Historical H1N1 Influenza Virus Imprinting Increases Vaccine Protection by Influencing the Activity and Sustained Production of Antibodies Elicited at Vaccination in Ferrets

Potential pandemic risk of circulating swine H1N2 influenza viruses

Influenza A viruses in swine have considerable genetic diversity and continue to pose a pandemic threat to humans due to a potential lack of population level immunity. Here we describe a pipeline to characterize and triage influenza viruses for their pandemic risk and examine the pandemic potential of two widespread swine origin viruses. Our analysis reveals that a panel of human sera collected from healthy adults in 2020 has no cross-reactive neutralizing antibodies against a α-H1 clade strain (α-swH1N2) but do against a γ-H1 clade strain. The α-swH1N2 virus replicates efficiently in human airway cultures and exhibits phenotypic signatures similar to the human H1N1 pandemic strain from 2009 (H1N1pdm09). Furthermore, α-swH1N2 is capable of efficient airborne transmission to both naïve ferrets and ferrets with prior seasonal influenza immunity. Ferrets with H1N1pdm09 pre-existing immunity show reduced α-swH1N2 viral shedding and less severe disease signs. Despite this, H1N1pdm09-immune ferrets that became infected via the air can still onward transmit α-swH1N2 with an efficiency of 50%. These results indicate that this α-swH1N2 strain has a higher pandemic potential, but a moderate level of impact since there is reduced replication fitness and pathology in animals with prior immunity.

Vitamin D and viral infections: Infectious diseases, autoimmune diseases, and cancers

Viruses can cause many human diseases. Three types of human diseases caused by viruses are discussed in this chapter: infectious diseases, autoimmune diseases, and cancers. The infectious diseases included in this chapter include three respiratory tract diseases: influenza, COVID-19, and respiratory syncytial virus. In addition, the mosquito-borne dengue virus diseases are discussed. Vitamin D can reduce risk, severity, and mortality of the respiratory tract diseases and possibly for dengue virus. Many autoimmune diseases are initiated by the body's reaction to a viral infection. The protective role of vitamin D in Epstein-Barr virus-related diseases such as multiple sclerosis is discussed. There are a few cancers linked to viral infections. Such cancers include cervical cancer, head and neck cancers, Hodgkin's and non-Hodgkin's lymphoma, and liver cancer. Vitamin D plays an important role in reducing risk of cancer incidence and mortality, although not as strongly for viral-linked cancers as for other types of cancer.

Influenza immune imprinting synergizes PEI-HA/CpG nanoparticle vaccine protection against heterosubtypic infection in mice

The first influenza virus infection (imprinting) can lead to long-term immune memory and influence subsequent vaccinations and infections. Previously, we reported a polyethyleneimine (PEI)-Aichi hemagglutinin (HA)/CpG (PHC) nanoparticle with cross-protective potential against homologous and heterologous influenza strains. Here we studied how influenza immune imprinting influences the antibody responses to the PHC vaccination and the protection against heterosubtypic virus challenges. We found that pre-existing virus immunity can maintain or synergize the vaccine-induced antibody titers, depending on the imprinting virus HA phylogenetic group. The HA group 1 virus (PR8, H1N1)-imprinted mice displayed comparable antigen-specific antibody responses to those without imprinting post-PHC vaccination. In contrast, the group 2 virus (Phi, H3N2)-imprinted mice showed significantly more robust and balanced antibodies post-vaccination, conferring complete protection against body weight loss and lung inflammation upon heterosubtypic reassortant A/Shanghai/2/2013 (rSH, H7N9) virus challenge. Our findings suggest that influenza imprinting from the same HA phylogenetic group can synergize subsequent vaccination, conferring heterosubtypic protection.

Influenza virus immune imprinting dictates the clinical outcomes in ferrets challenged with highly pathogenic avian influenza virus H5N1

Zoonotic transmission of H5N1 highly pathogenic avian influenza virus (HPAIV) into the human population is an increasing global threat. The recent 2022 HPAIV outbreak significantly highlighted this possibility, increasing concern in the general population. The clinical outcomes of H5N1 influenza virus exposure can be determined by an individual's primary influenza virus infection (imprinting) or vaccination status. Immunological imprinting with Group 1 - (H1N1, H2N2, and H2N3) increases survival rates following H5N1 viral infection compared to Group 2 - (H3N2) imprinted individuals. Vaccination against H5N1 influenza viruses can offer protection to at-risk populations; however, stockpiled inactivated H5N1 influenza vaccines are not readily available to the public. We hypothesize that the immunological response to vaccination and subsequent clinical outcome following H5N1 influenza virus infection is correlated with the immunological imprinting status of an individual. To test this hypothesis, our lab established a ferret pre-immune model of disease. Naïve ferrets were intranasally inoculated with seasonal influenza viruses and allowed to recover for 84 days prior to H5N1 virus infection. Ferrets imprinted following H1N1 and H2N3 virus infections were completely protected against lethal H5N1 influenza virus challenge (100% survival), with few to no clinical symptoms. In comparison, H3N2 influenza virus-imprinted ferrets had severe clinical symptoms, delayed disease progression, and a sublethal phenotype (40% mortality). Consecutive infections with H1N1 influenza viruses followed by an H3N2 influenza virus infection did not abrogate the immune protection induced by the original H1N1 influenza virus infection. In addition, ferrets consecutively infected with H1N1 and H2N3 viruses had no clinical symptoms or weight loss. H3N2 pre-immune ferrets were vaccinated with a broadly reactive H5 HA-based or H1 NA-based vaccine (Hu-CO 2). These ferrets were protected against H5N1 influenza virus challenge, whereas ferrets vaccinated with the H1N1 wild-type CA/09 rHA vaccine had similar phenotypes as non-vaccinated H3N2-imprinted ferrets with 40% survival. Overall, Group 2 imprinted ferrets, which were vaccinated with heterologous Group 1 HA vaccines, had redirected immune responses to Group 1 influenza viral antigens and rescued a sublethal phenotype to complete protection.

Sequential vaccinations with divergent H1N1 influenza virus strains induce multi-H1 clade neutralizing antibodies in swine

Vaccines that protect against any H1N1 influenza A virus strain would be advantageous for use in pigs and humans. Here, we try to induce a pan-H1N1 antibody response in pigs by sequential vaccination with antigenically divergent H1N1 strains. Adjuvanted whole inactivated vaccines are given intramuscularly in various two- and three-dose regimens. Three doses of heterologous monovalent H1N1 vaccine result in seroprotective neutralizing antibodies against 71% of a diverse panel of human and swine H1 strains, detectable antibodies against 88% of strains, and sterile cross-clade immunity against two heterologous challenge strains. This strategy outperforms any two-dose regimen and is as good or better than giving three doses of matched trivalent vaccine. Neutralizing antibodies are H1-specific, and the second heterologous booster enhances reactivity with conserved epitopes in the HA head. We show that even the most traditional influenza vaccines can offer surprisingly broad protection if they are administered in an alternative way.

Block the Spread: Barriers to Transmission of Influenza Viruses

Respiratory viruses, such as influenza viruses, cause significant morbidity and mortality worldwide through seasonal epidemics and sporadic pandemics. Influenza viruses transmit through multiple modes including contact (either direct or through a contaminated surface) and inhalation of expelled aerosols. Successful human to human transmission requires an infected donor who expels virus into the environment, a susceptible recipient, and persistence of the expelled virus within the environment. The relative efficiency of each mode can be altered by viral features, environmental parameters, donor and recipient host characteristics, and viral persistence. Interventions to mitigate transmission of influenza viruses can target any of these factors. In this review, we discuss many aspects of influenza virus transmission, including the systems to study it, as well as the impact of natural barriers and various nonpharmaceutical and pharmaceutical interventions.

The Intersection of Age and Influenza Severity: Utility of Ferrets for Dissecting the Age-Dependent Immune Responses and Relevance to Age-Specific Vaccine Development

Many factors impact the host response to influenza virus infection and vaccination. Ferrets have been an indispensable reagent for influenza virus research for almost one hundred years. One of the most significant and well-known factors affecting human disease after infection is host age. Another significant factor is the virus, as strain-specific disease severity is well known. Studying age-related impacts on viral infection and vaccination outcomes requires an animal model that reflects both the physiological and immunological changes that occur with human aging, and sensitivity to differentially virulent influenza viruses. The ferret is uniquely susceptible to a plethora of influenza viruses impacting humans and has proven extremely useful in studying the clinical and immunological pictures of influenza virus infection. Moreover, ferrets developmentally have several of the age-related physiological changes that occur in humans throughout infancy, adulthood, old age, and pregnancy. In this review, we discuss ferret susceptibility to influenza viruses, summarize previous influenza studies using ferrets as models of age, and finally, highlight the application of ferret age models in the pursuit of prophylactic and therapeutic agents to address age-related influenza disease severity.

Defining the root cause of reduced H1N1 live attenuated influenza vaccine effectiveness: low viral fitness leads to inter-strain competition

In the 2013-14 and 2015-16 influenza seasons, reduced vaccine effectiveness (VE) was observed for the H1N1 component of the FluMist quadrivalent live attenuated influenza vaccine (QLAIV) in the USA, leading to loss of Advisory Committee on Immunization Practices recommendation. Here we demonstrate in ferrets that 2015-16A/H1N1pdm09 vaccine strain A/Bolivia/559/2013 (A/BOL13) is outcompeted in trivalent (TLAIV) and QLAIV formulations, leading to reduced protection from wild-type challenge. While monovalent (MLAIV) A/BOL13 provided significant protection from wild-type virus shedding and fever at doses as low as 3.0 log₁₀ fluorescent focus units (FFU), it failed to provide a similar level of protection in TLAIV or QLAIV formulation, even at a 6.0 log₁₀ FFU dose. Conversely, clinically effective H1N1 strain A/New Caledonia/20/1999 provided significant protection in MLAIV, TLAIV, and QLAIV formulations. In conclusion, reduced A/BOL13 replicative fitness rendered it susceptible to inter-strain competition in QLAIV, contributing to its reduced VE in the 2015-16 season.

Broadly Reactive H2 Hemagglutinin Vaccines Elicit Cross-Reactive Antibodies in Ferrets Preimmune to Seasonal Influenza A Viruses

Influenza vaccines have traditionally been tested in naive mice and ferrets. However, humans are first exposed to influenza viruses within the first few years of their lives. Therefore, there is a pressing need to test influenza virus vaccines in animal models that have been previously exposed to influenza viruses before being vaccinated. In this study, previously described H2 computationally optimized broadly reactive antigen (COBRA) hemagglutinin (HA) vaccines (Z1 and Z5) were tested in influenza virus “preimmune” ferret models. Ferrets were infected with historical, seasonal influenza viruses to establish preimmunity. These preimmune ferrets were then vaccinated with either COBRA H2 HA recombinant proteins or wild-type H2 HA recombinant proteins in a prime-boost regimen. A set of naive preimmune or nonpreimmune ferrets were also vaccinated to control for the effects of the multiple different preimmunities. All of the ferrets were then challenged with a swine H2N3 influenza virus. Ferrets with preexisting immune responses influenced recombinant H2 HA-elicited antibodies following vaccination, as measured by hemagglutination inhibition (HAI) and classical neutralization assays. Having both H3N2 and H1N1 immunological memory regardless of the order of exposure significantly decreased viral nasal wash titers and completely protected all ferrets from both morbidity and mortality, including the mock-vaccinated ferrets in the group. While the vast majority of the preimmune ferrets were protected from both morbidity and mortality across all of the different preimmunities, the Z1 COBRA HA-vaccinated ferrets had significantly higher antibody titers and recognized the highest number of H2 influenza viruses in a classical neutralization assay compared to the other H2 HA vaccines.

IMPORTANCE H1N1 and H3N2 influenza viruses have cocirculated in the human population since 1977. Nearly every human alive today has antibodies and memory B and T cells against these two subtypes of influenza viruses. H2N2 influenza viruses caused the 1957 global pandemic and people born after 1968 have never been exposed to H2 influenza viruses. It is quite likely that a future H2 influenza virus could transmit within the human population and start a new global pandemic, since the majority of people alive today are immunologically naive to viruses of this subtype. Therefore, an effective vaccine for H2 influenza viruses should be tested in an animal model with previous exposure to influenza viruses that have circulated in humans. Ferrets were infected with historical influenza A viruses to more accurately mimic the immune responses in people who have preexisting immune responses to seasonal influenza viruses. In this study, preimmune ferrets were vaccinated with wild-type (WT) and COBRA H2 recombinant HA proteins in order to examine the effects that preexisting immunity to seasonal human influenza viruses have on the elicitation of broadly cross-reactive antibodies from heterologous vaccination.

Convergent antibody evolution and clonotype expansion following influenza virus vaccination

Recent advances in high-throughput single cell sequencing have opened up new avenues into the investigation of B cell receptor (BCR) repertoires. In this study, PBMCs were collected from 17 human participants vaccinated with the split-inactivated influenza virus vaccine during the 2016-2017 influenza season. A combination of Immune Repertoire Capture (IRCTM) technology and IgG sequencing was performed on ~7,800 plasmablast (PB) cells and preferential IgG heavy-light chain pairings were investigated. In some participants, a single expanded clonotype accounted for ~22% of their PB BCR repertoire. Approximately 60% (10/17) of participants experienced convergent evolution, possessing public PBs that were elicited independently in multiple participants. Binding profiles of one private and three public PBs confirmed they were all subtype-specific, cross-reactive hemagglutinin (HA) head-directed antibodies. Collectively, this high-resolution antibody repertoire analysis demonstrated the impact evolution can have on BCRs in response to influenza virus vaccination, which can guide future universal influenza prophylactic approaches.

Pre-existing heterosubtypic immunity provides a barrier to airborne transmission of influenza viruses

Human-to-human transmission of influenza viruses is a serious public health threat, yet the precise role of immunity from previous infections on the susceptibility to airborne infection is still unknown. Using the ferret model, we examined the roles of exposure duration and heterosubtypic immunity on influenza transmission. We demonstrate that a 48 hour exposure is sufficient for efficient transmission of H1N1 and H3N2 viruses. To test pre-existing immunity, a gap of 8–12 weeks between primary and secondary infections was imposed to reduce innate responses and ensure robust infection of donor animals with heterosubtypic viruses. We found that pre-existing H3N2 immunity did not significantly block transmission of the 2009 H1N1pandemic (H1N1pdm09) virus to immune animals. Surprisingly, airborne transmission of seasonal H3N2 influenza strains was abrogated in recipient animals with H1N1pdm09 pre-existing immunity. This protection from natural infection with H3N2 virus was independent of neutralizing antibodies. Pre-existing immunity with influenza B virus did not block H3N2 virus transmission, indicating that the protection was likely driven by the adaptive immune response. We demonstrate that pre-existing immunity can impact susceptibility to heterologous influenza virus strains, and implicate a novel correlate of protection that can limit the spread of respiratory pathogens through the air.

Influenza viruses pose a major public health threat through both seasonal epidemics and sporadic pandemics. An individual’s first influenza virus infection leaves long-lasting immunity, which plays an unknown role on susceptibility to airborne transmission of new viral strains. We show that pre-existing heterosubtypic immunity against the 2009 H1N1 pandemic virus protects recipient animals from airborne transmission of a seasonal H3N2 influenza virus, which is independent of cross-neutralizing antibodies. Pre-existing immunity with influenza B viruses was not protective suggesting that this phenomenon is driven by an adaptive response. Taken together, these data indicate that pre-existing immunity is an important barrier to airborne transmission and can influence the emergence and spread of potentially pandemic viruses.

The Power of First Impressions: Can Influenza Imprinting during Infancy Inform Vaccine Design?

Influenza virus infection causes severe respiratory illness in people worldwide, disproportionately affecting infants. The immature respiratory tract coupled with the developing immune system, and lack of previous exposure to the virus is thought to synergistically play a role in the increased disease severity in younger age groups. No influenza vaccines are available for those under six months, although maternal influenza immunization is recommended. In children aged six months to two years, vaccine immunogenicity is dampened compared to older children and adults. Unlike older children and adults, the infant immune system has fewer antigen-presenting cells and soluble immune factors. Paradoxically, we know that a person's first infection with the influenza virus during infancy or childhood leads to the establishment of life-long immunity toward that particular virus strain. This is called influenza imprinting. We contend that by understanding the influenza imprinting event in the context of the infant immune system, we will be able to design more effective influenza vaccines for both infants and adults. Working through the lens of imprinting, using infant influenza animal models such as mice and ferrets which have proven useful for infant immunity studies, we will gain a better understanding of imprinting and its implications regarding vaccine design. This review examines literature regarding infant immune and respiratory development, current vaccine strategies, and highlights the importance of research into the imprinting event in infant animal models to develop more effective and protective vaccines for all including young children.

Efficacy of Heterologous Prime-Boost Vaccination with H3N2 Influenza Viruses in Pre-Immune Individuals: Studies in the Pig Model

In a previous study in influenza-naïve pigs, heterologous prime-boost vaccination with monovalent, adjuvanted whole inactivated vaccines (WIV) based on the European swine influenza A virus (SwIAV) strain, A/swine/Gent/172/2008 (G08), followed by the US SwIAV strain, A/swine/Pennsylvania/A01076777/2010 (PA10), was shown to induce broadly cross-reactive hemagglutination inhibition (HI) antibodies against 12 out of 15 antigenically distinct H3N2 influenza strains. Here, we used the pig model to examine the efficacy of that particular heterologous prime-boost vaccination regimen, in individuals with pre-existing infection-immunity. Pigs were first inoculated intranasally with the human H3N2 strain, A/Nanchang/933/1995. Seven weeks later, they were vaccinated intramuscularly with G08 followed by PA10 or vice versa. We examined serum antibody responses against the hemagglutinin and neuraminidase, and antibody-secreting cell (ASC) responses in peripheral blood, draining lymph nodes, and nasal mucosa (NMC), in ELISPOT assays. Vaccination induced up to 10-fold higher HI antibody titers than in naïve pigs, with broader cross-reactivity, and protection against challenge with an antigenically distant H3N2 strain. It also boosted ASC responses in lymph nodes and NMC. Our results show that intramuscular administration of WIV can lead to enhanced antibody responses and cross-reactivity in pre-immune subjects, and recall of ASC responses in lymph nodes and NMC.

Original antigenic sin priming of influenza virus hemagglutinin stalk antibodies

Immunity to influenza viruses can be long-lived, but reinfections with antigenically distinct viral strains and subtypes are common. Reinfections can boost antibody responses against viral strains first encountered in childhood through a process termed "original antigenic sin." It is unknown how initial childhood exposures affect the induction of antibodies against the hemagglutinin (HA) stalk domain of influenza viruses. This is an important consideration since broadly reactive HA stalk antibodies can protect against infection, and universal vaccine platforms are being developed to induce these antibodies. Here we show that experimentally infected ferrets and naturally infected humans establish strong "immunological imprints" against HA stalk antigens first encountered during primary influenza virus infections. We found that HA stalk antibodies are surprisingly boosted upon subsequent infections with antigenically distinct influenza A virus subtypes. Paradoxically, these heterosubtypic-boosted HA stalk antibodies do not bind efficiently to the boosting influenza virus strain. Our results demonstrate that an individual's HA stalk antibody response is dependent on the specific subtype of influenza virus that they first encounter early in life. We propose that humans are susceptible to heterosubtypic influenza virus infections later in life since these viruses boost HA stalk antibodies that do not bind efficiently to the boosting antigen.

Comparative review of respiratory diseases caused by coronaviruses and influenza A viruses during epidemic season

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to sweep the world, causing infection of millions and death of hundreds of thousands. The respiratory disease that it caused, COVID-19 (stands for coronavirus disease in 2019), has similar clinical symptoms with other two CoV diseases, severe acute respiratory syndrome and Middle East respiratory syndrome (SARS and MERS), of which causative viruses are SARS-CoV and MERS-CoV, respectively. These three CoVs resulting diseases also share many clinical symptoms with other respiratory diseases caused by influenza A viruses (IAVs). Since both CoVs and IAVs are general pathogens responsible for seasonal cold, in the next few months, during the changing of seasons, clinicians and public heath may have to distinguish COVID-19 pneumonia from other kinds of viral pneumonia. This is a discussion and comparison of the virus structures, transmission characteristics, clinical symptoms, diagnosis, pathological changes, treatment and prevention of the two kinds of viruses, CoVs and IAVs. It hopes to provide information for practitioners in the medical field during the epidemic season.

Immune Imprinting in the Influenza Ferret Model

The initial exposure to influenza virus usually occurs during childhood. This imprinting has long-lasting effects on the immune responses to subsequent infections and vaccinations. Animal models that are used to investigate influenza pathogenesis and vaccination do recapitulate the pre-immune history in the human population. The establishment of influenza pre-immune ferret models is necessary for understanding infection and transmission and for designing efficacious vaccines.

Quantitative efficacy paradigms of the influenza clinical drug candidate EIDD-2801 in the ferret model

Seasonal influenza viruses cause major morbidity and mortality worldwide, threatening in particular older adults and the immunocompromised. Two classes of influenza therapeutics dominate current disease management, but both are compromised by pre-existing or rapidly emerging viral resistance. We have recently reported a novel ribonucleoside analog clinical candidate, EIDD-2801, that combines potent antiviral efficacy in ferrets and human airway epithelium cultures with a high barrier against viral escape. In this study, we established fundamental EIDD-2801 efficacy paradigms against pandemic and seasonal influenza A virus (IAV) strains in ferrets that can be used to inform exposure targets and treatment regimens. Based on reduction of shed virus titers, alleviation of clinical signs, and lowered virus burden in upper and lower respiratory tract tissues, lowest efficacious oral dose concentrations of EIDD-2801, given twice daily, were 2.3 and 7 mg/kg of body weight against seasonal and pandemic IAVs, respectively. The latest opportunity for initiation of efficacious treatment was 36 hours after infection of ferrets. Administered in 12-hour intervals, three 7 mg/kg doses of EIDD-2801 were sufficient for maximal therapeutic benefit against a pandemic IAV and significantly shortened the time to resolution of clinical signs. Ferrets infected with pandemic IAV and treated following the minimally efficacious EIDD-2801 regimen demonstrated significantly less shed virus and inflammatory cellular infiltrates in nasal lavages, but mounted a robust humoral antiviral response after recovery that was indistinguishable from that of vehicle-treated animals. These results provide an experimental basis in a human disease-relevant influenza animal model for clinical testing of EIDD-2801.

Predominance of a Drifted Influenza A (H3N2) Clade and its Association with Age-specific Influenza Vaccine Effectiveness Variations, Influenza Season 2018-2019

Background: Influenza A (H3N2) clade 3C.3a was the predominant influenza virus in Israel throughout the 2018-2019 season, constituting a drift from the influenza A (H3N2) vaccine. We estimated the end-of season vaccine effectiveness (VE) by age, among community patients with influenza-like illness (ILI), considering the hemagglutinin (HA) gene mutations and amino acid substitutions of influenza A (H3N2) viruses detected. Methods: Nose-throat samples were analyzed for the presence of influenza virus, type/subtype, and HA gene sequence. HA gene sequences and amino acid substitutions were compared to the influenza A/Singapore/INFIMH-16-0019/2016 (H3N2)-like 2018-2019 vaccine virus, and a phylogenetic tree was generated. Influenza VE against influenza A (H3N2) was estimated using the test-negative design. VE was estimated by age group and by 15 year moving age intervals. Results: In total, 90% of the influenza A (H3N2) viruses belonged to the 3C.3a clade, constituting a unique situation in the northern hemisphere. Adjusted all-age influenza A (H3N2) VE was −3.5% (95% CI: −51.2 to 29.1). Although adjusted VEs were very low among infants, children, and young adults, a VE of 45% (95% CI: −19.2 to 74.6) was estimated among adults aged ≥45 years old. Conclusions: The higher VE point estimates among older adults may be related to previous exposure to similar influenza viruses.

Paradoxical clade- and age-specific vaccine effectiveness during the 2018/19 influenza A(H3N2) epidemic in Canada: potential imprint-regulated effect of vaccine (I-REV)

IntroductionThe Canadian Sentinel Practitioner Surveillance Network reports vaccine effectiveness (VE) for the 2018/19 influenza A(H3N2) epidemic.AimTo explain a paradoxical signal of increased clade 3C.3a risk among 35-54-year-old vaccinees, we hypothesise childhood immunological imprinting and a cohort effect following the 1968 influenza A(H3N2) pandemic.MethodsWe assessed VE by test-negative design for influenza A(H3N2) overall and for co-circulating clades 3C.2a1b and 3C.3a. VE variation by age in 2018/19 was compared with amino acid variation in the haemagglutinin glycoprotein by year since 1968.ResultsInfluenza A(H3N2) VE was 17% (95% CI: -13 to 39) overall: 27% (95% CI: -7 to 50) for 3C.2a1b and -32% (95% CI: -119 to 21) for 3C.3a. Among 20-64-year-olds, VE was -7% (95% CI: -56 to 26): 6% (95% CI: -49 to 41) for 3C.2a1b and -96% (95% CI: -277 to -2) for 3C.3a. Clade 3C.3a VE showed a pronounced negative dip among 35-54-year-olds in whom the odds of medically attended illness were > 4-fold increased for vaccinated vs unvaccinated participants (p < 0.005). This age group was primed in childhood to influenza A(H3N2) viruses that for two decades following the 1968 pandemic bore a serine at haemagglutinin position 159, in common with contemporary 3C.3a viruses but mismatched to 3C.2a vaccine strains instead bearing tyrosine.DiscussionImprinting by the first childhood influenza infection is known to confer long-lasting immunity focused toward priming epitopes. Our findings suggest vaccine mismatch may negatively interact with imprinted immunity. The immunological mechanisms for imprint-regulated effect of vaccine (I-REV) warrant investigation.