A survey to measure seroresponses to seasonal influenza vaccine was conducted among military recruits at three RTCs in the eastern and southeastern United States March 2011, following intense transmission of pH1N1 in January, including one death, among vaccinated recruits in South Carolina 
. Serum antibody studies indicated that (1) compared with LAIV, TIV induced greater total serum antibodies, a more mature antibody response as measured by isotype distribution, and antibodies with greater affinity to HA1; (2) both LAIV and TIV induced seroresponses against a circulating 2011 pH1N1 clade, but responses were significantly lower than those against the vaccine 2009 pH1N1 strain; and (3) HA1 sequence analysis from concurrently circulating pH1N1 strains in 2011 demonstrated that a single clade with moderate drift from the pH1N1 vaccine strain was responsible for the outbreak. We hypothesize that increased pH1N1 infection among vaccinated recruits at Fort Jackson in early 2011 resulted from antigenic mismatch between the circulating pH1N1 strain and the pH1N1 vaccine strain.
Military recruits are homogeneously healthy and young. As a result of crowding and a challenging training environment, recruits experience higher respiratory disease rates than non-recruits 
. Vaccination reduces influenza rates in military populations providing that the vaccine is a good antigenic match 
. Ongoing syndromic and laboratory-based surveillance provide annual influenza vaccine effectiveness estimates 
. Vaccine effectiveness is a function of the antigenic match between circulating and vaccine influenza strains and host factors such as previous influenza exposure, age, and immune status 
. Recruits generally receive multiple, simultaneous vaccinations during the first days at the RTC. These factors must be taken into account when considering vaccine response and effectiveness data.
Both TIV and LAIV have been shown to be effective in children and adults, although multiple studies in children aged 6 months to 18 years have demonstrated that LAIV provides a greater seroresponse than TIV 
. While results in adults have been mixed 
, TIV vaccination has generally been reported as more effective than LAIV 
. When vaccines are staggered in a recruit setting rather than given simultaneously, clinic visits for respiratory disease decreased by up to 20% 
. A large, cross-sectional study during the 2005–2006 influenza season among military personnel determined that LAIV vaccination provided greater protection from laboratory-confirmed influenza than the TIV 
. In non-recruits TIV provided greater protection. Over the 2007–2008 influenza season, TIV was found to be 50% more effective than LAIV in adult populations 
. A monovalent LAIV pH1N1 vaccine showed differing vaccine efficacy based upon age groups when influenza-like symptoms were considered 
. In our study, geographic differences (vaccine type and circulating strain) and ongoing adenovirus transmission at the RTC precluded a clear comparison between vaccine types based on symptom data alone.
Our results show differential immune response between LAIV and TIV consistent with previous reports. The quantitative and qualitative robustness of T and B cell memory responses to viral antigens are mediated by previous exposure and/or vaccination. Recently, a study in children ages 6–35 months found that LAIV conferred broader heterotypic αβ and γδ T cell immunity against conserved influenza peptides than TIV 
. He et al. determined that the phenotypic changes of influenza-specific CD8+
differed significantly between LAIV and TIV depending on the age of the vaccinee. The authors of this study speculated that the route of vaccination influenced antigenic presentation 
. A prospective, randomized trial to compare the safety and efficacy of LAIV and TIV in adults in South Africa found that those ≥60 years old had better T cell responses to LAIV but superior humoral immune responses to TIV 
. In our work, TIV vaccinated recruits exhibited increased antibody class switching (IgM->IgG) and affinity maturation to 2009 pH1N1 compared with LAIV-vaccinated individuals. While serum antibody HI titers are a correlate of protection, modest antibody titers following LAIV vaccination do not necessarily indicate a failure of protection 
. However, the generation of anti-influenza antibodies with increased affinity among the TIV vaccinees likely allowed more effective clearance of infecting influenza viruses. Differential presentation in the two vaccines of similar antigens might have driven alternate immunoglobulin class switching and maturation routes. SPR on post-vaccination antibody binding to HA1 peptide demonstrated that TIV vaccination elicited a more mature response and that serum antibody binding antibodies to rHA1 after vaccination strongly correlated with the MN titers against the 2009 pH1N1 vaccine strain. As previously demonstrated in ferrets 
and humans 
, these findings suggested that rHA1 oligomer-binding antibodies are involved in virus neutralization.
Our results provided evidence for modest antigenic drift in pH1N1 viruses from the southeastern United States in 2011. There were significant differences in vaccine-induced serum antibody neutralization in the vaccine strain 2009 pH1N1 as compared to the circulating 2011 pH1N1. Contemporaneously collected viruses from nationwide surveillance were rooted against the vaccine and other circulating strains to infer divergence in the HA surface protein.
Recruits arrive at the RTCs from regions across the United States, resulting in the potential seeding of diverse viruses. This was demonstrated in our study as isolates from four of six subclades (Groups 2, 3, 6, and 7) were found at Fort Jackson during the first 2 months of 2011. Interestingly, only one of these, Group 3, circulated widely during the outbreak in January 2011.
In the spring of 2009, distinct spatial heterogeneity existed within pH1N1 viruses, resulting in strong regional founder effects. During this first wave, multiple phylogenetically distinct pH1N1 clades emerged globally 
. However, by the end of the second wave at the end of 2009, extensive viral migration and mixing resulted in the emergence of a single dominate viral lineage, in New York State 
. The international profile of New York likely contributed to the seeding of this virus to regions across the world over the next 12–18 months 
, although other clades continue to circulate.
Analysis of the HA genome elucidated a number of mutations in 2011 pH1N1 viruses. The S183P mutation in the Fort Jackson viruses has been shown in vitro
to inhibit the binding of the DFA monoclonal antibody from the WHO Influenza Detection Kit distributed in 2011 
. This mutation has been found in the 1918 pandemic influenza virus and shown to have increased virulence in mouse models 
. The reversion mutation S84N, evident in viruses from Fort Jackson, has been associated with decreased antigenic responses 
. These studies provide possible mechanisms underlying immune evasion in the pH1N1 viruses from the Fort Jackson region in 2011.
The outbreak of pH1N1 in vaccinated recruits occurred during a time when influenza A/H3N2 and B viruses and multiple subclades of A/pH1N1 circulated in the Fort Jackson. However, only one (Group 3) was evident during the outbreak. This circulating virus had important HA mutations that mediate antibody binding. Moderate antigenic divergence between circulating and vaccine influenza strains likely contributed to the outbreak of pH1N1 among recruits at Fort Jackson in the early weeks of 2011.
At Fort Jackson, the protective threshold was breached in LAIV vaccinees infected with antigenically divergent subclade 3 pH1N1 viruses. Increased TIV vaccination in the Fort Jackson recruit population could induce higher antibody titers and protective immunity. We speculate that TIV vaccination would increase overall vaccine effectiveness, thereby providing a herd immunity effect across the population.
The effectiveness of seasonal influenza vaccines varies by season. The risk of periodic influenza epidemics as a result of antigenic drift may best be ameliorated through the development of a universal influenza vaccine 
and/or therapeutics that address issues of antiviral resistance, such as multi-drug combinational therapy 
. Our work accentuates the need for intense surveillance tied to timely virus characterization and agile production of vaccines and therapeutics in response to ever-adapting influenza viruses.