The results of this assessment suggest there is an association with protection from the northern hemisphere 2008–09 seasonal influenza vaccine against clinically-apparent, laboratory-confirmed pH1N1-associated illness among active component US military service members. This association with protection may be more apparent for hospitalized (more severe) outcomes and warrants further investigation. Our findings further complement recent reports among civilian populations in Mexico 
and among health care professional in Ontario, Canada 
that demonstrate moderate to high (35 to 73%) vaccine effectiveness against pH1N1-associated illness from seasonal influenza vaccination. A very recent report from a Singaporean prospective study 
indicated a marked increased risk of pH1N1 infection in military personnel and has also provided further evidence of an association with protection from seasonal influenza vaccines among military personnel when compared to civilian populations.
In contrast, published findings from Australia, 
and three US-based studies 
showed either negligible association with protection (overall VE
−10 to 3%, 95% CI, −56 to 40%). Moreover, one recently published study out of Canada documented an increased risk for medically-attended, laboratory-confirmed pH1N1 after receipt of seasonal vaccine (VE
−68%, 95% CI, −174 to 3%) 
. The health care encounter-related findings of our vaccine effectiveness assessment also expand further on recently published laboratory-based serologic studies of the effect of TIV vaccines against pH1N1 
Our data also suggests that prior receipt of TIV or LAIV induces an association of protection against pH1N1-associated illness. This may reflect “priming” of the humoral immune system with influenza vaccine as demonstrated in immunologically-naïve children 
. Similar findings have also been observed in US military populations where the influenza vaccine increased the effectiveness of preventing pneumonia and influenza morbidity among vaccine-naïve service members compared to service members routinely immunized 
. Our findings also expand on the observations by Ohmit, et al, 
and Monto, et al, 
in their prospective, randomized, double-blind, placebo-controlled, 4-year study of efficacy demonstrating that TIV offers a higher degree of protection against laboratory-confirmed influenza in years 1 (2004–05) and 4 (2007–08) of their study.
Like the US CDC's recently published US serologic data 
our findings also strongly suggest an age-related association with protection. However, it appears that any association with protection may actually occur in those as young as 17 to 24 years of age. An unexpected finding of our study was the increased association with protection in those 40 years of age and older, perhaps reflecting an association with previous vaccine exposure and/or natural infection with other human H1N1 viruses in the setting of crowded living conditions prevalent in the military environment or in the population old enough to have been exposed to 1918-like H1N1 viruses 
. There are reports of cross-reactive protection induced by vaccination and infection with virus strains that are divergent between, and within, influenza A virus subtypes in animal models. It seems likely that, in addition to induced hemagglutinin (HA) strain-specific antibody responses, that cross-reactive epitopes on the HA and neuraminidase (NA) external proteins, as well as, immune responses to epitopes on internal proteins can contribute to protection against influenza 
. In addition to these specific epitopes, other studies have suggested that neutralizing capability depends also on the affinity and avidity of the antibodies such that quality may be more of a factor than quantity alone 
. To what degree host-specific, genetically-determined immune responses further confound vaccine effectiveness (or efficacy) has not been adequately studied and may represent an important biological/host confounder which is difficult to address in epidemiologic studies such as this.
Additional findings from our study support the notion that vaccination with seasonal influenza vaccines in the preceding four years (2004–08) also conferred a certain degree of protective immunological memory relevant to the new viral strain. Indeed, it has been shown in previous studies that both humoral and cell-mediated immune (CMI) responses may contribute to protection in influenza-vaccinated persons. In animal studies, the role of CMI in viral clearance and host survival has been shown and increasing evidence is available regarding T cell-mediated immune responses in humans after natural infection or vaccination 
. Thus, it is reasonable to think that CMI plays a significant role and that cross-protective CMI to pH1N1 virus may actually exist in individuals who have been frequently immunized and/or exposed to seasonal influenza 
. As recently described by Greenbaum, et al, 
it is also possible that some degree of pre-existing “memory” conferred by exposure to T-cell epitopes, similar to those found in previously circulating H1N1 strains in the past 20 years (1988–2008), may indeed work to elicit increased immunity of adults. This observation may explain the cumulative enhanced benefit of multiple prior influenza vaccines overlapping with increased potential seasonal exposures in older subjects.
There are several limitations with this study. First, tobacco exposure (e.g., smoking), an important co-factor in increasing the risk for influenza infection/disease, was not addressed in this study. There is animal, laboratory-based 
and human epidemiologic-based evidence 
which strongly suggests smoking as an important factor in predisposing to influenza infection and/or pneumonia. It is possible that lower rates of tobacco use among older military populations contrasted with higher use among younger military personnel outside of basic training (where tobacco use is more controlled) may have been an important confounder not adjusted for in this study.
Second, the vaccination status was based upon electronic data and relied on reporting by the vaccinating health care providers. The possibility of misclassification of vaccine status exists. However, we believe this to be minimal and non-differential between the cases and controls. Furthermore, this potential limitation would bias our results towards the null, therefore underestimating the overall effectiveness of the vaccines.
Third, it is possible that misclassification of cases into the control group may have occurred. Since laboratory testing requirements for pH1N1 confirmation have changed over the time period of this study (starting with universal testing to only testing severe cases), and since physicians may not have requested confirmatory testing for all suspected cases, the possibility exists that a control subject may have been infected with pH1N1 but did not get recorded as a laboratory-confirmed case. In order to decrease this risk of misclassification we excluded from our potential control pool anyone who had a wide range of respiratory-associated symptoms or diagnoses during their qualifying medical encounter. However, if this misclassification did occur we expect it would be non-differential in nature and, again, bias our results toward the null.
Fourth, both cases and controls were highly vaccinated; 80% and 89%, respectively. Thus, it is possible that the minority of the service members who did not receive the 2008–09 seasonal vaccine may have differed in the risk for influenza if they suffered from predisposing, co-morbid conditions which may have increased their risk of infection and/or illness as previously described 
. However, similar to findings from observational studies among the elderly, we actually found that vaccinated subjects were more likely to have history of an underlying medical condition compared to unvaccinated subjects 
. This could have potentially biased our study results towards the null (e.g. less VE), however, when added to the model we found this had no effect on the VE estimates due to the high correlation with age and prior receipt of an influenza vaccine.
Lastly, there are a number of studies that clearly illustrate the inherent bias in assessing influenza vaccine effectiveness when conducting observational, cross-sectional studies such as ours 
. Inherent biases in case ascertainment and access to care may have taken place, however, we feel these potential biases were minimized due to our study population. The active component military population receives universal health care coverage at military treatment facilities regardless of the nature of their underlying conditions or presenting medical symptoms (e.g., equal access to care for respiratory and non-respiratory complaints) and thus, would not have influenced our results in a significant manner.
Our finding of a greater association with protection against severe illness (e.g., hospitalization) suggests that the northern hemisphere 2008–09 influenza vaccine may have a more significant impact against overt pH1N1-associated illness compared to subclinical infection. This warrants further analysis with a larger sample size looking at age, sex, race and other factors, specifically for hospitalized individuals. Prospective monitoring of health care outcomes which may be indicative of severe pH1N1-associated illness, such as severe acute respiratory infections (SARI) and pneumonia is being implemented among all beneficiaries of the military health system.
Ongoing, systematic evaluations of seasonal and pH1N1-specific vaccination programs are critical to assess the overall public health impact of these interventions. Our data supports the importance of continued immunization coverage for all populations as recently recommended by the CDC's Advisory Committee on Immunization Practices 
. Expanded assessment of vaccine effectiveness among high-risk recruit populations who are immunologically-naïve and who traditionally sustain higher rates of acute respiratory infections, 
as well as, among young children and high-risk adults, are indicated and may further refine understanding of biological diversity based on age, sex and background disease states. In addition, the role of multiple previous influenza vaccines on immune response and vaccine efficacy/effectiveness deserves further investigation.
Further studies of the potential association between prior seasonal and pH1N1-specific influenza vaccinations (either single- or multi-year reception) and seasonal as well as pH1N1-associated illnesses are needed and should include prospective cohort as well as retrospective case-control studies using sentinel surveillance data 
. In addition, future immunologic assessments of any age-related protective effect (possibly due to the presence of natural infection or vaccine-induced cross-reactive antibodies) should also be conducted to further elucidate this relationship. A greater number of cases with a broader, older age representation are needed to study these hypotheses more thoroughly. In addition, the role of sex differences on vaccine immune responses and associated efficacy/effectiveness estimations needs to be evaluated through epidemiologic studies 
In summary, a moderate association with protection against clinically-apparent, laboratory-confirmed pH1N1-associated illness was found for immunization with either TIV or LAIV seasonal influenza vaccines. This association with protection was greater for severe disease as compared to milder outcomes. There was also a greater association with protection in the youngest (<25 years) and oldest (40+ years) compared to those 25 to 39 years. Prior vaccination in the 2004–08 timeframe was also independently associated with protection. Cross-protective immunity, as a result of natural influenza infections or prior influenza immunization in the military setting, may play a role in conferring a certain degree of enhanced host immunity as exposure takes place with each subsequent influenza season strain(s). Therefore, it is important to examine host-specific, genetically-determined factors in future assessments of influenza vaccine efficacy and/or effectiveness.