaccine viruses were significantly reduced due to the egg-adapted changes T160K, D225G, and L194P in the vaccine hemagglutinins. Vaccine egg adaptation had a differential impact on antibody responses across the different age groups. Immunologically naive children immunized with egg-adapted vaccines mostly mounted antibodies targeting egg-adapted epitopes, whereas those previously primed with infection produced broader responses even when vaccinated with egg-based vaccines. In the elderly, repeated boosts of vaccine egg-adapted epitopes significantly reduced antibody responses to the WT cell-grown viruses. Analysis with reverse genetic viruses suggested that the response to each egg-adapted substitution varied by age. No differences in antibody responses were observed between male and female vaccinees. Here, the combination of age-specific responses to vaccine egg-adapted substitutions, diverse host immune priming histories, and virus antigenic drift affected antibody responses following vaccination and may have led to the low and variable VE against A(H3N2) viruses across different age groups.

The impact of physical frailty on the response to inactivated influenza vaccine in older adults

Physical frailty's impact on hemagglutination inhibition antibody titers (HAI) and peripheral blood mononuclear cell (PBMC) transcriptional responses after influenza vaccination is unclear. Physical frailty was assessed using the 5-item Fried frailty phenotype in 168 community- and assisted-living adults ≥55 years of age during an observational study. Blood was drawn before, 3, 7, and 28 days post-vaccination with the 2017-2018 inactivated influenza vaccine. HAI response to the A/H1N1 strain was measured at Days 0 and 28 using seropositivity, seroconversion, log2 HAI titers, and fold-rise in log2 HAI titers. RNA sequencing of PBMCs from Days 0, 3 and 7 was measured in 28 participants and compared using pathway analyses. Frailty was not significantly associated with any HAI outcome in multivariable models. Compared with non-frail participants, frail participants expressed decreased cell proliferation, metabolism, antibody production, and interferon signaling genes. Conversely, frail participants showed elevated gene expression in IL-8 signaling, T-cell exhaustion, and oxidative stress pathways compared with non-frail participants. These results suggest that reduced effectiveness of influenza vaccine among older, frail individuals may be attributed to immunosenescence-related changes in PBMCs that are not reflected in antibody levels.

Development and Assessment of a Pooled Serum as Candidate Standard to Measure Influenza A Virus Group 1 Hemagglutinin Stalk-Reactive Antibodies

The stalk domain of the hemagglutinin has been identified as a target for induction of protective antibody responses due to its high degree of conservation among numerous influenza subtypes and strains. However, current assays to measure stalk-based immunity are not standardized. Hence, harmonization of assay readouts would help to compare experiments conducted in different laboratories and increase confidence in results. Here, serum samples from healthy individuals (n = 110) were screened using a chimeric cH6/1 hemagglutinin enzyme-linked immunosorbent assay (ELISA) that measures stalk-reactive antibodies. We identified samples with moderate to high IgG anti-stalk antibody levels. Likewise, screening of the samples using the mini-hemagglutinin (HA) headless construct #4900 and analysis of the correlation between the two assays confirmed the presence and specificity of anti-stalk antibodies. Additionally, samples were characterized by a cH6/1N5 virus-based neutralization assay, an antibody-dependent cell-mediated cytotoxicity (ADCC) assay, and competition ELISAs, using the stalk-reactive monoclonal antibodies KB2 (mouse) and CR9114 (human). A “pooled serum” (PS) consisting of a mixture of selected serum samples was generated. The PS exhibited high levels of stalk-reactive antibodies, had a cH6/1N5-based neutralization titer of 320, and contained high levels of stalk-specific antibodies with ADCC activity. The PS, along with blinded samples of varying anti-stalk antibody titers, was distributed to multiple collaborators worldwide in a pilot collaborative study. The samples were subjected to different assays available in the different laboratories, to measure either binding or functional properties of the stalk-reactive antibodies contained in the serum. Results from binding and neutralization assays were analyzed to determine whether use of the PS as a standard could lead to better agreement between laboratories. The work presented here points the way towards the development of a serum standard for antibodies to the HA stalk domain of phylogenetic group 1.

Impact of Immune Priming, Vaccination, and Infection on Influenza A(H3N2) Antibody Landscapes in Children

BACKGROUND

Preexisting antibodies to influenza, shaped by early infection and subsequent exposures, may impact responses to influenza vaccination.

METHODS

We enrolled 72 children (aged 7-17 years) in 2015-2016; all received inactivated influenza vaccines. Forty-one were also vaccinated in 2014-2015, with 12 becoming infected with A(H3N2) in 2014-2015. Thirty-one children did not have documented influenza exposures in the prior 5 seasons. Sera were collected pre- and postvaccination in both seasons. We constructed antibody landscapes using hemagglutination inhibition antibody titers against 16 A(H3N2) viruses representative of major antigenic clusters that circulated between 1968 and 2015.

RESULTS

The breadth of the antibody landscapes increased with age. Vaccine-induced antibody responses correlated with boosting of titers to previously encountered antigens. Postvaccination titers were the highest against vaccine antigens rather than the historic A(H3N2) viruses previously encountered. Prevaccination titers to the vaccine were the strongest predictors of postvaccination titers. Responses to vaccine antigens did not differ by likely priming virus. Influenza A(H3N2)-infected children in 2014-2015 had narrower antibody landscapes than those uninfected, but prior season infection status had little effect on antibody landscapes following 2015-2016 vaccination.

CONCLUSIONS

A(H3N2) antibody landscapes in children were largely determined by age-related immune priming, rather than recent vaccination or infection.

The Effect of Preexisting Immunity on Virus Detection and Immune Responses in a Phase II, Randomized Trial of a Russian-Backbone, Live, Attenuated Influenza Vaccine in Bangladeshi Children

Background

In a 2012 Phase II clinical trial, 300 Bangladeshi children aged 24 to 59 months with no prior influenza vaccine exposure were randomized to receive a single intranasally-administered dose of either trivalent, Russian-backbone, live, attenuated influenza vaccine (LAIV) or placebo. Protocol-defined analyses, presented in the companion manuscript, demonstrate decreased viral detection and immunogenicity for A/H1N1pdm09, relative to the A/H3N2 and B strains. This post hoc analysis of the trial data aims to investigate the LAIV strain differences by testing the hypothesis that preexisting humoral and mucosal immunity may influence viral recovery and immune responses after LAIV receipt.

Methods

We used logistic regressions to evaluate the relations between markers of preexisting immunity (ie, hemagglutination inhibition [HAI], microneutralization, and immunoglobulin G and immunoglobulin A (both serum and mucosal antibodies) and LAIV viral recovery in the week post-vaccination. We then tested for potential effect modification by baseline HAI titers (ie, <10 versus ≥10) and week 1 viral recovery on the LAIV-induced serum and mucosal immune responses, measured between days 0 and 21 post-vaccination.

Results

Higher levels of preexisting immunity to influenza A/H3N2 and B were strongly associated with strain-specific prevention of viral shedding upon LAIV receipt. While evidence of LAIV immunogenicity was observed for all 3 strains, the magnitudes of immune responses were most pronounced in children with no evidence of preexisting HAI and in those with detectable virus.

Conclusions

The results provide evidence for a bidirectional association between viral replication and immunity, and underscore the importance of accounting for preexisting immunity when evaluating virologic and immunologic responses to LAIVs.

Clinical Trials Registration

NCT01625689.

Immunogenicity and Viral Shedding of Russian-Backbone, Seasonal, Trivalent, Live, Attenuated Influenza Vaccine in a Phase II, Randomized, Placebo-Controlled Trial Among Preschool-Aged Children in Urban Bangladesh

Background

We evaluated a Russian-backbone, live, attenuated influenza vaccine (LAIV) for immunogenicity and viral shedding in a randomized, placebo-controlled trial among Bangladeshi children.

Methods

Healthy children received a single, intranasal dose of LAIV containing the 2011–2012 recommended formulation or placebo. Nasopharyngeal wash (NPW) specimens were collected on days 0, 2, 4, and 7. Reverse transcription polymerase chain reactions and sequencing identified the influenza virus (vaccine or wild-type). On days 0 and 21, blood specimens were collected to assess immunogenicity using hemagglutination inhibition, microneutralization, and immunoglobulin A (IgA) and G enzyme-linked immunosorbent assays (ELISAs); NPW specimens were also collected to assess mucosal immunogenicity using kinetic IgA ELISA.

Results

We enrolled 300 children aged 24 through 59 months in the immunogenicity and viral shedding analyses. Among children receiving LAIV, 45% and 67% shed A/H3N2 and B vaccine strains, respectively. No child shed A/H1N1 vaccine strain. There were significantly higher day 21 geometric mean titers (GMTs) for the LAIV, as compared to the placebo groups, in all immunoassays for A/H3N2 and B (log₁₀ titer P < .0001; GMT Ratio >2.0). Among immunoassays for A/H1N1, only the mucosal IgA GMT was significantly higher than placebo at day 21 (log₁₀ titer P = .0465).

Conclusions

Children vaccinated with LAIV had serum and mucosal antibody responses to A/H3N2 and B, but only a mucosal IgA response to A/H1N1. Many children shed A/H3N2 and B vaccine strains, but none shed A/H1N1. More research is needed to determine the reason for decreased LAIV A/H1N1 immunogenicity and virus shedding.

Clinical Trials Registration

NCT01625689.

A randomized controlled trial of antibody response to 2018-19 cell-based vs. egg-based quadrivalent inactivated influenza vaccine in children

BACKGROUND

Current influenza vaccine effectiveness (VE) improvement efforts focus on minimizing egg adaptation mutations during manufacture. This study compared immune response of two FDA-approved quadrivalent inactivated influenza vaccines in an unblinded randomized controlled trial.

METHODS

Participants were 144 community dwelling, healthy children/adolescents aged 4-20 years, randomized 1:1 in blocks of 4 to a vaccine grown in cell culture (ccIIV4 [Flucelvax®]; n = 85); or in egg medium (IIV4 [Fluzone ®]; n = 83). Blood was drawn at day 0 prevaccination and at day 28 (19-35 days) post vaccination. Hemagglutination inhibition (HI) assays against A/H1N1 and both B strains and microneutralization (MN) assays against egg-based and cell-based A/H3N2 strains were conducted. The primary outcome measure was seroconversion (day 28/day 0 titer ratio ≥ 4 with day 28 titer ≥ 40). Secondary outcomes were elevated titers (day 28 HI titer ≥ 1:110), geometric mean titers (GMTs) and mean fold rise (MFR) in titers. Outcomes were compared for 74 ccIIV4 recipients and 70 IIV4 recipients, and for those vaccinated and unvaccinated the previous year. Only the HI and MN laboratory analysis team was blinded to group assignment.

RESULTS

In this racially diverse (81% non-white) group of children with a median age of 14 years, baseline demographics did not differ between vaccine groups. At day 0, half or more in each vaccine group had elevated HI or MN titers. Low seroconversion rates (14%-35%) were found; they did not differ between groups. Among 2018-19 ccIIV4 recipients, those unvaccinated in the previous season showed significantly higher MFR against A/H1N1 and A/H3N2 cell-grown virus than the previously vaccinated. Similar results were found for MFR against B/Victoria among 2018-2019 IIV4 recipients.

CONCLUSION

In mostly older children with high baseline titers, no differences in seroconversion or other measures of antibody titers were found between ccIIV4 and IIV4 recipients against egg- and cell-grown influenza vaccine viruses.

CLINICAL TRIALS NO

NCT03614975.

Minimal transmission in an influenza A (H3N2) human challenge-transmission model within a controlled exposure environment

Uncertainty about the importance of influenza transmission by airborne droplet nuclei generates controversy for infection control. Human challenge-transmission studies have been supported as the most promising approach to fill this knowledge gap. Healthy, seronegative volunteer ‘Donors’ (n = 52) were randomly selected for intranasal challenge with influenza A/Wisconsin/67/2005 (H3N2). ‘Recipients’ randomized to Intervention (IR, n = 40) or Control (CR, n = 35) groups were exposed to Donors for four days. IRs wore face shields and hand sanitized frequently to limit large droplet and contact transmission. One transmitted infection was confirmed by serology in a CR, yielding a secondary attack rate of 2.9% among CR, 0% in IR (p = 0.47 for group difference), and 1.3% overall, significantly less than 16% (p<0.001) expected based on a proof-of-concept study secondary attack rate and considering that there were twice as many Donors and days of exposure. The main difference between these studies was mechanical building ventilation in the follow-on study, suggesting a possible role for aerosols.

Understanding the relative importance of influenza modes of transmission informs strategic use of preventive measures to reduce influenza risk in high-risk settings such as hospitals and is important for pandemic preparedness. Given the increasing evidence from epidemiological modelling, exhaled viral aerosol, and aerobiological survival studies supporting a role for airborne transmission and the potential benefit of respirators (and other precautions designed to prevent inhalation of aerosols) versus surgical masks (mainly effective for reducing exposure to large droplets) to protect healthcare workers, more studies are needed to evaluate the extent of risk posed airborne versus contact and large droplet spray transmission modes. New human challenge-transmission studies should be carefully designed to overcome limitations encountered in the current study. The low secondary attack rate reported herein also suggests that the current challenge-transmission model may no longer be a more promising approach to resolving questions about transmission modes than community-based studies employing environmental monitoring and newer, state-of-the-art deep sequencing-based molecular epidemiological methods.

Stockpiled Avian Influenza A(H7N9) Vaccines Induce Robust, Nonneutralizing Functional Antibodies Against Antigenically Drifted Fifth-Wave A(H7N9) Viruses

Human infections caused by avian influenza A(H7N9) viruses have raised concerns of a pandemic. The capability of the current stockpiled A(H7N9) vaccines to induce cross-protective, nonneutralizing functional antibodies against antigenically drifted A(H7N9) viruses has not been evaluated before. Here we show that vaccination with either MF59- or AS03-adjuvanted inactivated A(H7N9) vaccines elicited robust, cross-reactive antibody-dependent cell-mediated cytotoxicity-mediating and neuraminidase-inhibiting functional antibodies against the antigenically drifted A(H7N9) viruses that emerged recently during the fifth-wave outbreak in China, including a highly pathogenic A(H7N9) human isolate. Such cross-reactive humoral immunity may provide vital first-line defense against fatal outcomes in case of an A(H7N9) pandemic.

Birth Cohort Effects in Influenza Surveillance Data: Evidence That First Influenza Infection Affects Later Influenza-Associated Illness

BACKGROUND

The evolution of influenza A viruses results in birth cohorts that have different initial influenza virus exposures. Historically, A/H3 predominant seasons have been associated with more severe influenza-associated disease; however, since the 2009 pandemic, there are suggestions that some birth cohorts experience more severe illness in A/H1 predominant seasons.

METHODS

United States influenza virologic, hospitalization, and mortality surveillance data during 2000-2017 were analyzed for cohorts born between 1918 and 1989 that likely had different initial influenza virus exposures based on viruses circulating during early childhood. Relative risk/rate during H3 compared with H1 predominant seasons during prepandemic versus pandemic and later periods were calculated for each cohort.

RESULTS

During the prepandemic period, all cohorts had more influenza-associated disease during H3 predominant seasons than H1 predominant seasons. During the pandemic and later period, 4 cohorts had higher hospitalization and mortality rates during H1 predominant seasons than H3 predominant seasons.

CONCLUSIONS

Birth cohort differences in risk of influenza-associated disease by influenza A virus subtype can be seen in US influenza surveillance data and differ between prepandemic and pandemic and later periods. As the population ages, the amount of influenza-associated disease may be greater in future H1 predominant seasons than H3 predominant seasons.

Expression and characterization of a monoclonal IgA isotype antibody against group 1 and group 2 influenza of the HV1-18 QxxV class

Examining cross-reactive influenza-specific human monoclonal antibodies can inform the rational design of universal flu vaccines. The vast majority of monoclonal antibodies studied to-date are of the IgG isotype or cloned into IgG vectors for expression. However, IgA isotype antibodies are also of interest, especially given their increased concentrations in the respiratory tract where the influenza virus replicates. To acquire anti-influenza monoclonal antibodies that are of both IgG and IgA isotypes, we used a fluorescently labeled hemagglutinin (HA) for fluorescence-activated cell sorting on memory B cells 14 days post-influenza vaccination. We then sequenced paired heavy and light chains with the 10× Genomics platform and chose relevant sequences to synthesize for expression and characterization. An IgA1 antibody of the HV1-18 QxxV class demonstrated binding to both group 1 and group 2 HA subtypes in enzyme-linked immunosorbent assays. Although Hemagglutinin inhibition assays were negative, neutralizing activity was positive in a microneutralization activity, at levels comparable to other published stalk-binding and neutralizing monoclonal antibodies, FI6v3 and CR9114. By testing the antibody against a panel of HA mutants, we found that the antibody bound to a region of the HA stalk previously reported for IgG1 antibodies of this class. Studies are on-going, but thus far demonstrate a broadly cross-reactive and neutralizing IgA antibody that recognizes the HA stalk of both group 1 and group 2 influenza viruses.

Prospective cohort study of influenza vaccine effectiveness among healthcare personnel in Lima, Peru: Estudio Vacuna de Influenza Peru, 2016-2018

Background

The Estudio Vacuna de Influenza Peru (VIP) cohort aims to describe the frequency of influenza virus infection, identify predictors of vaccine acceptance, examine the effects of repeated influenza vaccination on immunogenicity, and evaluate influenza vaccine effectiveness among HCP.

Methods

The VIP cohort prospectively followed HCP in Lima, Peru, during the 2016‐2018 influenza seasons; a fourth year is ongoing. Participants contribute blood samples before and after the influenza season and after influenza vaccination (for vaccinees). Weekly surveillance is conducted to identify acute respiratory or febrile illnesses (ARFI). When an ARFI is identified, participants self‐collect nasal swabs that are tested for influenza viruses by real‐time reverse transcriptase‐polymerase chain reaction. Influenza vaccination status and 5‐year vaccination history are ascertained. We analyzed recruitment and enrollment results for 2016‐2018 and surveillance participation for 2016‐2017.

Results

In the first 3 years of the cohort, VIP successfully contacted 92% of potential participants, enrolled 76% of eligible HCP, and retained >90% of participants across years. About half of participants are medical assistants (54%), and most provide “hands‐on” medical care (76%). Sixty‐nine percent and 52% of participants completed surveillance for >70% of weeks in years 1 and 2, respectively. Fewer weeks of completed surveillance was associated with older age (≥50 years), being a medical assistant, self‐rated health of fair or poor, and not receiving the influenza vaccine during the current season (P‐values < .05).

Conclusions

The VIP cohort provides an opportunity to address knowledge gaps about influenza virus infection, vaccination uptake, effectiveness and immunogenicity among HCP.

Antibodies Against Egg- and Cell-Grown Influenza A(H3N2) Viruses in Adults Hospitalized During the 2017-2018 Influenza Season

BACKGROUND

Influenza vaccine effectiveness was low in 2017-2018, yet circulating influenza A(H3N2) viruses were antigenically similar to cell-grown vaccine strains. Notably, most influenza vaccines are egg propagated.

METHODS

Serum specimens were collected shortly after illness onset from 15 influenza A(H3N2) virus-infected cases and 15 uninfected hospitalized adults. Geometric mean titers against egg- and cell-grown influenza A/Hong Kong/4801/2014(H3N2) virus vaccine strains and representative circulating viruses (including A/Washington/16/2017) were determined by a microneutralization (MN) assay. Independent effects of strain-specific titers on susceptibility were estimated by logistic regression.

RESULTS

MN titers against egg-grown influenza A/Hong Kong virus were significantly higher among vaccinated individuals (173 vs 41; P = 0.01). In unadjusted models, a 2-fold increase in titers against egg-grown influenza A/Hong Kong virus was not significantly protective (29% reduction; P = .09), but a similar increase in the cell-grown influenza A/Washington virus antibody titer (3C.2a2) was protective (60% reduction; P = .02). Higher egg-grown influenza A/Hong Kong virus titers were not significantly associated with infection, when adjusted for antibody titers against influenza A/Washington virus (15% reduction; P = .61). A 54% reduction in the odds of infection was observed with a 2-fold increase in titer against influenza A/Washington virus (P = not significant), adjusted for the titer against egg-grown influenza A/Hong Kong virus titer.

CONCLUSION

Individuals vaccinated in 2017-2018 had high antibody titers against the egg-adapted vaccine strain and lower titers against circulating viruses. Titers against circulating but not egg-adapted strains were correlated with protection.

Identification of novel influenza A virus exposures by an improved high-throughput multiplex MAGPIX platform and serum adsorption

Background

The development of serologic assays that can rapidly assess human exposure to novel influenza viruses remains a public health need. Previously, we developed an 11‐plex magnetic fluorescence microsphere immunoassay (MAGPIX) by using globular head domain recombinant hemagglutinins (rHAs) with serum adsorption using two ectodomain rHAs.

Methods

We compared sera collected from two cohorts with novel influenza exposures: animal shelter staff during an A(H7N2) outbreak in New York City in 2016‐2017 (n = 119 single sera) and poultry workers from a live bird market in Bangladesh in 2012‐2014 (n = 29 pairs). Sera were analyzed by microneutralization (MN) assay and a 20‐plex MAGPIX assay with rHAs from 19 influenza strains (11 subtypes) combined with serum adsorption using 8 rHAs from A(H1N1) and A(H3N2) viruses. Antibody responses were analyzed to determine the novel influenza virus exposure.

Results

Among persons with novel influenza virus exposures, the median fluorescence intensity (MFI) against the novel rHA from exposed influenza virus had the highest correlation with MN titers to the same viruses and could be confirmed by removal of cross‐reactivity from seasonal H1/H3 rHAs following serum adsorption. Interestingly, in persons with exposures to novel influenza viruses, age and MFIs against exposed novel HA were negatively correlated, whereas in persons without exposure to novel influenza viruses, age and MFI against novel HAs were positively correlated.

Conclusions

This 20‐plex high‐throughput assay with serum adsorption will be a useful tool to detect novel influenza virus infections during influenza outbreak investigations and surveillance, especially when well‐paired serum samples are not available.

Influence of Immune Priming and Egg Adaptation in the Vaccine on Antibody Responses to Circulating A(H1N1)pdm09 Viruses After Influenza Vaccination in Adults

Background

Although ferret antisera used in influenza surveillance did not detect antigenic drift of A(H1N1)pdm09 viruses during the 2015-2016 season, low vaccine effectiveness was reported in adults. We investigated the immune basis of low responses to circulating A(H1N1)pdm09 viruses after vaccination.

Methods

Prevaccination and postvaccination serum samples collected from >300 adults (aged 18-49 years) in 6 seasons (2010-2011 to 2015-2016) were analyzed using hemagglutination inhibition assays to evaluate the antibody responses to 13 A(H1N1) viruses circulated from 1977 to 2016. Microneutralization and serum adsorption assays were used to verify the 163K and 223R specificity of antibodies.

Results

Individual antibody profiles to A(H1N1) viruses revealed 3 priming patterns: USSR/77, TW/86, or NC/99 priming. More than 20% of adults had reduced titers to cell-propagated circulating 6B.1 and 6B.2 A(H1N1)pdm09 viruses compared with the A/California/07/2009 vaccine virus X-179A. Significantly reduced antibody reactivity to circulating viruses bearing K163Q was observed only in the USSR/77-primed cohort, whereas significantly lower reactivity caused by egg-adapted Q223R change was detected across all 3 cohorts.

Conclusion

Both 163K specificity driven by immune priming and 223R specificity from egg-adapted changes in the vaccine contributed to low responses to circulating A(H1N1)pdm09 viruses after vaccination. Our study highlights the need to incorporate human serology in influenza surveillance and vaccine strain selection.

Underdetection of laboratory-confirmed influenza-associated hospital admissions among infants: a multicentre, prospective study

BACKGROUND

Since influenza often presents non-specifically in infancy, we aimed to assess the extent to which existing respiratory surveillance platforms might underestimate the frequency of severe influenza disease among infants.

METHODS

The Influenza and Respiratory Syncytial Virus in Infants (IRIS) study was a prospective observational study done at four hospitals in Albania, Jordan, Nicaragua, and the Philippines. We included acutely ill infants aged younger than 1 year admitted to hospital within 10 days or less of illness onset during two influenza seasons (2015-16 and 2016-17) in Albania, Jordan, and Nicaragua, and over a continuous 34 week period (2015-16) in the Philippines. We assessed the frequency of influenza virus infections by real-time RT-PCR (rRT-PCR) and serology. The main study outcome was seroconversion, defined as convalescent antibody titres more than or equal to four-fold higher than acute sera antibody titres, and convalescent antibody titres of 40 or higher. Seroconverison was confirmed by haemagglutination inhibition assay for influenza A viruses, and by hemagglutination inhibition assay and microneutralisation for influenza B viruses.

FINDINGS

Between June 27, 2015, and April 21, 2017, 3634 acutely ill infants were enrolled, of whom 1943 were enrolled during influenza seasons and had complete acute-convalescent pairs and thus were included in the final analytical sample. Of the 1943 infants, 94 (5%) were influenza-positive by both rRT-PCR and serology, 58 (3%) were positive by rRT-PCR-only, and 102 (5%) were positive by serology only. Seroconversion to at least one of the influenza A or B viruses was observed among 196 (77%) of 254 influenza-positive infants. Of the 254 infants with influenza virus, 84 (33%) only had non-respiratory clinical discharge diagnoses (eg, sepsis, febrile seizures, dehydration, or other non-respiratory viral illness). A focus on respiratory diagnoses and rRT-PCR-confirmed influenza underdetects influenza-associated hospital admissions among infants by a factor of 2·6 (95% CI 2·0-3·6). Findings were unchanged when syndromic severe acute respiratory infection criteria were applied instead of clinical diagnosis.

INTERPRETATION

If the true incidence of laboratory-confirmed influenza-associated hospital admissions among infants is at least twice that of previous estimates, this substantially increases the global burden of severe influenza and expands our estimates of the preventive value of maternal and infant influenza vaccination programmes.

FUNDING

US Centers for Disease Control and Prevention.

Kinetics of Serological Responses in Critically Ill Patients Hospitalized With 2009 Pandemic Influenza A(H1N1) Virus Infection in Canada, 2009-2011

Background

The kinetics of the antibody response during severe influenza are not well documented.

Methods

Critically ill patients infected with 2009 pandemic influenza A(H1N1) virus (A[H1N1]pdm09), confirmed by reverse-transcription polymerase chain reaction analysis or seroconversion (defined as a ≥4-fold rise in titers), during 2009-2011 in Canada were prospectively studied. Antibody titers in serially collected sera were determined using hemagglutinin inhibition (HAI) and microneutralization assays. Average antibody curves were estimated using linear mixed-effects models and compared by patient outcome, age, and corticosteroid treatment.

Results

Of 47 patients with A(H1N1)pdm09 virus infection (median age, 47 years), 59% had baseline HAI titers of <40, and 68% had baseline neutralizing titers of <40. Antibody titers rose quickly after symptom onset, and, by day 14, 83% of patients had HAI titers of ≥40, and 80% had neutralizing titers ≥40. Baseline HAI titers were significantly higher in patients who died compared with patients who survived; however, the antibody kinetics were similar by patient outcome and corticosteroid treatment. Geometric mean titers over time in older patients were lower than those in younger patients.

Conclusions

Critically ill patients with influenza A(H1N1)pdm09 virus infection had strong HAI and neutralizing antibody responses during their illness. Antibody kinetics differed by age but were not associated with patient outcome.

Repeated vaccination against matched H3N2 influenza virus gives less protection than single vaccination in ferrets

Epidemiological studies suggest that humans who receive repeated annual immunization with influenza vaccine are less well protected against influenza than those who receive vaccine in the current season only. To better understand potential mechanisms underlying these observations, we vaccinated influenza-naive ferrets either twice, 10 months apart (repeated vaccination group; RV), or once (current season only group; CS), using a prime-boost regimen, and then challenged the ferrets with A/Hong Kong/4801/2014(H3N2). Ferrets that received either vaccine regimen were protected against influenza disease and infection relative to naive unvaccinated ferrets, but the RV group shed more virus, especially at the peak of virus shedding 2 days post infection (p < 0.001) and regained weight more slowly (p < 0.05) than those in the CS group. Qualitative, rather than quantitative, differences in the antibody response may affect protection after repeated influenza vaccination.

Heterologous prime-boost with A(H5N1) pandemic influenza vaccines induces broader cross-clade antibody responses than homologous prime-boost

Highly pathogenic avian influenza (HPAI) A(H5Nx) viruses continue to pose a pandemic threat. US national vaccine stockpiles are a cornerstone of the influenza pandemic preparedness plans. However, continual genetic and antigenic divergence of A(H5Nx) viruses requires the development of effective vaccination strategies using stockpiled vaccines and adjuvants for pandemic preparedness. Human sera collected from healthy adults who received either homologous (2 doses of a AS03_A-adjuvanted A/turkey/Turkey/1/2005, A/Turkey), or heterologous (primed with AS03_A-adjuvanted A/Indonesia/5/2005, A/Indo, followed by A/Turkey boost) prime-boost vaccination regimens were analyzed by hemagglutination inhibition and microneutralization assays against 8 wild-type HPAI A(H5Nx) viruses from 6 genetic clades. Molecular, structural and antigenic features of the A(H5Nx) viruses that could influence the cross-clade antibody responses were also explored. Compared with homologous prime-boost vaccinations, priming with a clade 2.1.3.2 antigen (A/Indo) followed by one booster dose of a clade 2.2.1 antigen (A/Turkey) administered 18 months apart did not compromise the antibody responses to the booster vaccine (A/Turkey), it also broadened the cross-clade antibody responses to several antigenically drifted variants from 6 heterologous clades, including an antigenically distant A(H5N8) virus (A/gyrfalcon/Washington/410886/2014, clade 2.3.4.4) that caused recent outbreaks in US poultry. The magnitude and breadth of the cross-clade antibody responses against emerging HPAI A(H5Nx) viruses are associated with genetic, structural and antigenic differences from the vaccine viruses and enhanced by the inclusion of an adjuvant. Heterologous prime-boost vaccination with AS03_A adjuvanted vaccine offers a vaccination strategy to use existing stockpiled vaccines for pandemic preparedness against new emerging HPAI A(H5Nx) viruses.

Influenza viruses are highly variable and display continuous antigenic drift, limiting the effectiveness of vaccine stockpiles and demanding new strategies to enhance vaccine effectiveness. Here, Min Levine from the Centers for Disease Control and Prevention and colleagues report a heterologous prime-boost A(H5N1) vaccination regimen that induced a broader cross-clade response when compared with homologous vaccination. In the study, adults primed with a clade 2.1.3.2 antigen (A/Indo) followed by one booster dose of a clade 2.2.1 antigen (A/Turkey) presented with enhanced hemagglutinin inhibition and neutralizing antibody titers to eight A(H5Nx) viruses without limiting the antibody response to the A/Turkey booster vaccine. Given that no individual H5 clade has led to protection against all H5 viruses, heterologous vaccination strategies that provide cross-clade reactivity may lead to more effective protection against influenza virus infection.

Detection of Avian Influenza A(H7N2) Virus Infection Among Animal Shelter Workers Using a Novel Serological Approach-New York City, 2016-2017

BACKGROUND

In 2016, an influenza A(H7N2) virus outbreak occurred in cats in New York City's municipal animal shelters. One human infection was initially detected.

METHODS

We conducted a serological survey using a novel approach to rule out cross-reactive antibodies to other seasonal influenza viruses to determine whether additional A(H7N2) human infections had occurred and to assess exposure risk.

RESULTS

Of 121 shelter workers, one had serological evidence of A(H7N2) infection, corresponding to a seroprevalence of 0.8% (95% confidence interval, .02%-4.5%). Five persons exhibited low positive titers to A(H7N2) virus, indicating possible infection; however, we could not exclude cross-reactive antibody responses to seasonal influenza viruses. The remaining 115 persons were seronegative. The seropositive person reported multiple direct cat exposures without using personal protective equipment and mild illness with subjective fever, runny nose, and sore throat.

CONCLUSIONS

We identified a second case of A(H7N2) infection from this outbreak, providing further evidence of cat-to-human transmission of A(H7N2) virus.

Effect of Previous-Season Influenza Vaccination on Serologic Response in Children During 3 Seasons, 2013-2014 Through 2015-2016

BACKGROUND

The effects of repeated influenza vaccination in children are not well understood. In this study, we evaluated previous vaccination effects on antibody response after vaccination with trivalent inactivated influenza vaccine (IIV) or quadrivalent live-attenuated influenza vaccine (LAIV) among school-aged children (5-17 years) across 3 seasons.

METHODS

Children were enrolled in the fall of 2013, 2014, and 2015. The participants received IIV or LAIV according to parent preference (2013-2014) or our randomization scheme (2014-2015). All study children received IIV in 2015-2016. Hemagglutination-inhibition assays measured antibody response to egg-grown vaccine strains from prevaccination and postvaccination serum samples. Geometric mean titers (GMTs) and increases in GMTs from before to after vaccination (geometric mean fold rise [GMFR]) were estimated from repeated-measures linear mixed models.

RESULTS

We enrolled 161 children in 2013-2014, 128 in 2014-2015, and 126 in 2015-2016. Among the IIV recipients, responses to the influenza A(H1N1)pdm09 and B vaccine strains were lowest among children who had received a previous-season IIV. The GMFRs for strains A(H1N1)pdm09 and B were 1.5 to 2.3 for previous-season IIV and 4.3 to 12.9 for previous-season LAIV or no previous vaccine. GMFRs were lower for strain A(H3N2), and differences according to previous-season vaccination history were smaller and not significant in most seasons. Most children had a post-IIV vaccination titer of ≥40 for vaccine strains in all seasons, regardless of previous-season vaccination history. Little to no increase in antibody levels was observed after vaccination with LAIV.

CONCLUSIONS

Serologic response to vaccination was greatest for IIV, but previous-season vaccination modified IIV response to A(H1N1)pdm09 and B. Influenza A(H3N2) responses were low in all groups, and LAIV generated minimal serologic response against all strains.

Neutralization and hemagglutination-inhibition antibodies following influenza vaccination of HIV-infected and HIV-uninfected pregnant women

Background

We previously reported that despite HIV-infected pregnant women had modest humoral immune responses to inactivated influenza vaccine (IIV) measured by hemagglutination-inhibition (HAI) assay, the observed vaccine efficacy against influenza disease was higher than predicted by HAI; suggesting that IIV may confer protection to HIV-infected individuals by additional mechanisms. We evaluated the response to IIV by microneutralization (MN) and HAI assays and correlated both methods in HIV-infected and HIV-uninfected pregnant women.

Methods

MN and HAI antibodies were measured pre-vaccination and approximately one-month post-vaccination in 80 HIV-infected and 75 HIV-uninfected women who received IIV. Geometric mean titers (GMTs), fold-change in titers and seroconversion rates were determined for the three influenza stains in the vaccine.

Results

After vaccination there were significant increases in MN and HAI GMTs for the three vaccine strains in both HIV-infected and HIV-uninfected women. HIV-infected women had, however, a lower immune response compared to HIV-uninfected. Fold-increases were 2 to 3-times higher for MN assay compared to HAI assay for the influenza-A strains. Also a higher percentage of women seroconverted by MN than by HAI assay for the influenza-A strains. There was high positive correlation between MN and HAI assays, except for the B/Victoria strain at pre-vaccination.

Conclusions

In general, the MN assay was more sensitive than the HAI assay. Microneutralization antibodies might correlate better with protection against influenza infection.

Serological response to influenza vaccination among adults hospitalized with community-acquired pneumonia

Ninety‐five adults enrolled in the Etiology of Pneumonia in the Community study with negative admission influenza polymerase chain reaction (PCR) tests received influenza vaccination during hospitalization. Acute and convalescent influenza serology was performed. After vaccination, seropositive (≥1:40) hemagglutination antibody titers (HAI) were achieved in 55% to influenza A(H1N1)pdm09, 58% to influenza A(H3N2), 77% to influenza B (Victoria), and 74% to influenza B (Yamagata) viruses. Sixty‐six (69%) patients seroconverted (≥4‐fold HAI rise) to ≥1 strain. Failure to seroconvert was associated with diabetes, bacterial detection, baseline seropositive titers for influenza B (Yamagata), and influenza vaccination in the previous season.

Use of Multiple Imputation to Estimate the Proportion of Respiratory Virus Detections Among Patients Hospitalized With Community-Acquired Pneumonia

Background

Real-time polymerase chain reaction (PCR) on respiratory specimens and serology on paired blood specimens are used to determine the etiology of respiratory illnesses for research studies. However, convalescent serology is often not collected. We used multiple imputation to assign values for missing serology results to estimate virus-specific prevalence among pediatric and adult community-acquired pneumonia hospitalizations using data from an active population-based surveillance study.

Methods

Presence of adenoviruses, human metapneumovirus, influenza viruses, parainfluenza virus types 1-3, and respiratory syncytial virus was defined by positive PCR on nasopharyngeal/oropharyngeal specimens or a 4-fold rise in paired serology. We performed multiple imputation by developing a multivariable regression model for each virus using data from patients with available serology results. We calculated absolute and relative differences in the proportion of each virus detected comparing the imputed to observed (nonimputed) results.

Results

Among 2222 children and 2259 adults, 98.8% and 99.5% had nasopharyngeal/oropharyngeal specimens and 43.2% and 37.5% had paired serum specimens, respectively. Imputed results increased viral etiology assignments by an absolute difference of 1.6%-4.4% and 0.8%-2.8% in children and adults, respectively; relative differences were 1.1-3.0 times higher.

Conclusions

Multiple imputation can be used when serology results are missing, to refine virus-specific prevalence estimates, and these will likely increase estimates.

Development of a high-throughput assay to detect antibody inhibition of low pH induced conformational changes of influenza virus hemagglutinin

Many broadly neutralizing antibodies (bnAbs) bind to conserved areas of the hemagglutinin (HA) stalk region and can inhibit the low pH induced HA conformational changes necessary for viral membrane fusion activity. We developed and evaluated a high-throughput virus-free and cell-free ELISA based low pH induced HA Conformational Change Inhibition Antibody Detection Assay (HCCIA) and a complementary proteinase susceptibility assay. Human serum samples (n = 150) were tested by HCCIA using H3 recombinant HA. Optical density (OD) ratios of mAb HC31 at pH 4.8 to pH 7.0 ranged from 0.87 to 0.09. Our results demonstrated that low pH induced HA conformational change inhibition antibodies (CCI) neutralized multiple H3 strains after removal of head-binding antibodies. The results suggest that HCCIA can be utilized to detect and characterize CCI in sera, that are potentially broadly neutralizing, and serves as a useful tool for evaluating universal vaccine candidates targeting the HA stalk.

Influence of Birth Cohort on Effectiveness of 2015-2016 Influenza Vaccine Against Medically Attended Illness Due to 2009 Pandemic Influenza A(H1N1) Virus in the United States

Background

The effectiveness of influenza vaccine during 2015-2016 was reduced in some age groups as compared to that in previous 2009 pandemic influenza A(H1N1) virus (A[H1N1]pdm09 virus)-predominant seasons. We hypothesized that the age at first exposure to specific influenza A(H1N1) viruses could influence vaccine effectiveness (VE).

Methods

We estimated the effectiveness of influenza vaccine against polymerase chain reaction-confirmed influenza A(H1N1)pdm09-associated medically attended illness from the 2010-2011 season through the 2015-2016 season, according to patient birth cohort using data from the Influenza Vaccine Effectiveness Network. Birth cohorts were defined a priori on the basis of likely immunologic priming with groups of influenza A(H1N1) viruses that circulated during 1918-2015. VE was calculated as 100 × [1 - adjusted odds ratio] from logistic regression models comparing the odds of vaccination among influenza virus-positive versus influenza test-negative patients.

Results

A total of 2115 A(H1N1)pdm09 virus-positive and 14 696 influenza virus-negative patients aged ≥6 months were included. VE was 61% (95% confidence interval [CI], 56%-66%) against A(H1N1)pdm09-associated illness during the 2010-2011 through 2013-2014 seasons, compared with 47% (95% CI, 36%-56%) during 2015-2016. During 2015-2016, A(H1N1)pdm09-specific VE was 22% (95% CI, -7%-43%) among adults born during 1958-1979 versus 61% (95% CI, 54%-66%) for all other birth cohorts combined.

Conclusion

Findings suggest an association between reduced VE against influenza A(H1N1)pdm09-related illness during 2015-2016 and early exposure to specific influenza A(H1N1) viruses.

Measuring Influenza Neutralizing Antibody Responses to A(H3N2) Viruses in Human Sera by Microneutralization Assays Using MDCK-SIAT1 Cells

Neutralizing antibodies against hemagglutinin (HA) of influenza viruses are considered the main immune mechanism that correlates with protection for influenza infections. Microneutralization (MN) assays are often used to measure neutralizing antibody responses in human sera after influenza vaccination or infection. Madine Darby Canine Kidney (MDCK) cells are the commonly used cell substrate for MN assays. However, currently circulating 3C.2a and 3C.3a A(H3N2) influenza viruses have acquired altered receptor binding specificity. The MDCK-SIAT1 cell line with increased α-2,6 sialic galactose moieties on the surface has proven to provide improved infectivity and more faithful replications than conventional MDCK cells for these contemporary A(H3N2) viruses. Here, we describe a MN assay using MDCK-SIAT1 cells that has been optimized to quantify neutralizing antibody titers to these contemporary A(H3N2) viruses. In this protocol, heat inactivated sera containing neutralizing antibodies are first serially diluted, then incubated with 100 TCID50/well of influenza A(H3N2) viruses to allow antibodies in the sera to bind to the viruses. MDCK-SIAT1 cells are then added to the virus-antibody mixture, and incubated for 18 - 20 h at 37 °C, 5% CO2 to allow A(H3N2) viruses to infect MDCK-SIAT1 cells. After overnight incubation, plates are fixed and the amount of virus in each well is quantified by an enzyme-linked immunosorbent assay (ELISA) using anti-influenza A nucleoprotein (NP) monoclonal antibodies. Neutralizing antibody titer is defined as the reciprocal of the highest serum dilution that provides ≥50% inhibition of virus infectivity.

Cross-Reactive Antibody Responses to Novel H5Nx Influenza Viruses Following Homologous and Heterologous Prime-Boost Vaccination with a Prepandemic Stockpiled A(H5N1) Vaccine in Humans

Recently, novel highly pathogenic avian influenza H5Nx viruses (clade 2.3.4.4) caused outbreaks in US poultry. We evaluated the potential of a stockpiled A(H5N1) A/Anhui/1/2005 (clade 2.3.4) vaccine to elicit cross-reactive antibody responses to these emerging viruses. Sera from subjects who received 2 doses of MF59-adjuvanted A/Anhui/1/2005, or 1 dose of MF59-adjuvanted A/Anhui/1/2005 following priming with a clade 1 vaccine were characterized by microneutralization assays and modified hemagglutination inhibition (HI) assays. Only heterologous prime-boost vaccination induced modest cross-reactive HI antibody responses to H5Nx viruses. Heterologous prime-boost may provide a more effective vaccination strategy to broaden the antibody responses to emerging viruses.