Influenza vaccination strategies in older adults

Abstract

Older adults are one of the priority groups for influenza vaccination each year. Health care personnel are another priority group for vaccination, to provide protection against occupational exposures and to reduce the risk of transmitting infection to vulnerable patients. Current inactivated influenza vaccines provide a variable level of protection against infection that depends on the degree of “match” between vaccine strains and circulating strains. However, important questions have been raised over the possible reductions in immune response with repeated annual vaccination, and whether these might lead to reductions in vaccine effectiveness.

The overall aim of my thesis is to explore alternative influenza vaccination strategies that could overcome the negative effects of repeated vaccination, and identify better vaccination strategies to reduce the impact of influenza in older adults in Hong Kong and elsewhere in the world. I undertook a number of specific studies to examine repeated vaccination effects and the potential improvements offered by enhanced influenza vaccines.

First, using data from a test-negative study, I identified variability in influenza vaccine effectiveness estimates over time, with a tendency towards higher estimates during winter epidemics than summer epidemics that was consistent with intra-season waning. Lower vaccine effectiveness was observed among older adults.

Second, I conducted a meta-analysis to compare the immunogenicity of these strategies with conventional standard-dose vaccine. Enhanced influenza vaccines had a comparable immunogenicity profile and all elicited improved immunogenicity compared with the standard-dose vaccine.

Third, I analysed data from an observational study of twice-annual vaccination and from a randomised controlled trial of enhanced influenza vaccines. Both studies showed blunted immune responses with repeated vaccination, that were more pronounced among participants who had been previously vaccinated with the same components as the current vaccine. However, I observed less blunting in immune response in enhanced vaccine groups than in the standard-dose vaccine group in the proportions of participants with post-vaccination titre ≥160.

In addition, I conducted a longitudinal study to examine the psychosocial determinants of influenza vaccination among healthcare personnel. I found that attitude towards influenza vaccination was the strongest predictor of intention and actual receipt of influenza vaccination. An indirect association between norms and vaccination intention through attitude towards influenza vaccination was observed.

Overall, my findings support that enhanced influenza vaccines may be considered as strategies to increase immune response in older adults and possibly overcome the negative effect of repeated vaccination. Head-to-head trials across multiple years would provide valuable data on the immune responses to repeated vaccinations with these enhanced influenza vaccines, and larger studies would allow us to examine efficacy against a variety of circulating strains with different degrees of match between vaccine strains and circulating strains. Promotion of influenza vaccination in healthcare personnel would also provide indirect protection to vulnerable patients especially older adults. A social norm approach may be an intervention strategy to shape their attitude towards influenza vaccination and their subsequent decision-making for influenza vaccination.