Each year, influenza viruses circulate worldwide and are responsible for a considerable burden of medical visits, hospitalizations and deaths, especially at the extremes of age, among the very young and the very old. Older adults are more vulnerable to influenza illness and less likely to benefit from preventive measures such as influenza vaccination due to the increased prevalence of chronic medical conditions with age and the systemic degeneration of immune functioning, known as “immunosenescence”. Indeed, although influenza vaccines have been used since the 1940s and the World Health Organization (WHO) recommends vaccinating older adults, there is limited evidence on vaccine efficacy in this age group and numerous reports of lower immunogenicity and clinical vaccine effectiveness (VE) in older compared to younger and middle-aged adults. We consider two strategies that may overcome some of these challenges.
I will discuss the potential for vaccinating older adults every 6 months, instead of every year. This would increase the likelihood that vaccine components would match circulating influenza viruses, and may be particularly valuable in subtropical or tropical areas where there are multiple epidemics each year or prolonged periods of influenza activity. In a randomized controlled trial, we found that twice annual vaccination did provide higher year-round antibody titers in the first year, when vaccine strains were unchanged. The trial is ongoing. Then I will discuss the potential for using enhanced influenza vaccines in older adults. Vaccines which include a higher antigen content, or are adjuvanted, generate stronger immune responses, and may provide prolonged protection, and broader protection against mismatched viruses. A randomized trial is now being conducted to examine in more detail the immune responses to enhanced vaccination in older adults in Hong Kong, including the use of these vaccines in alternating/combination strategies.
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