The public health importance of the effects of aging on influenza vaccine responses has become increasingly more evident as the global elderly population continues to dramatically rise. Development of effective influenza vaccines targeted specifically to this high-risk group remains a high priority. The protection elicited by a successful influenza vaccine for the elderly will likely need both humoral and cellular-mediated immunity, requiring full engagement of the host immune system. However, during aging the coordination and regulation of immunological functions are impaired as a result of immunesenescence [30
In this study, we identified significant differences in the immune response to an inactivated subunit influenza vaccine between seniors and young adults. We also determined the extent to which a four-fold higher dosage vaccine might enhance the immune response despite these age-related immunological defects. Our results showed that there was a clear age-related impairment in the HAI and VN antibody responses to the standard-dose influenza vaccine for each of the three vaccine virus strains. Although the high-dosage vaccine improved antibody responses measured by GMT, fold-increase in titers over baseline, and seroconversion and seroprotection rates in elderly subjects, this group was unable to attain the magnitude of responses elicited by the young adults who received the standard-dose vaccine. The relatively small sample sizes of the groups, might have underpowered the study, limiting our capacity to detect statistically significant differences for some of the immunological outcomes. Because elderly patients are usually under medication or have underlying health conditions, recruitment of participants that met the study requirements was difficult. Nonetheless, the HAI results from the parent Phase 2 trial [32
], which was a multi-site study that included a much larger number of elderly subjects, were similar to those of the elderly group reported here, and yet still failed to exceed the HAI responses of the YS, by both GMT and fold-rise in serum levels. Because we studied a healthy and active elderly population as opposed to senior community subjects in the parent study, our data support the notion of a general decline in the immune capacity of medically stable, active elderly people and challenges the concept that the poor immune responses of the elderly are due to reduced health status [33
]. In previous studies of multivalent higher dosage influenza vaccines, increases in antibody levels were not observed for all the components [34
A limitation when interpreting our data is the vaccination history of the subjects enrolled, since prior exposure to influenza vaccine can influence responses to a subsequent immunization [37
]. It is also possible that some of the discrepancy between our data and the immune responses reported previously by others, result from differences in priming background. The enrollment criteria of the parent study that recruited the elderly subjects did not exclude participants based on prior vaccination history; in fact it sought a broader inclusion to obtain data representative of the general population. In that study, the responses of elderly subjects with a recent prior influenza vaccination were found to be lower than those of elderly without recent vaccination [40
]. The majority of the elderly subjects we examined received influenza vaccination 5–6 months prior to our immunization, during which the same 3 virus strains were administered (). Conversely, a minority of the young individuals received influenza vaccination during the same time period. When we examined our data adjusting for prior immunization through multiple linear regression analysis we found superior antibody responses in the YS group compared with the elderly for most of the virus strains. However, due to the low numbers of subjects in some of the groups, we failed to demonstrate statistiscally significant differences in all comparisons. The small number of subjects also precluded us from further dissecting the influence of priming history on the immune responses in relation to age.
Several studies have examined the effect of higher dosage levels of influenza vaccine on the elderly [16
]. Only one of these studies attempted to compare these responses to those of healthy young adults [16
], whereas none of them examined CMI responses.
Because a previous report had suggested that ELISA might be superior to HAI in distinguishing differences to influenza vaccination [45
], we further explored the humoral immune responses by examining influenza-specific IgG, IgA, IgG subclasses and avidity by ELISA. The serum IgG and IgA antibody responses of the elderly were lower than those of the young; the high-dose vaccine resulted in some improvement of those immune responses but still fell short of those induced by the standard dose in young adults. In contrast, none of the elderly subjects that received the standard-dose vaccine achieved a four-fold rise in IgG antibodies, stressing again the need for a higher dose of antigen to trigger a stronger response during immunesenesence.
Aging has been shown to be associated with a significant impairment of IgG1, but not of IgG3 [46
]. We did not find significant differences in the IgG subclasses between the young and elderly subjects in response to influenza vaccination. IgG1 was the main subclass produced, followed by IgG2, in both groups and for all strains. This increase in IgG1 levels following immunization, particularly in the EH group, may reflect a predominant Th2-type response. A major limitation of our study was the low number of subjects that could be included in this analysis. Nevertheless, the trends were consistent and clear. In addition, we were unable to provide evidence for or against a defect in avidity maturation as another feature of age-related effect on vaccination [47
]. It is possible that IgG subclass distribution and avidity are more dependent on priming and boosting by natural infection or vaccination rather than on age [48
Interferon-γ production and other CMI responses may represent complementary and relevant effector mechanisms involved in protection against influenza, especially in the elderly. Although we were unable to demonstrate significant differences in IFN-γ production among the age and dose groups, there was a trend for the young to have higher post-vaccination IFN-γ peak SFC. We did not observe improved SFC in the higher dose vaccine group among the elderly. These findings seem to support the view that there are significant age-related defects in lymphocyte function and/or Th2-biased responses [50
]. The absence of an IFN-γ response may reflect a Th2-type response to this vaccine, although an association was not examined. Notably, we observed significant or almost significant increases in vaccine-induced IFN-γ production in all groups of volunteers following immunization, suggesting that the elderly still maintain the ability to respond to TIV vaccination with CMI responses of moderate magnitude. The IFN-γ production described in our study most likely results from vaccine-primed CD4+
Th1 cells, with limited contribution of CD8+
T cells. We are further exploring the potential for T cell responses in these subjects, including the involvement of specific CD4+
subsets and the activation of vaccine-specific effector and memory T cells. The highest risk of influenza disease was previously demonstrated to be among individuals that had neither a humoral (HAI) or CMI response to vaccination [53
]. We showed that immunization of the elderly with the standard-dose vaccine was associated with high numbers of complete non-responders (i.e., individuals lacking both antibodies and CMI) compared with immunization with the high-dose vaccine. In contrast, none of the young adults receiving the standard-dose were complete non-responders.
In conclusion, there is an urgent need to develop newer generation influenza vaccines to specifically address the limitations of the current vaccines in protecting elderly people. Our study shows a clear age-associated impairment in host immunity against influenza vaccination and an improvement of humoral responses with the use of a higher dose vaccine. The superior antibody levels induced by the higher dose vaccine should lead to increased sterilizing immunity, yet this might still be lower than that expected in healthy young adults receiving the standard dose vaccine. Recently, a high-dose influenza vaccine (Fluzone® High-dose) was approved for use in elderly adults. Currently, the ACIP does not favor vaccination with the high-dose over the standard dose vaccine [54
] and the level of protection of the high dose in comparison to the standard dose vaccine has yet to be determined. Adjuvanted inactivated influenza vaccines, some of which have been licensed in Europe since 1997, may provide superior protection. Annual vaccination with either the standard dose or high dose vaccine, with or without adjuvant, remains of utmost importance for individuals who may indeed benefit from vaccination; it maintains herd immunity and reduces the chances of infection of susceptible (perhaps unprotected) groups.