Correlates of protection to influenza virus, where do we go from here?

Secukinumab treatment and vaccination efficacy

Dear Editor, Secukinumab, a monoclonal antibody against interleukin (IL)-17A, is used to treat psoriasis and psoriatic arthritis. The effect of this treatment on COVID-19 vaccination is not well characterized. While IL-17 deficiency states are associated with mucocutaneous infection with Candida albicans, there is no apparent increased risk of viral infection, and redundancy between IL-17A and IL-17F limits the impact of IL-17A deficiency on immune responsiveness.

Pandemic Preparedness Against Influenza: DNA Vaccine for Rapid Relief

The 2009 “swine flu” pandemic outbreak demonstrated the limiting capacity for egg-based vaccines with respect to global vaccine supply within a timely fashion. New vaccine platforms that efficiently can quench pandemic influenza emergences are urgently needed. Since 2009, there has been a profound development of new vaccine platform technologies with respect to prophylactic use in the population, including DNA vaccines. These vaccines are particularly well suited for global pandemic responses as the DNA format is temperature stable and the production process is cheap and rapid. Here, we show that by targeting influenza antigens directly to antigen presenting cells (APC), DNA vaccine efficacy equals that of conventional technologies. A single dose of naked DNA encoding hemagglutinin (HA) from influenza/A/California/2009 (H1N1), linked to a targeting moiety directing the vaccine to major histocompatibility complex class II (MHCII) molecules, raised similar humoral immune responses as the adjuvanted split virion vaccine Pandemrix, widely administered in the 2009 pandemic. Both vaccine formats rapidly induced serum antibodies that could protect mice already 8 days after a single immunization, in contrast to the slower kinetics of a seasonal trivalent inactivated influenza vaccine (TIV). Importantly, the DNA vaccine also elicited cytotoxic T-cell responses that reduced morbidity after vaccination, in contrast to very limited T-cell responses seen after immunization with Pandemrix and TIV. These data demonstrate that DNA vaccines has the potential as a single dose platform vaccine, with rapid protective effects without the need for adjuvant, and confirms the relevance of naked DNA vaccines as candidates for pandemic preparedness.

Safety, Immunogenicity, Efficacy and Effectiveness of Inactivated Influenza Vaccines in Healthy Pregnant Women and Children Under 5 Years: An Evidence-Based Clinical Review

Annual influenza vaccination is often recommended for pregnant women and young children to reduce the risk of severe influenza. However, most studies investigating the safety, immunogenicity, and efficacy or effectiveness of influenza vaccines are conducted in healthy adults. In this evidence-based clinical review, we provide an update on the safety profile, immunogenicity, and efficacy/effectiveness of inactivated influenza vaccines (IIVs) in healthy pregnant women and children <5 years old. Six electronic databases were searched until May 27, 2021. We identified 3,731 articles, of which 93 met the eligibility criteria and were included. The IIVs were generally well tolerated in pregnant women and young children, with low frequencies of adverse events following IIV administration; however, continuous vaccine safety monitoring systems are necessary to detect rare adverse events. IIVs generated good antibody responses, and the seroprotection rates after IIVs were moderate to high in pregnant women (range = 65%-96%) and young children (range = 50%-100%), varying between the different influenza types/subtypes and seasons. Studies show vaccine efficacy/effectiveness values of 50%-70% in pregnant women and 20%-90% in young children against lab-confirmed influenza, although the efficacy/effectiveness depended on the study design, host factors, vaccine type, manufacturing practices, and the antigenic match/mismatch between the influenza vaccine strains and the circulating strains. Current evidence suggests that the benefits of IIVs far outweigh the potential risks and that IIVs should be recommended for pregnant women and young children.

Mosaic Hemagglutinin-Based Whole Inactivated Virus Vaccines Induce Broad Protection Against Influenza B Virus Challenge in Mice

Influenza viruses undergo antigenic changes in the immuno-dominant hemagglutinin (HA) head domain, necessitating annual re-formulation of and re-vaccination with seasonal influenza virus vaccines for continuing protection. We previously synthesized mosaic HA (mHA) proteins of influenza B viruses which redirect the immune response towards the immuno-subdominant conserved epitopes of the HA via sequential immunization. As ~90% of current influenza virus vaccines are manufactured using the inactivated virus platform, we generated and sequentially vaccinated mice with inactivated influenza B viruses displaying either the homologous (same B HA backbones) or the heterologous (different B HA backbones) mosaic HAs. Both approaches induced long-lasting and cross-protective antibody responses showing strong antibody-dependent cellular cytotoxicity (ADCC) activity. We believe the B virus mHA vaccine candidates represent a major step towards a universal influenza B virus vaccine.

Prevalence of antibodies against seasonal influenza A and B viruses among older adults in rural Thailand: A cross-sectional study

Assessing the seroprevalence of the high-risk individuals against the influenza virus is essential to evaluate the progress of vaccine implementation programs and establish influenza virus interventions. Herein, we identified the pre-existing cross-protection of the circulating seasonal influenza viruses among the older-aged population. A cross-sectional study was performed base on the 176 residual sera samples collected from older adults aged 60 to 95 years without a history of vaccination in rural Thailand in 2015. Sera antibody titers against influenza A and B viruses circulating between 2016 and 2019 were determined by hemagglutination inhibition assay. These findings indicated the low titers of pre-existing antibodies to circulating influenza subtypes and showed age-independent antibody titers among the old adults. Moderate seropositive rates (HAI ≥ 1:40) were observed in influenza A viruses (65.9%A(H3N2), 50.0% for A(H1N1) pdm09), and found comparatively lower rates in influenza B viruses (14% B/Yam2, 21% B/Yam3 and 25% B/Vic). Only 5% of individuals possessed broadly protective antibodies against both seasonal influenza A and B virus in this region. Our findings highlighted the low pre-existing antibodies to circulating influenza strains in the following season observed in older adults. The serological study will help inform policy-makers for health care planning and guide control measures concerning vaccination programs.

Inflammasome Activity in Response to Influenza Vaccination Is Maintained in Monocyte-Derived Peripheral Blood Macrophages in Older Adults

Introduction: Each year, a disproportionate number of the total seasonal influenza-related hospitalizations (90%) and deaths (70%) occur among adults who are &gt;65 years old. Inflammasome activation has been shown to be important for protection against influenza infection in animal models but has not yet been demonstrated in humans. We hypothesized that age-related dysfunction (immunosenescence) of the inflammasome may be associated with poor influenza-vaccine response among older adults.

Methods: A cohort of younger (18–40 years of age) and older (≥65 years of age) adults was recruited prior to the 2014–2015 influenza season. We measured hemagglutination inhibition (HAI) titers in serum before and 28 days after receipt of the seasonal inactivated influenza vaccine. Inflammasome-related gene expression and protein secretion were quantified in monocyte-derived macrophages following stimulation with influenza A/H1N1 virus.

Results: Younger adults exhibited higher HAI titers compared to older adults following vaccination, although inflammasome-related protein secretion in response to influenza stimulation was similar between the age groups. Expression of P2RX7 following influenza stimulation was lower among older adults. Interestingly, CFLAR expression was significantly higher among females (p = 2.42 × 10−5) following influenza stimulation and this gene may play an important role in the development of higher HAI antibody titers among older females.

Conclusion: Inflammasome activation in response to influenza vaccination appears to be maintained in monocyte-derived macrophages from older adults and does not explain the poor influenza vaccine responses generally observed among this age group.

Antibody Responsiveness to Influenza: What Drives It?

The induction of a specific antibody response has long been accepted as a serological hallmark of recent infection or antigen exposure. Much of our understanding of the influenza antibody response has been derived from studying antibodies that target the hemagglutinin (HA) protein. However, growing evidence points to limitations associated with this approach. In this review, we aim to highlight the issue of antibody non-responsiveness after influenza virus infection and vaccination. We will then provide an overview of the major factors known to influence antibody responsiveness to influenza after infection and vaccination. We discuss the biological factors such as age, sex, influence of prior immunity, genetics, and some chronic infections that may affect the induction of influenza antibody responses. We also discuss the technical factors, such as assay choices, strain variations, and viral properties that may influence the sensitivity of the assays used to measure influenza antibodies. Understanding these factors will hopefully provide a more comprehensive picture of what influenza immunogenicity and protection means, which will be important in our effort to improve influenza vaccines.

Exploiting Pan Influenza A and Pan Influenza B Pseudotype Libraries for Efficient Vaccine Antigen Selection

We developed an influenza hemagglutinin (HA) pseudotype library encompassing Influenza A subtypes HA1-18 and Influenza B subtypes (both lineages) to be employed in influenza pseudotype microneutralization (pMN) assays. The pMN is highly sensitive and specific for detecting virus-specific neutralizing antibodies against influenza viruses and can be used to assess antibody functionality in vitro. Here we show the production of these viral HA pseudotypes and their employment as substitutes for wildtype viruses in influenza neutralization assays. We demonstrate their utility in detecting serum responses to vaccination with the ability to evaluate cross-subtype neutralizing responses elicited by specific vaccinating antigens. Our findings may inform further preclinical studies involving immunization dosing regimens in mice and may help in the creation and selection of better antigens for vaccine design. These HA pseudotypes can be harnessed to meet strategic objectives that contribute to the strengthening of global influenza surveillance, expansion of seasonal influenza prevention and control policies, and strengthening pandemic preparedness and response.

Comparison of hemagglutination inhibition and hemagglutinin pseudovirus neutralization titres in relation to protection against influenza in a mouse model

The hemagglutination inhibition (HI) test has long been used as a standard measure of antibody response for inactivated influenza vaccines. However, the HI test has limitations, such as insensitivity when using some H3N2 virus strains and failure to detect neutralizing antibodies that target regions distant from the receptor binding site. We therefore examined a hemagglutinin pseudovirus neutralization (PVN) test as a possible supplement or alternative to the HI test. We evaluated the association of HI or PVN titres with protection against influenza infection in mice based on morbidity (where the illness was defined as 25% body weight loss). We assessed this relationship using dose-response models incorporating HI or PVN titres as a variable. The morbidity was correlated with the pre-exposure titres, and such a correlation was well described by a modified dose-response model. The mathematical modelling suggests that PVN titres consistently show a stronger association with in vivo protection as compared to HI titres in mice. Given our findings, the PVN test warrants further investigation as a tool for evaluating antibody responses to influenza vaccines containing hemagglutinin. The resulting models may also be useful for analyzing human clinical data to identify potentially protective antibody titres against influenza illness.

Evolution of immune responses to SARS-CoV-2 in mild-moderate COVID-19

The durability of infection-induced SARS-CoV-2 immunity has major implications for reinfection and vaccine development. Here, we show a comprehensive profile of antibody, B cell and T cell dynamics over time in a cohort of patients who have recovered from mild-moderate COVID-19. Binding and neutralising antibody responses, together with individual serum clonotypes, decay over the first 4 months post-infection. A similar decline in Spike-specific CD4+ and circulating T follicular helper frequencies occurs. By contrast, S-specific IgG+ memory B cells consistently accumulate over time, eventually comprising a substantial fraction of circulating the memory B cell pool. Modelling of the concomitant immune kinetics predicts maintenance of serological neutralising activity above a titre of 1:40 in 50% of convalescent participants to 74 days, although there is probably additive protection from B cell and T cell immunity. This study indicates that SARS-CoV-2 immunity after infection might be transiently protective at a population level. Therefore, SARS-CoV-2 vaccines might require greater immunogenicity and durability than natural infection to drive long-term protection.

Postvaccination Serum Antirotavirus Immunoglobulin A as a Correlate of Protection Against Rotavirus Gastroenteritis Across Settings

BACKGROUND

A correlate of protection for rotavirus gastroenteritis would facilitate rapid assessment of vaccination strategies and the next generation of rotavirus vaccines. We aimed to quantify a threshold of postvaccine serum antirotavirus immunoglobulin A (IgA) as an individual-level immune correlate of protection against rotavirus gastroenteritis.

METHODS

Individual-level data on 5074 infants in 9 GlaxoSmithKline Rotarix Phase 2/3 clinical trials from 16 countries were pooled. Cox proportional hazard models were fit to estimate hazard ratios (HRs) describing the relationship between IgA thresholds and occurrence of rotavirus gastroenteritis.

RESULTS

Seroconversion (IgA ≥ 20 U/mL) conferred substantial protection against any and severe rotavirus gastroenteritis to age 1 year. In low child mortality settings, seroconversion provided near perfect protection against severe rotavirus gastroenteritis (HR, 0.04; 95% confidence interval [CI], .01-.31). In high child mortality settings, seroconversion dramatically reduced the risk of severe rotavirus gastroenteritis (HR, 0.46; 95% CI, .25-.86). As IgA threshold increased, risk of rotavirus gastroenteritis generally decreased. A given IgA threshold provided better protection in low compared to high child mortality settings.

DISCUSSION

Postvaccination antirotavirus IgA is a valuable correlate of protection against rotavirus gastroenteritis to age 1 year. Seroconversion provides an informative threshold for assessing rotavirus vaccine performance.

Adenoviral Vectors as Vaccines for Emerging Avian Influenza Viruses

It is evident that the emergence of infectious diseases, which have the potential for spillover from animal reservoirs, pose an ongoing threat to global health. Zoonotic transmission events have increased in frequency in recent decades due to changes in human behavior, including increased international travel, the wildlife trade, deforestation, and the intensification of farming practices to meet demand for meat consumption. Influenza A viruses (IAV) possess a number of features which make them a pandemic threat and a major concern for human health. Their segmented genome and error-prone process of replication can lead to the emergence of novel reassortant viruses, for which the human population are immunologically naïve. In addition, the ability for IAVs to infect aquatic birds and domestic animals, as well as humans, increases the likelihood for reassortment and the subsequent emergence of novel viruses. Sporadic spillover events in the past few decades have resulted in human infections with highly pathogenic avian influenza (HPAI) viruses, with high mortality. The application of conventional vaccine platforms used for the prevention of seasonal influenza viruses, such as inactivated influenza vaccines (IIVs) or live-attenuated influenza vaccines (LAIVs), in the development of vaccines for HPAI viruses is fraught with challenges. These issues are associated with manufacturing under enhanced biosafety containment, and difficulties in propagating HPAI viruses in embryonated eggs, due to their propensity for lethality in eggs. Overcoming manufacturing hurdles through the use of safer backbones, such as low pathogenicity avian influenza viruses (LPAI), can also be a challenge if incompatible with master strain viruses. Non-replicating adenoviral (Ad) vectors offer a number of advantages for the development of vaccines against HPAI viruses. Their genome is stable and permits the insertion of HPAI virus antigens (Ag), which are expressed in vivo following vaccination. Therefore, their manufacture does not require enhanced biosafety facilities or procedures and is egg-independent. Importantly, Ad vaccines have an exemplary safety and immunogenicity profile in numerous human clinical trials, and can be thermostabilized for stockpiling and pandemic preparedness. This review will discuss the status of Ad-based vaccines designed to protect against avian influenza viruses with pandemic potential.

Correlation of Influenza B Haemagglutination Inhibiton, Single-Radial Haemolysis and Pseudotype-Based Microneutralisation Assays for Immunogenicity Testing of Seasonal Vaccines

Influenza B is responsible for a significant proportion of the global morbidity, mortality and economic loss caused by influenza-related disease. Two antigenically distinct lineages co-circulate worldwide, often resulting in mismatches in vaccine coverage when vaccine predictions fail. There are currently operational issues with gold standard serological assays for influenza B, such as lack of sensitivity and requirement for specific antigen treatment. This study encompasses the gold standard assays with the more recent Pseudotype-based Microneutralisation assay in order to study comparative serological outcomes. Haemagglutination Inhibition, Single Radial Haemolysis and Pseudotype-based Microneutralisation correlated strongly for strains in the Yamagata lineage; however, it correlated with neither gold standard assays for the Victoria lineage.

Looking beyond COVID-19 vaccine phase 3 trials

After the recent announcement of COVID-19 vaccine efficacy in clinical trials by several manufacturers for protection against severe disease, a comprehensive post-efficacy strategy for the next steps to ensure vaccination of the global population is now required. These considerations should include how to manufacture billions of doses of high-quality vaccines, support for vaccine purchase, coordination of supply, the equitable distribution of vaccines and the logistics of global vaccine delivery, all of which are a prelude to a massive vaccination campaign targeting people of all ages. Furthermore, additional scientific questions about the vaccines remain that should be answered to improve vaccine efficacy, including questions regarding the optimization of vaccination regimens, booster doses, the correlates of protection, vaccine effectiveness, safety and enhanced surveillance. The timely and coordinated execution of these post-efficacy tasks will bring the pandemic to an effective, and efficient, close.

Immunogenicity of Viral Vaccines in the Italian Military

Military personnel of all armed forces receive multiple vaccinations and have been doing so since long ago, but relatively few studies have investigated the possible negative or positive interference of simultaneous vaccinations. As a contribution to fill this gap, we analyzed the response to the live trivalent measles/mumps/rubella (MMR), the inactivated hepatitis A virus (HAV), the inactivated trivalent polio, and the trivalent subunits influenza vaccines in two cohorts of Italian military personnel. The first cohort was represented by 108 students from military schools and the second by 72 soldiers engaged in a nine-month mission abroad. MMR and HAV vaccines had never been administered before, whereas inactivated polio was administered to adults primed at infancy with a live trivalent oral polio vaccine. Accordingly, nearly all subjects had baseline antibodies to polio types 1 and 3, but unexpectedly, anti-measles/-mumps/-rubella antibodies were present in 82%, 82%, and 73.5% of subjects, respectively (43% for all of the antigens). Finally, anti-HAV antibodies were detectable in 14% and anti-influenza (H1/H3/B) in 18% of the study population. At mine months post-vaccination, 92% of subjects had protective antibody levels for all MMR antigens, 96% for HAV, 69% for the three influenza antigens, and 100% for polio types 1 and 3. An inverse relationship between baseline and post-vaccination antibody levels was noticed with all the vaccines. An excellent vaccine immunogenicity, a calculated long antibody persistence, and apparent lack of vaccine interference were observed.

Impact of Pre-Existing Immunity to Influenza on Live-Attenuated Influenza Vaccine (LAIV) Immunogenicity

During the previous influenza seasons, between 2010 and 2016, the live attenuated influenza vaccine (LAIV) provided variable efficacy against influenza in the U.S., causing the recommendation against the use of the LAIV. In striking contrast, pre-clinical studies have repeatedly demonstrated superior efficacy of LAIV against mismatched influenza viruses, compared to inactivated influenza vaccines (IIV). This disparity in reported vaccine efficacies between pre-clinical and clinical studies may in part be explained by limitations of the animal models of influenza. In particular, the absence of pre-existing immunity in animal models has recently emerged as a potential explanation for the discrepancies between preclinical findings and human studies. This commentary focuses on the potential impact of pre-existing immunity on LAIV induced immunogenicity with an emphasis on cross-protective immunity.

Recombinant HA-based vaccine outperforms split and subunit vaccines in elicitation of influenza-specific CD4 T cells and CD4 T cell-dependent antibody responses in humans

Although traditional egg-based inactivated influenza vaccines can protect against infection, there have been significant efforts to develop improved formats to overcome disadvantages of this platform. Here, we have assessed human CD4 T cell responses to a traditional egg-based influenza vaccine with recently available cell-derived vaccines and recombinant baculovirus-derived vaccines. Adults were administered either egg-derived Fluzone^®, mammalian cell-derived Flucelvax^® or recombinant HA (Flublok^®). CD4 T cell responses to each HA protein were assessed by cytokine EliSpot and intracellular staining assays. The specificity and magnitude of antibody responses were quantified by ELISA and HAI assays. By all criteria, Flublok vaccine exhibited superior performance in eliciting both CD4 T cell responses and HA-specific antibody responses, whether measured by mean response magnitude or percent of responders. Although the mechanism(s) underlying this advantage is not yet clear, it is likely that both qualitative and quantitative features of the vaccines impact the response.

COVID-19 vaccines: Knowing the unknown

Vaccine development against SARS-CoV-2 has drawn attention around the globe due to the exploding pandemic. Although COVID-19 is caused by a new coronavirus, SARS-CoV-2, previous research on other coronavirus vaccines, such as FIPV, SARS, and MERS, has provided valuable information for the rapid development of COVID-19 vaccine. However, important knowledge gaps remain - some are specific to SARS-CoV-2, others are fundamental to immunology and vaccinology. Here, we discuss areas that need to be addressed for COVID-19 vaccine development, and what can be learned from examples of vaccine development in the past. Since the beginning of the outbreak, the research progress on COVID-19 has been remarkable. We are therefore optimistic about the rapid development of COVID-19 vaccine.

Combined Intranasal Nanoemulsion and RIG-I Activating RNA Adjuvants Enhance Mucosal, Humoral, and Cellular Immunity to Influenza Virus

Current influenza virus vaccines are focused on humoral immunity and are limited by the short duration of protection, narrow cross-strain efficacy, and suboptimal immunogenicity. Here, we combined two chemically and biologically distinct adjuvants, an oil-in-water nanoemulsion (NE) and RNA-based agonists of RIG-I, to determine whether the diverse mechanisms of these adjuvants could lead to improved immunogenicity and breadth of protection against the influenza virus. NE activates TLRs, stimulates immunogenic apoptosis, and enhances cellular antigen uptake, leading to a balanced T_H1/T_H2/T_H17 response when administered intranasally. RIG-I agonists included RNAs derived from Sendai and influenza viral defective interfering RNAs (IVT DI, 3php, respectively) and RIG-I/TLR3 agonist, poly(I:C) (pIC), which induce IFN-Is and T_H1-polarized responses. NE/RNA combined adjuvants potentially allow for costimulation of multiple innate immune receptor pathways, more closely mimicking patterns of activation occurring during natural viral infection. Mice intranasally immunized with inactivated A/Puerto Rico/8/1934 (H1N1) (PR/8) adjuvanted with NE/IVT DI or NE/3php (but not NE/pIC) showed synergistic enhancement of systemic PR/8-specific IgG with significantly greater avidity and virus neutralization activity than the individual adjuvants. Notably, NE/IVT DI induced protective neutralizing titers after a single immunization. Hemagglutinin stem-specific antibodies were also improved, allowing recognition of heterologous and heterosubtypic hemagglutinins. All NE/RNAs elicited substantial PR/8-specific sIgA. Finally, a unique cellular response with enhanced T_H1/T_H17 immunity was induced with the NE/RNAs. These results demonstrate that the enhanced immunogenicity of the adjuvant combinations was synergistic and not simply additive, highlighting the potential value of a combined adjuvant approach for improving the efficacy of vaccination against the influenza virus.

Non-sterilizing, Infection-Permissive Vaccination With Inactivated Influenza Virus Vaccine Reshapes Subsequent Virus Infection-Induced Protective Heterosubtypic Immunity From Cellular to Humoral Cross-Reactive Immune Responses

Conventional influenza vaccines aim at the induction of virus-neutralizing antibodies that provide with sterilizing immunity. However, influenza vaccination often confers protection from disease but not from infection. The impact of infection-permissive vaccination on the immune response elicited by subsequent influenza virus infection is not well-understood. Here, we investigated to what extent infection-permissive immunity, in contrast to virus-neutralizing immunity, provided by a trivalent inactivated virus vaccine (TIV) modulates disease and virus-induced host immune responses after sublethal vaccine-matching H1N1 infection in a mouse model. More than one TIV vaccination was needed to induce a serum HI titer and provide sterilizing immunity upon homologous virus infection. However, single TIV administration provided infection-permissive immunity, characterized by lower viral lung titers and faster recovery. Despite the presence of replicating virus, single TIV vaccination prevented induction of pro-inflammatory cyto- and chemokines, alveolar macrophage depletion as well as the establishment of lung-resident B and T cells after infection. To investigate virus infection-induced cross-protective heterosubtypic immune responses in vaccinated and unvaccinated animals, mice were re-infected with a lethal dose of H3N2 virus 4 weeks after H1N1 infection. Single TIV vaccination did not prevent H1N1 virus infection-induced heterosubtypic cross-protection, but shifted the mechanism of cross-protection from the cellular to the humoral branch of the immune system. These results suggest that suboptimal vaccination with conventional influenza vaccines may still positively modulate disease outcome after influenza virus infection, while promoting humoral heterosubtypic immunity after virus infection.

Efficacy of FLU-v, a broad-spectrum influenza vaccine, in a randomized phase IIb human influenza challenge study

FLU-v, developed by PepTcell (SEEK), is a peptide vaccine aiming to provide a broadly protective cellular immune response against influenza A and B. A randomized, double-blind, placebo-controlled, single-center, phase IIb efficacy and safety trial was conducted. One hundred and fifty-three healthy individuals 18-55 years of age were randomized to receive one or two doses of adjuvanted FLU-v or adjuvanted placebo subcutaneously on days -43 and -22, prior to intranasal challenge on day 0 with the A/California/04/2009/H1N1 human influenza A challenge virus. The primary objective of the study was to identify a reduction in mild to moderate influenza disease (MMID) defined as the presence of viral shedding and clinical influenza symptoms. Single-dose adjuvanted FLU-v recipients (n = 40) were significantly less likely to develop MMID after challenge vs placebo (n = 42) (32.5% vs 54.8% p = 0.035). FLU-v should continue to be evaluated and cellular immunity explored further as a possible important correlate of protection against influenza.

Comparison of influenza-specific neutralizing antibody titers determined using different assay readouts and hemagglutination inhibition titers: good correlation but poor agreement

Determination of influenza-specific antibody titers is commonly done using the hemagglutination inhibition assay (HAI) and the viral microneutralization assay (MN). Both assays are characterized by high intra- and inter-laboratory variability. The HAI assay offers little opportunity for standardization. For the MN assay, variability might be due to the use of different assay protocols employing different readouts. We therefore aimed at investigating which of the MN assay readout methods currently in use would be the most suitable choice for a standardized MN assay that could serve as a substitute for the HAI assay. For this purpose, human serum samples were tested for the presence of influenza specific neutralizing antibodies against A/California/7/09 H1N1 (49 sera) or A/Hong Kong/4801/2014 (50 sera) using four different infection readout methods for the MN assay (cytopathic effect, hemagglutination, ELISA, RT qPCR) and using the HAI assay. The results were compared by correlation analysis and by determining the level of agreement before and after normalization to a standard serum. Titers as measured by the 4 MN assay readouts showed good correlation, with high Person's r for most comparisons. However, agreement between nominal titers varied with readouts compared and virus strain used. In addition, Pearson's correlation of MN titers with HAI titers was high but agreement of nominal titers was moderate and the average difference between the readings of two assays (bias) was virus strain-dependent. Normalization to a standard serum did not result in better agreement of assay results. Our study demonstrates that different MN readouts result in nominally different antibody titers. Accordingly, the use of a common and standardized MN assay protocol will be crucial to minimize inter-laboratory variability. Based on reproducibility, cost effectiveness and unbiased assessment of results we elected the MN assay with ELISA readout as most suitable for a possible replacement of the HAI assay.

Distinguishing Causation From Correlation in the Use of Correlates of Protection to Evaluate and Develop Influenza Vaccines

There is increasing attention to the need to identify new immune markers for the evaluation of existing and new influenza vaccines. Immune markers that could predict individual protection against infection and disease, commonly called correlates of protection (CoPs), play an important role in vaccine development and licensing. Here, we discuss the epidemiologic considerations when evaluating immune markers as potential CoPs for influenza vaccines and emphasize the distinction between correlation and causation. While an immune marker that correlates well with protection from infection can be used as a predictor of vaccine efficacy, it should be distinguished from an immune marker that plays a mechanistic role in conferring protection against a clinical endpoint-the latter might be a more reliable predictor of vaccine efficacy and a more appropriate target for rational vaccine design. To clearly distinguish mechanistic and nonmechanistic CoPs, we suggest using the term "correlates of protection" for nonmechanistic CoPs, and ''mediators of protection'' for mechanistic CoPs. Furthermore, because the interactions among and relative importance of correlates or mediators of protection can vary according to age or prior vaccine experience, the effect sizes and thresholds for protective effects for CoPs could also vary in different segments of the population.

Tailoring mRNA Vaccine to Balance Innate/Adaptive Immune Response

mRNA vaccine platforms present numerous advantages, such as versatility, rapid production, and induction of cellular and humoral responses. Moreover, mRNAs have inherent adjuvant properties due to their complex interaction with pattern recognition receptors (PRRs). This recognition can be either beneficial in activating antigen-presenting cells (APCs) or detrimental by indirectly blocking mRNA translation. To decipher this Janus effect, we describe the different innate response mechanisms triggered by mRNA molecules and how each element from the 5' cap to the poly-A tail interferes with innate/adaptive immune responses. Then, we emphasize the importance of some critical steps such as production, purification, and formulation as key events to further improve the quality of immune responses and balance innate and adaptive immunity.

Poly(I:C) augments inactivated influenza virus-chitosan nanovaccine induced cell mediated immune response in pigs vaccinated intranasally

To improve the innate and adaptive immune responses elicited by a killed/inactivated swine influenza virus antigen (KAg)-loaded chitosan nanoparticles (CS NPs-KAg), we used the adjuvant, poly(I:C). The formulated CS NPs-KAg and CS NPs-poly(I:C) had a net surface charge of +30.7 mV and +25.1 mV, respectively. The CS NPs-KAg was coadministered with CS NPs-poly(I:C) (chitosan nanovaccine) as intranasal mist. Vaccinations enhanced homologous (H1N2-OH10) and heterologous (H1N1-OH7) hemagglutination inhibition (HI) titers in both vaccinated and virus-challenged animals compared to the control soluble poly(I:C) vaccinated pigs. In addition, the chitosan nanovaccine induced the proliferation of antigen-specific IFNγ secreting T-helper/memory and γδ T cells compared to control poly(I:C) group; and an increased Th1 (IFNγ, IL-6 and IL-2) and Th2 (IL-10 and IL-13) cytokines mRNA expression in the tracheobronchial lymph nodes compared to lymphoid tissues obtained from pigs given commercial influenza vaccine. The virus load in nasal passages and microscopic lung lesions were partially reduced by both chitosan nanovaccine and commercial vaccine. The HA gene homology between the vaccine and challenge viruses indicated that the chitosan nanovaccine induced a cross-protective immune response. In conclusion, coadministration of CS NPs-poly(I:C) with CS NPs-KAg augmented the cross-reactive specific HI titers and the cell-mediated immune responses in pigs.

Innate immunemodulator containing adjuvant formulated HA based vaccine protects mice from lethal infection of highly pathogenic avian influenza H5N1 virus

The highly pathogenic avian influenza (HPAI) H5N1 viruses and their spillover into the human population pose substantial economic and public health threats. Although antiviral drugs have some effect in treating influenza infection, vaccination is still the most effective intervention to prevent possible pandemic outbreaks. We have developed a novel H5 influenza vaccine to improve the world's pandemic preparedness. We produced a hemagglutinin (HA) of HPAI H5N1 virus A/Alberta/01/2014 (AB14) using both mammalian (m) and bacterial (b) expression systems. The purified recombinant proteins were formulated with a proprietary adjuvant (TriAdj) and their efficacy as vaccine candidates was evaluated in mice. Intramuscular delivery of two doses of TriAdj formulated mammalian expressed HA (m-HA/TriAdj) was shown to provide full protection against a lethal challenge of AB14 in mice. In contrast, bacterially expressed HA with TriAdj (b-HA/TriAdj), b-HA without adjuvant, and m-HA without adjuvant resulted in no protection in immunized mice. Furthermore, m-HA/TriAdj elicited significantly higher levels of balanced Th1 and Th2 responses and neutralizing antibody titres. All the mice in the m-HA/TriAdj group survived a lethal AB14 H5N1 challenge and showed no signs of disease or infection as demonstrated by no loss of body weight or detectable virus in the lungs. Our results suggest that m-HA formulated with TriAdj has potential to protect against pandemic H5N1 in the event of its cross over to the human host.

Meeting report and review: Immunological assays and correlates of protection for next-generation influenza vaccines

BACKGROUND

This report summarizes the discussions and conclusions from the "Immunological Assays and Correlates of Protection for Next-Generation Influenza Vaccines" meeting which took place in Siena, Italy, from March 31, 2019, to April 2, 2019.

CONCLUSIONS

Furthermore, we review current correlates of protection against influenza virus infection and disease and their usefulness for the development of next generation broadly protective and universal influenza virus vaccines.

Quadrivalent Influenza Vaccine-Induced Antibody Response and Influencing Determinants in Patients ≥ 55 Years of Age in the 2018/2019 Season

The effects of immunization with subunit inactivated quadrivalent influenza vaccine (QIV) are not generally well assessed in the elderly Polish population. Therefore, this study evaluated vaccine-induced antibody response and its determinants. Methods: Consecutive patients ≥ 55 years old, attending a Primary Care Clinic in Gryfino, Poland, received QIV (A/Michigan/ 45/2015(H1N1)pdm09, A/Singapore/INFIMH-16-0019/2016 (H3N2), B/Colorado/06/2017, B/Phuket/ 3073/2013) between October-December 2018. Hemagglutination inhibition assays measured antibody response to vaccine strains from pre/postvaccination serum samples. Geometric mean titer ratio (GMTR), protection rate (PR) and seroconversion rate (SR) were also calculated. Results: For 108 patients (54.6% males, mean age: 66.7 years) the highest GMTR (61.5-fold) was observed for A/H3N2/, then B/Colorado/06/2017 (10.3-fold), A/H1N1/pdm09 (8.4-fold) and B/Phuket/ 3073/2013 (3.0-fold). Most patients had post-vaccination protection for A/H3N2/ and B/Phuket/3073/ 2013 (64.8% and 70.4%, respectively); lower PRs were observed for A/H1N1/pdm09 (41.8%) and B/Colorado/06/ 2017 (57.4%). The SRs for A/H3N2/, A/H1N1/pdm09, B Victoria and B Yamagata were 64.8%, 38.0%, 46.8%, and 48.2%, respectively. Patients who received QIV vaccination in the previous season presented lower (p < 0.001 and p = 0.03, respectively) response to B Victoria and B Yamagata. Conclusions: QIV was immunogenic against the additional B lineage strain (B Victoria) without significantly compromising the immunogenicity of the other three vaccine strains, therefore, adding a second B lineage strain in QIV could broaden protection against influenza B infection in this age group. As the QIV immunogenicity differed regarding the four antigens, formulation adjustments to increase the antigen concentration of the serotypes that have lower immunogenicity could increase effectiveness. Prior season vaccination was associated with lower antibody response to a new vaccine, although not consistent through the vaccine strains.

Maternally-Derived Antibodies Protect against Challenge with Highly Pathogenic Avian Influenza Virus of the H7N3 Subtype

Vaccination of hens against influenza leads to the transfer of protective maternally-derived antibodies (MDA) to hatchlings. However, little is known about the transfer of H7N3 vaccine-induced MDA. Here, we evaluated transfer, duration, and protective effect of MDA in chickens against H7N3 HPAIV. To generate chickens with MDA (MDA (+)), 15-week-old White Leghorn hens were vaccinated and boosted twice with an inactivated H7N3 low pathogenic avian influenza virus vaccine, adjuvanted with Montanide ISA 71 VG. One week after the final boost, eggs were hatched. Eggs from non-vaccinated hens were hatched for chickens without MDA (MDA (-)). Both MDA (+) and MDA (-) hatchlings were monitored weekly for antibody levels. Anti-HA MDA were detected by hemagglutination inhibition assay mostly until day 7 post-hatch. However, anti-nucleoprotein MDA were still detected three weeks post-hatch. Three weeks post-hatch, chickens were challenged with 106 EID50/bird of Mexican-origin H7N3 HPAIV. Interestingly, while 0% of the MDA (-) chickens survived the challenge, 95% of the MDA (+) chickens survived. Furthermore, virus shedding was significantly reduced by day 5 post-challenge in the MDA (+) group. In conclusion, MDA confers partial protection against mortality upon challenge with H7N3 HPAIV, as far as three weeks post-hatch, even in the absence of detectable anti-HA antibodies, and reduce virus shedding after challenge.

Population Serologic Immunity to Human and Avian H2N2 Viruses in the United States and Hong Kong for Pandemic Risk Assessment

Background

Influenza A pandemics cause significant mortality and morbidity. H2N2 viruses have caused a prior pandemic, and are circulating in avian reservoirs. The age-related frequency of current population immunity to H2 viruses was evaluated.

Methods

Hemagglutinin inhibition (HAI) assays against historical human and recent avian influenza A(H2N2) viruses were performed across age groups in Rochester, New York, and Hong Kong, China. The impact of existing cross-reactive HAI immunity on the effective reproduction number was modeled.

Results

One hundred fifty individual sera from Rochester and 295 from Hong Kong were included. Eighty-five percent of patients born in Rochester and Hong Kong before 1968 had HAI titers ≥1:40 against A/Singapore/1/57, and >50% had titers ≥1:40 against A/Berkeley/1/68. The frequency of titers ≥1:40 to avian H2N2 A/mallard/England/727/06 and A/mallard/Netherlands/14/07 in subjects born before 1957 was 62% and 24%, respectively. There were no H2 HAI titers >1:40 in individuals born after 1968. These levels of seroprevalence reduce the initial reproduction number of A/Singapore/1/1957 or A/Berkeley/1/68 by 15%-20%. A basic reproduction number (R0) of the emerging transmissible virus <1.2 predicts a preventable pandemic.

Conclusions

Population immunity to H2 viruses is insufficient to block epidemic spread of H2 virus. An H2N2 pandemic would have lower impact in those born before 1968.

Prime-Boost Vaccination With a Novel Hemagglutinin Protein Produced in Bacteria Induces Neutralizing Antibody Responses Against H5-Subtype Influenza Viruses in Commercial Chickens

The highly pathogenic (HP) avian influenza virus (AIV), H5N1 and reassortant H5-subtype HPAIVs, H5N2, H5N6, and H5N8, cause high mortality in domestic birds, resulting in economic losses in the poultry industry. H5N1 and H5N6 also pose significant public health risks and H5N1 viruses are a permanent pandemic threat. To control HPAIVs, eukaryotic expression systems have traditionally been exploited to produce vaccines based on hemagglutinin (HA), a protective viral antigen. In contrast, we used a bacterial expression system to produce vaccine targeting the HA protein. A fragment of the HA ectodomain from H5N1, with a multibasic cleavage site deletion, was expressed in Escherichia coli, refolded, and chromatographically purified from inclusion bodies. The resulting antigen, rH5-E. coli, was validated in terms of conformational integrity and oligomerization status. Previously, the protective efficacy of rH5-E. coli adjuvanted with aluminum hydroxide, has been positively verified by challenging the specific pathogen-free layer chickens with homologous and heterologous H5N1 HPAIVs. Protection was provided primarily by the H5 subtype-specific antibodies, as detected in the FluAC H5 test. The present studies were conducted to assess the performance of alum-adjuvanted rH5-E. coli in commercial birds. Broiler chickens were vaccinated twice with 25 μg of rH5-E. coli at 2- and 4-week intervals, while the layer chickens were vaccinated with 5- to 25-μg antigen doses at 4- and 6-week intervals. Post-vaccination sera were analyzed for anti-H5 HA antibodies, using homologous ELISA and heterologous FluAC H5 and hemagglutination inhibition (HI) tests. Prime-boost immunizations with rH5-E. coli elicited H5 HA-specific antibodies in all the chickens tested. Two antigen doses administered at 4- or 6-week intervals were sufficient to induce neutralizing antibodies against H5-subtype HAs; however, they were ineffective when applied with a 2-week delay. In the layers, 80% to 100% of individuals developed antibodies that were active in the FluAC H5 and/or HI tests. A dose-sparing effect was seen when using the longer prime-boost interval. In the broiler chickens, 62.5% positivity was achieved in the FluAC H5 and/or HI tests. The trials confirmed the vaccine potential of rH5-E. coli and provided indications for anti-influenza vaccination with respect to the chicken type and immunization scheme.

A Randomized, Double-blind, Active-controlled Phase III Trial of a Cell Culture-derived Quadrivalent Inactivated Influenza Vaccine in Healthy South Korean Children and Adolescents 6 Months to 18 Years of Age

BACKGROUND

Cell culture-derived influenza vaccines have several important advantages over egg-based influenza vaccines. The quadrivalent influenza vaccine may offer broader protection against seasonal influenza than trivalent influenza vaccine by containing 1 more B strain. The purpose of this study was to evaluate the immunogenicity and safety of NBP607-QIV, a novel cell culture-derived inactivated quadrivalent influenza vaccine (cIIV4), in children and adolescents.

METHODS

This phase III, randomized, double-blind, multicenter trial in children/adolescents (6 mo to 18 yr) was conducted in South Korea during 2014-2015 season. Subjects were randomized 4:1 to receive either NBP607-QIV or control inactivated trivalent influenza vaccine. Hemagglutination inhibition antibody titers were assessed in prevaccination and 28 days postvaccination sera. Safety data were collected for up to 6 months postvaccination.

RESULTS

A total of 454 participants completed the study. Three-hundred sixty-six subjects received cIIV4 and 88 subjects received inactivated trivalent influenza vaccine. Overall, NBP607-QIV met the immunogenicity criteria of Committee for Medicinal Products for Human Use for each of the 4 strains. Between the NBP607-QIV and control groups, immunogenicity endpoints were comparable. Participants younger than 3 years of age had lower immunologic responses to 2 influenza B strains in both NBP607-QIV and control group. No deaths, vaccine-related serious adverse events (AEs) or withdrawals because of AEs were reported. The solicited AEs reported were generally of mild intensity.

CONCLUSIONS

NBP607-QIV, a novel cIIV4, showed good immunogenicity to all 4 influenza strains and had tolerable safety profiles in children and adolescents. Moreover, NBP607-QIV was more immunogenic against influenza B compared with the control, an egg-based subunit vaccine.

Comparative Immunogenicity of the 2014-2015 Northern Hemisphere Trivalent IIV and LAIV against Influenza A Viruses in Children

Both inactivated influenza vaccines (IIV) and live-attenuated influenza vaccines (LAIV) have been recommended for administration to children. Children are a high-risk group for severe influenza, and a major source of transmission. Therefore, prevention of infection by vaccination is particularly important. However, efficacy and immunogenicity of these vaccines are known to vary by season and geographic location. We compared the immunogenicity of the 2014–2015 Northern Hemisphere trivalent IIV and LAIV against influenza A virus in Canadian Hutterite children aged 2 to 17 using hemagglutination inhibition (HAI) assays, and enzyme-linked immunosorbent assays to measure hemagglutinin-specific serum IgA and mucosal IgA. Both vaccine formulations induced significant increases in HAI titers against H1N1 and H3N2 vaccine strains. Serum IgA titers against H3N2 were significantly boosted by both IIV and LAIV, while only IIV induced a significant increase in serum IgA specific to the H1N1 vaccine strain. While HAI titers correlated with protection conferred by IIV, mucosal IgA titers correlated with protection conferred by LAIV (mucosal IgA titers could not be established as a correlate for IIV due to sample size limitations). IIV and LAIV were previously reported to be equally efficacious in this cohort, although the immunogenicity of IIV was generally superior.

Serum anti-neuraminidase antibody responses in human influenza A(H1N1)pdm09 virus infections

Haemagglutination inhibition (HAI) antibody titres are a correlate of protection for influenza virus infection, but several studies have also demonstrated the protective role of anti-neuraminidase (anti-NA) antibodies. However, there is limited data on anti-NA antibody responses in naturally occurring human influenza. We investigated anti-NA antibody responses to pandemic N1 and seasonal N1 in 18 RT-PCR-confirmed patients with naturally acquired pandemic influenza A (H1N1) 2009 disease detected as part of a prospective community study of influenza. There were increases in neuraminidase inhibition (NAI) antibody titres to both pandemic and seasonal N1 antigens, with greater fold increases in those who had low levels of anti-pandemic N1 titres in acute sera. Of 18 patients with pandemic H1N1 infection, fourfold increases in antibody were observed by HAI in 11 (61%) patients, by anti-pandemic N1 inhibition in 13 (72%) or either in 15 of them (83%). Prior seasonal H1N1 virus infections had elicited cross-reactive anti-pandemic N1 antibody titres in some people prior to the emergence of the 2009 pandemic H1N1 virus. Antibody responses to the anti-N1 pandemic 2009 virus and cross-reactive responses to anti-seasonal N1 antibody were seen in influenza A pandemic 2009 infections. NAI antibodies can complement HAI antibody in sero-diagnosis and sero-epidemiology.

Humoral response to natural influenza infection in solid organ transplant recipients

The humoral immune response of transplant recipients to influenza vaccination has been studied in detail. In contrast, the hemagglutinin inhibiting (HI) antibody response evoked by natural influenza infection and its impact on viral kinetics is unknown. In this prospective, multicenter, cohort study of natural influenza infection in transplant recipients, we measured HI antibody titers at presentation and 4 weeks later. Serial nasopharyngeal viral loads were determined using a quantitative influenza A polymerase chain reaction (PCR). We analyzed 196 transplant recipients with influenza infection. In the cohort of organ transplant patients with influenza A (n = 116), seropositivity rates for strain-specific antibodies were 44.0% (95% confidence interval [CI] 31.5-53.2%) at diagnosis and 64.7% (95% CI 55.4-72.9%) 4 weeks postinfection. Seroconversion was observed in 32.8% (95% CI 24.7-41.9%) of the cases. Lung transplant recipients were more likely to seroconvert (P = .002) and vaccine recipients were less likely to seroconvert (P = .024). A subset of patients (n = 30) who were unresponsive to prior vaccination were also unresponsive to natural infection. There was no correlation between viral kinetics and antibody response. This study provides novel data on the seroresponse to influenza infection in transplant patients and its relationship to a number of parameters including a prior vaccination status, virologic measures, and clinical variables.

A mosaic hemagglutinin-based influenza virus vaccine candidate protects mice from challenge with divergent H3N2 strains

Current seasonal influenza virus vaccines only provide limited, short-lived protection, and antigenic drift in the hemagglutinin surface glycoprotein necessitates their annual re-formulation and re-administration. To overcome these limitations, universal vaccine strategies that aim at eliciting broadly protective antibodies to conserved epitopes of the hemagglutinin show promise for protecting against diverse and drifted influenza viruses. Here a vaccination strategy that focuses antibody responses to conserved epitopes of the H3 hemagglutinin is described. The approach is based on antigenic silencing of the immunodominant major antigenic sites of an H3 protein from 2014 by replacing them with corresponding sequences of exotic avian hemagglutinins, yielding "mosaic" hemagglutinins. In mice, vaccination with inactivated viruses expressing mosaic hemagglutinins induced highly cross-reactive antibodies against the H3 stalk domain that elicited Fc-mediated effector functions in vitro. In addition, the mosaic viruses elicited head-specific antibodies with neutralizing and hemagglutination-inhibiting activity against recent H3N2 viruses in vitro. Immune sera protected mice from heterologous challenge with viruses carrying H3 proteins from 1968 and 1982, whereas immune sera generated with a seasonal vaccine did not protect. Consequently, the mosaic vaccination approach provides a promising avenue toward a universal influenza virus vaccine.

Protective Antibodies Against Influenza Proteins

The influenza A virus infection continues to be a threat to the human population. The seasonal variation of the virus and the likelihood of periodical pandemics caused by completely new virus strains make it difficult to produce vaccines that efficiently protect against this infection. Antibodies (Abs) are very important in preventing the infection and in blocking virus propagation once the infection has taken place. However, the precise protection mechanism provided by these Abs still needs to be established. Furthermore, most research has focused on Abs directed to the globular head domain of hemagglutinin (HA). However, other domains of HA (like the stem) and other proteins are also able to elicit protective Ab responses. In this article, we review the current knowledge about the role of both neutralizing and non-neutralizing anti-influenza proteins Abs that play a protective role during infection or vaccination.

The Effect of a Lipopolysaccharide from Rhodobacter capsulatus PG on Inflammation Caused by Various Influenza Strains

The development of a specific inflammation in mice that had been infected by two influenza virus strains, A/chicken/Kurgan/5/2005 (H5N1) and A/Hamburg/2009 MA (H1N1), was studied. We investigated the effect of a non-toxic lipopolysaccharide from Rhodobacter capsulatus PG on the survival and body weight of the mice, production of IgG antibodies, and the induction of pro- and anti-inflammatory cytokines in blood serum. The administration of the R. capsulatus PG lipopolysaccharide was shown to induce interferon-β synthesis, both in healthy and influenza A virus-infected mice, and to promote production of antiviral antibodies in the blood of the influenza-infected animals.

Differential Effects of Influenza Virus NA, HA Head, and HA Stalk Antibodies on Peripheral Blood Leukocyte Gene Expression during Human Infection

In this study, we examined the relationships between anti-influenza virus serum antibody titers, clinical disease, and peripheral blood leukocyte (PBL) global gene expression during presymptomatic, acute, and convalescent illness in 83 participants infected with 2009 pandemic H1N1 virus in a human influenza challenge model. Using traditional statistical and logistic regression modeling approaches, profiles of differentially expressed genes that correlated with active viral shedding, predicted length of viral shedding, and predicted illness severity were identified. These analyses further demonstrated that challenge participants fell into three peripheral blood leukocyte gene expression phenotypes that significantly correlated with different clinical outcomes and prechallenge serum titers of antibodies specific for the viral neuraminidase, hemagglutinin head, and hemagglutinin stalk. Higher prechallenge serum antibody titers were inversely correlated with leukocyte responsiveness in participants with active disease and could mask expression of peripheral blood markers of clinical disease in some participants, including viral shedding and symptom severity. Consequently, preexisting anti-influenza antibodies may modulate PBL gene expression, and this must be taken into consideration in the development and interpretation of peripheral blood diagnostic and prognostic assays of influenza infection.IMPORTANCE Influenza A viruses are significant human pathogens that caused 83,000 deaths in the United States during 2017 to 2018, and there is need to understand the molecular correlates of illness and to identify prognostic markers of viral infection, symptom severity, and disease course. Preexisting antibodies against viral neuraminidase (NA) and hemagglutinin (HA) proteins play a critical role in lessening disease severity. We performed global gene expression profiling of peripheral blood leukocytes collected during acute and convalescent phases from a large cohort of people infected with A/H1N1pdm virus. Using statistical and machine-learning approaches, populations of genes were identified early in infection that correlated with active viral shedding, predicted length of shedding, or disease severity. Finally, these gene expression responses were differentially affected by increased levels of preexisting influenza antibodies, which could mask detection of these markers of contagiousness and disease severity in people with active clinical disease.

mRNA vaccines against H10N8 and H7N9 influenza viruses of pandemic potential are immunogenic and well tolerated in healthy adults in phase 1 randomized clinical trials

BACKGROUND

We evaluated safety and immunogenicity of the first mRNA vaccines against potentially pandemic avian H10N8 and H7N9 influenza viruses.

METHODS

Two randomized, placebo-controlled, double-blind, phase 1 clinical trials enrolled participants between December 2015 and August 2017 at single centers in Germany (H10N8) and USA (H7N9). Healthy adults (ages 18-64 years for H10N8 study; 18-49 years for H7N9 study) participated. Participants received vaccine or placebo in a 2-dose vaccination series 3 weeks apart. H10N8 intramuscular (IM) dose levels of 25, 50, 75, 100, and 400 µg and intradermal dose levels of 25 and 50 µg were evaluated. H7N9 IM 10-, 25-, and 50-µg dose levels were evaluated; 2-dose series 6 months apart was also evaluated. Primary endpoints were safety (adverse events) and tolerability. Secondary immunogenicity outcomes included humoral (hemagglutination inhibition [HAI], microneutralization [MN] assays) and cell-mediated responses (ELISPOT assay).

RESULTS

H10N8 and H7N9 mRNA IM vaccines demonstrated favorable safety and reactogenicity profiles. No vaccine-related serious adverse event was reported. For H10N8 (N = 201), 100-µg IM dose induced HAI titers ≥ 1:40 in 100% and MN titers ≥ 1:20 in 87.0% of participants. The 25-µg intradermal dose induced HAI titers > 1:40 in 64.7% of participants compared to 34.5% of participants receiving the IM dose. For H7N9 (N = 156), IM doses of 10, 25, and 50 µg achieved HAI titers ≥ 1:40 in 36.0%, 96.3%, and 89.7% of participants, respectively. MN titers ≥ 1:20 were achieved by 100% in the 10- and 25-µg groups and 96.6% in the 50-µg group. Seroconversion rates were 78.3% (HAI) and 87.0% (MN) for H10N8 (100 µg IM) and 96.3% (HAI) and 100% (MN) in H7N9 (50 µg). Significant cell-mediated responses were not detected in either study.

CONCLUSIONS

The first mRNA vaccines against H10N8 and H7N9 influenza viruses were well tolerated and elicited robust humoral immune responses. ClinicalTrials.gov NCT03076385 and NCT03345043.

Heterotypic Neuraminidase Antibodies Against Different A(H1N1) Strains are Elicited after Seasonal Influenza Vaccination

Neuraminidase (NA) content is not standardized in current seasonal influenza vaccines; neither anti-NA antibodies (anti-NA Abs) are measured nor is it well-defined as a correlate of humoral protection. In this work, the presence of NA1 antibodies against classical A(H1N1) and A(H1N1) pdm09 subtypes was studied before and after vaccination with seasonal vaccines containing A/California/07/2009 strain (A(H1N1) pdm09 subtype). By Enzyme-Linked Lectin Assay (ELLA; Consortium for the Standardization of Influenza Seroepidemiology), we analyzed serum samples from two different cohorts (adults and elderly). The presence of anti-NA Abs at titers ≥1/40 against classical A(H1N1) and A(H1N1) pdm09 subtypes were frequently found in both age groups, in 81.3% and 96.3% of adults and elderly, respectively. The higher titers of anti-NA Abs (NAI titers) were detected more frequently against classical A(H1N1) strains according to the expected age when the first flu infection takes place. In this way, an Original Antigenic Sin phenomenon related to NA seems to be part of the immune response against flu. Seasonal-vaccination induced homologous seroconversion against NA of A(H1N1) pdm09 subtype in 52.5% and 55.0%, and increased the Geometric Mean Titers (GMTs) in 70.0% and 78.8% of adults and elderly, respectively. Seasonal vaccination also induced a heterotypic anti-NA Abs response against classical A(H1N1) strains (seroconversion at least in 8.8% and 11.3% of adults and elderly, respectively, and an increase in GMTs of at least 28.0% in both age groups). These anti-NA Abs responses occur even though the seasonal vaccine does not contain a standardized amount of NA. This work demonstrates that seasonal vaccines containing the A(H1N1) pdm09 subtype induce a broad antibody response against NA1, that may be a target for future influenza vaccines. Our study is one of the first to analyze the presence of Abs against NA and the response mediated by NAI titers after seasonal influenza vaccination.

Safety and Immunogenicity of a Full-dose, Split-virion, Inactivated, Quadrivalent Influenza Vaccine in Healthy Children 6-35 Months of Age: A Randomized Controlled Clinical Trial

BACKGROUND

For children <3 years of age, a half dose of inactivated influenza vaccine (7.5 μg hemagglutinin per strain) has been used for more than 30 years, but several studies indicate that a full dose (15 μg hemagglutinin per strain) can be used in this population without increasing the rate of fever or other reactions. Here, we compare the safety and immunogenicity of full and half doses of quadrivalent, split-virion, inactivated influenza vaccine (IIV4) in children 6-35 months of age.

METHODS

In this phase IV, randomized, observer-blinded, multi-center study, healthy children 6-35 months of age were randomized 1:1 to be vaccinated with a half or full dose of IIV4 (NCT02915302). The primary objective was to demonstrate that the rate of any fever (≥38.0°C) up to 7 days after a full dose of IIV4 was noninferior to the rate of fever after a half dose.

RESULTS

The study included 1950 children. Noninferiority in the rate of fever was demonstrated for the full dose versus the half dose of IIV4 (difference in rate = 0.84%; 95% confidence interval, -2.13% to 3.80%). Solicited reactions and unsolicited adverse events were similar between the dose groups. No vaccine-related serious adverse events were reported. Noninferiority of both hemagglutination inhibition geometric mean titers and seroconversion rates was demonstrated for all 4 vaccine strains for the full dose versus the half dose.

CONCLUSIONS

In children 6-35 months of age, a full dose of IIV4 was immunogenic and had a safety profile comparable to that of a half dose, with no new safety concerns observed.

The use of cell-mediated immunity for the evaluation of influenza vaccines: an upcoming necessity

Influenza vaccines are a fundamental tool for preventing the disease and reducing its consequences, particularly in specific high-risk groups. In order to be licensed, influenza vaccines have to meet strict criteria established by European Medicines Agency. Although the licensure of influenza vaccines started 65 years ago, Hemagglutination Inhibition and Single Radial Hemolysis are the only serological assays that can ascertain correlates of protection. However, they present evident limitations. The present review focuses on the evaluation of cell-mediated immunity (CMI), which plays an important role in the host immune response in protecting against virus-related illness and in the establishment of long-term immunological memory. Although correlates of protection are not currently available for CMI, it would be advisable to investigate this kind of immunological response for the evaluation of next-generation vaccines.

Efficient protection of mice from influenza A/H1N1pdm09 virus challenge infection via high avidity serum antibodies induced by booster immunizations with inactivated whole virus vaccine

The immunogenicities of inactivated whole and split virus vaccines derived from influenza A/H1N1pdm09 virus were compared in a mouse model. We demonstrated the unique properties of whole virus vaccine boosters on the serum memory antibody response in mice. Consistent with previous studies, booster immunization with either whole or split virus vaccines of A/H1N1pdm09 virus produced comparable titers of serum antibodies with hemagglutination inhibition and virus-neutralizing activities. However, superior protection against the challenge infection was unexpectedly observed in mice primed and boosted with whole virus vaccines compared with those treated with split virus vaccines, despite similar levels of antibody titers in each group. Immune serum antibodies were shown to be primarily responsible for this protection via passive transfer experiments of immune serum antibodies to naive recipient mice. Moreover, this protection correlated with elevated affinity maturation of the antibodies. Thus, booster immunization with whole virus vaccines elicited a robust serum antibody response with high avidity to the virus, which was not measurable via conventional serological assays.

Augmented CD4+ T-cell and humoral responses after repeated annual influenza vaccination with the same vaccine component A/H1N1pdm09 over 5 years

Annual seasonal influenza vaccination is recommended for high-risk populations and often occupational groups such as healthcare workers (HCWs). Repeated annual vaccination has been reported to either have no impact or reduce antibody responses or protection. However, whether repeated vaccination influences T-cell responses has not been sufficiently studied, despite the increasing evidence of the protective roles of T-cell immunity. Here, we explored the impact of repeated annual vaccination with the same vaccine strain (H1N1pdm09) over multiple seasons in the post-2009 pandemic era and showed that repeated vaccination increased both T-cell and humoral responses. Using the T-cell FluroSpot and intracellular cytokine-staining, the hemagglutination inhibition (HI), and the memory B-cell (MBC) ELISpot assays, we investigated pre- and postvaccination T cells, antibodies, and MBCs in a cohort of HCWs repeatedly vaccinated with H1N1pdm09 for 5 years (pandemic vaccination in 2009 and subsequently annual seasonal vaccination containing H1N1pdm09 during 2010-2013). We found that the prevaccination H1N1pdm09-specific T cells, antibodies, and MBCs were significantly increased after 3-4 repeated vaccinations and maintained at high levels throughout seasons 2012 and 2013. The cross-reactive IFN-γ-secreting CD4⁺ cells recognizing conserved viral external or internal epitopes were also maintained throughout 2012 and 2013. Repeated vaccination improved the multifunctional memory CD4⁺ responses. Particularly, the IFN-γ⁺TNF-α⁺CD4⁺ T cells were boosted following each vaccination. HI antibodies were significantly induced after each vaccination over 5 years. Our findings indicate a broad impact of repeated annual vaccination, even with the same vaccine component, on the influenza-specific T-cell and humoral immunity and support the continuing recommendation of annual influenza vaccination. Despite the widespread implementation of annual influenza vaccination, the effect of repeated vaccinations on the immune response in subsequent seasons is poorly understood. A team led by Rebecca Jane Cox at the University of Bergen examined the humoral and T-cell response elicited by 2009's H1N1pdm09 seasonal vaccine. Since the H1N1pdm09 strain continued to circulate in subsequent years it was included in later vaccine formulations and the authors could therefore examine the effects of repeated annual vaccination over multiple seasons. They observed that H1N1pdm09-specific polyfunctional T-cell responses and antibodies were enhanced by multiple annual vaccinations. In particular, T cells showed progressive skewing to IFN-γ⁺TNF⁺ double producers, but the magnitude of the T-cell response tended to plateau after 3-4 repeated vaccinations. The findings suggest that including the same component in subsequent annual vaccines can significantly impact the influenza immune response.

Cross-reactive mouse monoclonal antibodies raised against the hemagglutinin of A/Shanghai/1/2013 (H7N9) protect against novel H7 virus isolates in the mouse model

Influenza viruses remain a major global public health risk. In addition to seasonal influenza viruses, epizootic influenza A H7 subtype viruses of both the Asian and North American lineage are of concern due to their pandemic potential. In China, the simultaneous occurrence of H7N9 zoonotic episodes and seasonal influenza virus epidemics could potentially lead to novel reassortant viruses with the ability to efficiently spread among humans. Recently, the H7N9 virus has evolved into two new lineages, the Pearl River Delta and the Yangtze River Delta clade. This development has also resulted in viruses with a polybasic cleavage site in the hemagglutinin that are highly pathogenic in avian species and have caused human infections. In addition, an outbreak of a highly pathogenic H7N8 strain was reported in the US state of Indiana in 2016. Furthermore, an H7N2 feline virus strain caused an outbreak in cats in an animal shelter in New York City in 2016, resulting in one human zoonotic event. In this study, mouse monoclonal antibodies previously raised against the hemagglutinin of the A/Shanghai/1/2013 (H7N9) virus were tested for their (cross-) reactivity to these novel H7 viruses. Moreover, the functionality of these antibodies was assessed in vitro in hemagglutination inhibition and microneutralization assays. The therapeutic and prophylactic efficacy of the broadly reactive antibodies against novel H7 viruses was determined in vivo in mouse passive transfer-viral challenge experiments. Our results provide data about the conservation of critical H7 epitopes and could inform the selection of pre-pandemic H7 vaccine strains.

Clinical Expectations for Better Influenza Virus Vaccines-Perspectives from the Young Investigators' Point of View

The influenza virus is one of a few viruses that is capable of rendering an otherwise healthy person acutly bedridden for several days. This impressive knock-out effect, without prodromal symptoms, challenges our immune system. The influenza virus undergoes continuous mutations, escaping our pre-existing immunity and causing epidemics, and its segmented genome is subject to reassortment, resulting in novel viruses with pandemic potential. The personal and socieoeconomic burden from influenza is high. Vaccination is the most cost-effective countermeasure, with several vaccines that are available. The current limitations in vaccine effectivness, combined with the need for yearly updating of vaccine strains, is a driving force for research into developing new and improved influenza vaccines. The lack of public concern about influenza severity, and misleading information concerning vaccine safety contribute to low vaccination coverage even in high-risk groups. The success of future influeza vaccines will depend on an increased public awarness of the disease, and hence, the need for vaccination-aided through improved rapid diagnositics. The vaccines must be safe and broadly acting, with new, measurable correlates of protection and robust post-marketing safety studies, to improve the confidence in influenza vaccines.

Safety and immunogenicity of a quadrivalent influenza vaccine in adults 65 y of age and older

Frequent mismatches between the predominant circulating B strain lineage and the B strain lineage in trivalent influenza vaccines have resulted in missed opportunities to prevent influenza illness. Quadrivalent influenza vaccines containing B strains from each of the 2 lineages have been developed for improved prevention of influenza B infections. Here, we describe the results of a phase III, randomized, double-blind, active-controlled, multicenter trial examining the safety and immunogenicity of a split-virion inactivated quadrivalent influenza vaccine (IIV4) in 675 adults ≥ 65 y of age (NCT01218646). Participants were randomly assigned 1:1:1 to receive a single intramuscular injection with the investigational IIV4, or one of 2 split-virion trivalent inactivated influenza vaccines (IIV3s): a licensed IIV3 containing a B Victoria-lineage strain or an investigational IIV3 containing a B Yamagata-lineage strain. Post-vaccination (day 21) hemagglutinin inhibition titers to all strains induced by IIV4 were statistically non-inferior to those induced by the 2 IIV3s. In addition, for each B strain, rates of seroconversion in the IIV4 group were superior to those induced by the comparator IIV3 not containing that B strain. For all vaccines, the most common solicited reaction was injection-site pain, and most reactions were mild to moderate in intensity and transient. Overall safety profiles were similar between IIV4 and the IIV3s, and no vaccine-related serious adverse events were reported. These results confirm that in adults ≥ 65 y of age, IIV4 was well tolerated and immunogenic against the additional B lineage strain without compromising the immunogenicity of the other 3 vaccine strains.