The evidence for the degree to which indirect effects (i.e. the protection conferred to unvaccinated individuals) of influenza vaccination reduce the risk of infection is lacking. Much of our current knowledge of indirect effects of influenza vaccination comes from ecologic studies comparing communities in which school children were vaccinated to those in which they were not. Subsequently, several randomized clinical trials vaccinated children and then compared the risk in their contacts to the risk in contacts of unvaccinated children. This previous work is generally limited to one year of follow-up and has not used laboratory-confirmed infection, limiting the quality of the evidence and the ability to draw conclusions about different influenza types and subtypes. The objective of this project is to determine the indirect effects of influenza vaccination in households over multiple years. We hypothesize that indirect vaccine effects can manifest in two ways at the household level. First, vaccinated individuals may be protected against community acquired infection thus preventing them from exposing unvaccinated household contacts. Second, unvaccinated household members can be protected through reduction in infectiousness of a vaccinated household index case, and a subsequent reduction in the secondary infection risk among household contacts. Using longitudinally collected data from the Household Influenza Vaccine Effectiveness (HIVE) cohort study we plan to conduct analyses to address the following specific aims:
Aim 1) Quantify the indirect and total effects of influenza vaccination in reducing the risk of influenza infection;
Aim 2) Estimate the indirect effect of influenza vaccination in reducing the risk of exposure to influenza at the household level;
and Aim 3) Estimate the indirect effect of influenza vaccination in reducing the risk of influenza infection in unvaccinated members following introduction of influenza to the household. The expected outcome of the work proposed in these aims is to quantify the protection of unvaccinated individuals by influenza vaccination of household contacts by influenza type/subtype. We further expect to clarify how that protection is conferred, provide clearer evidence for targeting vaccination by age in the event of vaccine shortage, and to estimate the total effect of vaccination in preventing infection. Indirect effects of influenza vaccination are assumed, despite insufficient evidence. Thus, the impact of these findings will be to improve public health messaging by informing vaccine advisory bodies and other decision makers of the total benefits influenza vaccination.
Public health messaging frequently mentions that influenza vaccination can protect unvaccinated individuals, but the evidence supporting this claim is limited. The proposed project will use data collected from multiple years of an established, ongoing cohort study to quantify the protection afforded to unvaccinated individuals by the level of vaccine coverage in their close contacts. This work is relevant to NIH?s mission in that it will inform vaccine advisory bodies and other decision makers of the potential benefits of influenza vaccination.