The Doctrine of Original Antigenic Sin: Separating Good From Evil

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

Background

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

Methods

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

Results

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

Conclusion

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

Pre-existing yellow fever immunity impairs and modulates the antibody response to tick-borne encephalitis vaccination

Flaviviruses have an increasing global impact as arthropod-transmitted human pathogens, exemplified by Zika, dengue, yellow fever (YF), West Nile, Japanese encephalitis, and tick-borne encephalitis (TBE) viruses. Since all flaviviruses are antigenically related, they are prone to phenomena of immunological memory ('original antigenic sin'), which can modulate immune responses in the course of sequential infections and/or vaccinations. In our study, we analyzed the influence of pre-existing YF vaccine-derived immunity on the antibody response to TBE vaccination. By comparing samples from YF pre-vaccinated and flavivirus-naive individuals, we show that YF immunity not only caused a significant impairment of the neutralizing antibody response to TBE vaccination but also a reduction of the specific TBE virus neutralizing activities (NT/ELISA-titer ratios). Our results point to a possible negative effect of pre-existing cross-reactive immunity on the outcome of flavivirus vaccination that may also pertain to other combinations of sequential flavivirus infections and/or vaccinations.

Impact of age and pre-existing immunity on the induction of human antibody responses against influenza B viruses

Pre-existing immunity to influenza is dependent on a number of factors and can vary greatly within and across influenza subtypes. In this study, volunteers (aged 18–85 years) were vaccinated with split, inactivated Fluzone^TM in four consecutive influenza seasons from 2013 to 2016. The impact of repeat vaccination on breadth and durability of functional antibodies was assessed for total IgG and IgA anti-hemagglutinin (HA) binding antibodies and hemagglutination-inhibition (HAI) activity against both influenza B lineages. Many subjects were able to maintain high seroprotective titers to the vaccine strains in subsequent years, which resulted in low vaccine-induced seroconversion rates. This was especially evident in younger subjects who typically had higher titers and maintained these titers into the following season. In contrast, the HAI titers in elderly subjects were generally lower and more likely to decline prior to the start of the next influenza season. Immunological recall or ‘back-boosting’ to antigenically related viruses was associated with seroconversion. Overall, influenza vaccination in both younger and older people elicited broadly reactive immune responses within a lineage, as well as cross-reactive immune responses between lineages. This study exemplified the impact that age and influenza exposure history have on determining an individual’s ability to respond to future influenza infections.

Acquired immune responses to the seasonal trivalent influenza vaccination in COPD

Epidemiological data suggest that influenza vaccination protects against all‐cause mortality in chronic obstructive pulmonary disease (COPD) patients. However, recent work has suggested there is a defect in the ability of some COPD patients to mount an adequate humoral response to influenza vaccination. The aim of our study was to investigate humoral and cell‐mediated vaccine responses to the seasonal trivalent influenza vaccination (TIV) in COPD subjects and healthy controls. Forty‐seven subjects were enrolled into the study; 23 COPD patients, 13 age‐matched healthy controls (HC ≥ 50) and 11 young healthy control subjects (YC ≤ 40). Serum and peripheral blood mononuclear cells (PBMC) were isolated pre‐TIV vaccination and at days 7 and 28 and 6 months post‐vaccine for haemagglutinin inhibition (HAI) titre, antigen‐specific T cell and antibody‐secreting cell analysis. The kinetics of the vaccine response were similar between YC, HC and COPD patients and there was no significant difference in antibody titres between these groups at 28 days post‐vaccine. As we observed no disease‐dependent differences in either humoral or cellular responses, we investigated if there was any association of these measures with age. H1N1 (r = −0·4253, P = 0·0036) and influenza B (r = −0·344, P = 0·0192) antibody titre at 28 days negatively correlated with age, as did H1N1‐specific CD4⁺ T helper cells (r = −0·4276, P = 0·0034). These results suggest that age is the primary determinant of response to trivalent vaccine and that COPD is not a driver of deficient responses per se. These data support the continued use of the yearly trivalent vaccine as an adjunct to COPD disease management.

Influenza Vaccine Effectiveness: Defining the H3N2 Problem

Observational studies have consistently shown that influenza vaccine effectiveness (VE) is lower for H3N2 relative to H1N1pdm09 and type B, and this is not entirely explained by antigenic match. The triad of virus, vaccine, and host immunity provides a framework to examine contributing factors. Antigenic evolution facilitates H3N2 immune escape, and increasing glycosylation of the hemagglutinin shields antigenic sites from antibody binding. Egg passage adaptation of vaccine viruses generates mutations that alter glycosylation, impair the neutralizing antibody response, and reduce VE. Complex host immune factors may also influence H3N2 VE, including early childhood imprinting and repeated vaccination, but their role is uncertain. Of the triad of contributing factors, only changes to the vaccine are readily achievable. However, it is unclear whether current licensed non–egg-based vaccines generate superior protection against H3N2. The optimal strategy remains to be defined, but newer vaccine technology platforms offer great potential.

Continuous Readout versus Titer-Based Assays of Influenza Vaccine Trials: Sensitivity, Specificity, and False Discovery Rates

The current gold standard for measuring antibody-based immunity to influenza viruses relies on the hemagglutinin inhibition assay (HAI), an 80-year-old technology, and the microneutralization assay (MN). Both assays use serial dilution to provide a discrete, ranked readout of 8-14 categorical titer values for each sample. In contrast to other methods of measuring vaccine antibody levels that produce a continuous readout (i.e., mPLEX-Flu and ELISA), titering methods introduce imprecision and increase false discovery rates (FDR). In this paper, we assess the degree of such statistical errors, first with simulation studies comparing continuous data with titer data in influenza vaccine study group comparison analyses and then by analyzing actual sample data from an influenza vaccine trial. Our results show the superiority of using continuous, rather than discrete, readout assays. Compared to continuous readout assays, titering assays have a lower statistical precision and a higher FDR. The results suggested that traditional titering assays could lead to increased Type-II errors in the comparison of different therapeutic arms of an influenza vaccine trial. These statistical issues are related to the mathematical nature of titer-based assays, which we examine in detail in the simulation studies. Continuous readout assays are free of this issue, and thus it is possible that comparisons of study groups could provide different results with these two methods as we have shown in our case study.

Imprinting and Editing of the Human CD4 T Cell Response to Influenza Virus

Immunity to influenza is unique among pathogens, in that immune memory is established both via intermittent lung localized infections with highly variable influenza virus strains and by intramuscular vaccinations with inactivated protein-based vaccines. Studies in the past decades have suggested that the B cell responses to influenza infection and vaccination are highly biased by an individual's early history of influenza infection. This reactivity likely reflects both the competitive advantage that memory B cells have in an immune response and the relatively limited diversity of epitopes in influenza hemagglutinin that are recognized by B cells. In contrast, CD4 T cells recognize a wide array of epitopes, with specificities that are heavily influenced by the diversity of influenza antigens available, and a multiplicity of functions that are determined by both priming events and subsequent confrontations with antigens. Here, we consider the events that prime and remodel the influenza-specific CD4 T cell response in humans that have highly diverse immune histories and how the CD4 repertoire may be edited in terms of functional potential and viral epitope specificity. We discuss the consequences that imprinting and remodeling may have on the potential of different human hosts to rapidly respond with protective cellular immunity to infection. Finally, these issues are discussed in the context of future avenues of investigation and vaccine strategies.

The Role of Pre-existing Cross-Reactive Central Memory CD4 T-Cells in Vaccination With Previously Unseen Influenza Strains

Influenza vaccination is a common approach to prevent seasonal and pandemic influenza. Pre-existing antibodies against close viral strains might impair antibody formation against previously unseen strains-a process called original antigenic sin. The role of this pre-existing cellular immunity in this process is, despite some hints from animal models, not clear. Here, we analyzed cellular and humoral immunity in healthy individuals before and after vaccination with seasonal influenza vaccine. Based on influenza-specific hemagglutination inhibiting (HI) titers, vaccinees were grouped into HI-negative and -positive cohorts followed by in-depth cytometric and TCR repertoire analysis. Both serological groups revealed cross-reactive T-cell memory to the vaccine strains at baseline that gave rise to the majority of vaccine-specific T-cells post vaccination. On the contrary, very limited number of vaccine-specific T-cell clones was recruited from the naive pool. Furthermore, baseline quantity of vaccine-specific central memory helper T-cells and clonotype richness of this population directly correlated with the vaccination efficacy. Our findings suggest that the deliberate recruitment of pre-existing cross-reactive cellular memory might help to improve vaccination outcome.

Beyond Antigenic Match: Possible Agent-Host and Immuno-epidemiological Influences on Influenza Vaccine Effectiveness During the 2015-2016 Season in Canada

Background

Vaccine effectiveness (VE) estimates for 2015-2016 seasonal influenza vaccine are reported from Canada's Sentinel Practitioner Surveillance Network (SPSN). This season was characterized by a delayed 2009 pandemic influenza A(H1N1) virus (A[H1N1]pdm09) epidemic and concurrent influenza B(Victoria) virus activity. Potential influences on VE beyond antigenic match are explored, including viral genomic variation, birth cohort effects, prior vaccination, and epidemic period.

Methods

VE was estimated by a test-negative design comparing the adjusted odds ratio for influenza test positivity among vaccinated compared to unvaccinated participants. Vaccine-virus relatedness was assessed by gene sequencing and hemagglutination inhibition assay.

Results

Analyses included 596 influenza A(H1N1)pdm09 and 305 B(Victoria) cases and 926 test-negative controls. A(H1N1)pdm09 viruses were considered antigenically related to vaccine (unchanged since 2009), despite phylogenetic clustering within emerging clade 6B.1. The adjusted VE against A(H1N1)pdm09 was 43% (95% confidence interval [CI], 25%-57%). Compared to other age groups, VE against A(H1N1)pdm09 was lower for adults born during 1957-1976 (25%; 95% CI, -16%-51%). The VE against A(H1N1)pdm09 was also lower for participants consecutively vaccinated during both the current and prior seasons (41%; 95% CI, 18%-57%) than for those vaccinated during the current season only (75%; 95% CI, 45%-88%), and the VE among participants presenting in March-April 2016 (19%; 95% CI, -15%-44%) was lower than that among those presenting during January-February 2016 (62%; 95% CI, 44%-74%). The adjusted VE for B(Victoria) viruses was 54% (95% CI, 32%-68%), despite lineage-level mismatch to B(Yamagata) vaccine. The further variation in VE as observed for A(H1N1)pdm09 was not observed for B(Victoria).

Conclusions

Influenza VE findings may require consideration of other agent-host and immuno-epidemiologic influences on vaccine performance beyond antigenic match, including viral genomic variation, repeat vaccination, birth (immunological) cohort effects, and potential within-season waning of vaccine protection.

Serological Number for Characterization of Circulating Antibodies

The dissociation constant of the circulating IgG antibodies is suggested to be proportional to the partial concentrations of these antibodies in blood serum in equilibrium. This coefficient, called serological number, is a dimensionless parameter and may be equal for all antibodies in a serum. Based on the serological number, we derived the equilibrium equation of the humoral immune system which allows estimating the number of different binding motifs in a serum. This equation also allows estimating the number of binding motifs of posttranslational and conformational nature. The feasibility of measuring the serological number via peptide arrays was demonstrated. Fifteen peptides with unique binding motifs were incubated and stained with the blood serum of a healthy adult at different dilutions. From these experiments, the serological number was determined. The serological number may explain the pre-existing antibody response after vaccination.

Back to the Future for Influenza Preimmunity-Looking Back at Influenza Virus History to Infer the Outcome of Future Infections

The influenza virus-host interaction is a classic arms race. The recurrent and evolving nature of the influenza virus family allows a single host to be infected several times. Locked in co-evolution, recurrent influenza virus infection elicits continual refinement of the host immune system. Here we give historical context of circulating influenza viruses to understand how the individual immune history is mirrored by the history of influenza virus circulation. Original Antigenic Sin was first proposed as the negative influence of the host’s first influenza virus infection on the next and Imprinting modernizes Antigenic Sin incorporating both positive and negative outcomes. Building on imprinting, we refer to preimmunity as the continual refinement of the host immune system with each influenza virus infection. We discuss imprinting and the interplay of influenza virus homology, vaccination, and host age establishing preimmunity. We outline host signatures and outcomes of tandem infection according to the sequence of virus and classify these relationships as monosubtypic homologous, monosubtypic heterologous, heterosubtypic, or heterotypic sequential infections. Finally, the preimmunity knowledge gaps are highlighted for future investigation. Understanding the effects of antigenic variable recurrent influenza virus infection on immune refinement will advance vaccination strategies, as well as pandemic preparedness.

Influenza, evolution, and the next pandemic

Mortality rates in influenza appear to have been shaped by evolution. During the 1918 pandemic, mortality rates were lower in children compared with adults. This mortality difference occurs in a wide variety of infectious diseases. It has been replicated in mice and might be due to greater tolerance of infection, not greater resistance. Importantly, combination treatment with inexpensive and widely available generic drugs (e.g. statins and angiotensin receptor blockers) might change the damaging host response in adults to a more tolerant response in children. These drugs might work by modifying endothelial dysfunction, mitochondrial biogenesis and immunometabolism. Treating the host response might be the only practical way to reduce global mortality during the next influenza pandemic. It might also help reduce mortality due to seasonal influenza and other forms of acute critical illness. To realize these benefits, we need laboratory and clinical studies of host response treatment before and after puberty.

Seasonal influenza immunisation: Strategies for older adults

Adults over the age of 60-65 years suffer disproportionally from seasonal influenza, experiencing high rates of complications, exacerbation of underlying medical comorbidities, and excess mortality. Thus, older adults are an important priority for influenza immunisation campaigns. Unfortunately, older adults generally display lower immune responses to standard influenza vaccines because of immunosenescence, with resulting suboptimal vaccine effectiveness. Thus, the development of improved vaccines that heighten immune responses and improve effectiveness is an important medical need. To this end, enhanced influenza vaccines specifically targeting this age group have been developed, which seek to overcome the inherent limitations in the immune responses of older adults. Both the licensed high-dose trivalent influenza vaccine (hdTIV) containing fourfold higher antigen contents than standard vaccine, and the MF59^® -adjuvanted trivalent influenza vaccine (aTIV) have been proven to be safe and well-tolerated while enhancing the immune response. Healthcare providers for populations of older adults should be advised to routinely use these enhanced influenza vaccines in seasonal immunisation campaigns to provide improved immunity against influenza and its consequences in this particularly susceptible age group.

Inter-Pathogen Peptide Sharing and the Original Antigenic Sin: Solving a Paradox

Aims:

To analyse the peptide commonality among viral, bacterial, and protozoan pathogens, and the immunopathologic consequences in the human host.

Methods:

HPV16, HCMV,C. diphtheriae, B. pertussis, C. tetani, T. gondii,andT. cruziwere analysed for common amino acid sequences that are additionally shared with the human host. The pentapeptide, a minimal immune determinant in humoral and cellular immune recognition, was used as a measurement unit of the peptide similarity level. Molecular modeling was applied to compare the amino acid contexts containing common minimal determinants.

Results:

Twenty-nine pentapeptides were found to occur, even hundreds of times, throughout the analyzed pathogen proteomes as well as in the human proteome. Such vast peptide commonalities together with molecular modeling data support the possibility that a pre-existing immune response to a first pathogen can be boosted by a successive exposure to a second different pathogen,i.e., the primary response to a pathogen can be transformed into a secondary response to a previously encountered different pathogen. Two possible consequences emerge. Firstly, no responses might be elicited against the pathogen lastly encountered either by infection or active immunization, but reactions could occur only with the early sensitizing pathogen, which is no more present in the organism. Secondly, the immune response boosted by the pathogen lastly encountered will find a way out by cross-reacting with human proteins.

Conclusion:

This study might explain the “original antigenic sin” phenomenon described seven decades ago [Francis T. Jr. Ann Intern Med 1953;39:203], thus providing explanations for vaccine failures and offering possible clues for designing successful vaccines.

The Effects of Birth Year, Age and Sex on Hemagglutination Inhibition Antibody Responses to Influenza Vaccination

The first exposure to influenza is thought to impact subsequent immune responses later in life. The consequences of this can be seen during influenza epidemics and pandemics with differences in morbidity and mortality for different birth cohorts. There is a need for better understanding of how vaccine responses are affected by early exposures to influenza viruses. In this analysis of hemagglutination inhibition (HI) antibody responses in two cohorts of military personnel we noticed differences related to age, sex, prior vaccination, deployment and birth year. These data suggest that HI antibody production, in response to influenza vaccination, is affected by these factors. The magnitude of this antibody response is associated with, among other factors, the influenza strain that circulated following birth.

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

Background

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

Methods

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

Results

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

Conclusion

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

Characterization of Hemagglutinin Antigens on Influenza Virus and within Vaccines Using Electron Microscopy

Influenza viruses affect millions of people worldwide on an annual basis. Although vaccines are available, influenza still causes significant human mortality and morbidity. Vaccines target the major influenza surface glycoprotein hemagglutinin (HA). However, circulating HA subtypes undergo continual variation in their dominant epitopes, requiring vaccines to be updated annually. A goal of next-generation influenza vaccine research is to produce broader protective immunity against the different types, subtypes, and strains of influenza viruses. One emerging strategy is to focus the immune response away from variable epitopes, and instead target the conserved stem region of HA. To increase the display and immunogenicity of the HA stem, nanoparticles are being developed to display epitopes in a controlled spatial arrangement to improve immunogenicity and elicit protective immune responses. Engineering of these nanoparticles requires structure-guided design to optimize the fidelity and valency of antigen presentation. Here, we review electron microscopy applied to study the 3D structures of influenza viruses and different vaccine antigens. Structure-guided information from electron microscopy should be integrated into pipelines for the development of both more efficacious seasonal and universal influenza vaccine antigens. The lessons learned from influenza vaccine electron microscopic research could aid in the development of novel vaccines for other pathogens.

Epidemiological Studies to Support the Development of Next Generation Influenza Vaccines

The National Institute of Allergy and Infectious Diseases recently published a strategic plan for the development of a universal influenza vaccine. This plan focuses on improving understanding of influenza infection, the development of influenza immunity, and rational design of new vaccines. Epidemiological studies such as prospective, longitudinal cohort studies are essential to the completion of these objectives. In this review, we discuss the contributions of epidemiological studies to our current knowledge of vaccines and correlates of immunity, and how they can contribute to the development and evaluation of the next generation of influenza vaccines. These studies have been critical in monitoring the effectiveness of current influenza vaccines, identifying issues such as low vaccine effectiveness, reduced effectiveness among those who receive repeated vaccination, and issues related to egg adaptation during the manufacturing process. Epidemiological studies have also identified population-level correlates of protection that can inform the design and development of next generation influenza vaccines. Going forward, there is an enduring need for epidemiological studies to continue advancing knowledge of correlates of protection and the development of immunity, to evaluate and monitor the effectiveness of next generation influenza vaccines, and to inform recommendations for their use.

Towards a universal influenza vaccine: different approaches for one goal

Influenza virus infection is an ongoing health and economic burden causing epidemics with pandemic potential, affecting 5–30% of the global population annually, and is responsible for millions of hospitalizations and thousands of deaths each year. Annual influenza vaccination is the primary prophylactic countermeasure aimed at limiting influenza burden. However, the effectiveness of current influenza vaccines are limited because they only confer protective immunity when there is antigenic similarity between the selected vaccine strains and circulating influenza isolates. The major targets of the antibody response against influenza virus are the surface glycoprotein antigens hemagglutinin (HA) and neuraminidase (NA). Hypervariability of the amino acid sequences encoding HA and NA is largely responsible for epidemic and pandemic influenza outbreaks, and are the consequence of antigenic drift or shift, respectively. For this reason, if an antigenic mismatch exists between the current vaccine and circulating influenza isolates, vaccinated people may not be afforded complete protection. There is currently an unmet need to develop an effective “broadly-reactive” or “universal” influenza vaccine capable of conferring protection against both seasonal and newly emerging pre-pandemic strains. A number of novel influenza vaccine approaches are currently under evaluation. One approach is the elicitation of an immune response against the “Achille’s heel” of the virus, i.e. conserved viral proteins or protein regions shared amongst seasonal and pre-pandemic strains. Alternatively, other approaches aim toward eliciting a broader immune response capable of conferring protection against the diversity of currently circulating seasonal influenza strains.

In this review, the most promising under-development universal vaccine approaches are discussed with an emphasis on those targeting the HA glycoprotein. In particular, their strengths and potential short-comings are discussed. Ultimately, the upcoming clinical evaluation of these universal vaccine approaches will be fundamental to determine their effectiveness against preventing influenza virus infection and/or reducing transmission and disease severity.

Influenza

Influenza is an infectious respiratory disease that, in humans, is caused by influenza A and influenza B viruses. Typically characterized by annual seasonal epidemics, sporadic pandemic outbreaks involve influenza A virus strains of zoonotic origin. The WHO estimates that annual epidemics of influenza result in ~1 billion infections, 3–5 million cases of severe illness and 300,000–500,000 deaths. The severity of pandemic influenza depends on multiple factors, including the virulence of the pandemic virus strain and the level of pre-existing immunity. The most severe influenza pandemic, in 1918, resulted in >40 million deaths worldwide. Influenza vaccines are formulated every year to match the circulating strains, as they evolve antigenically owing to antigenic drift. Nevertheless, vaccine efficacy is not optimal and is dramatically low in the case of an antigenic mismatch between the vaccine and the circulating virus strain. Antiviral agents that target the influenza virus enzyme neuraminidase have been developed for prophylaxis and therapy. However, the use of these antivirals is still limited. Emerging approaches to combat influenza include the development of universal influenza virus vaccines that provide protection against antigenically distant influenza viruses, but these vaccines need to be tested in clinical trials to ascertain their effectiveness.