Vaccine effectiveness in older individuals: What has been learned from the influenza-vaccine experience

Vaccination policies in most high-income countries attempt to reduce the adverse impact of influenza targeting people aged at least 60 years. However, while it is widely believed that the current immunization strategy saves many lives, influenza infection still remains a severe burden in aged individuals leading to a wide debate on the exact magnitude of the benefit of vaccination in this population. The first aim of the present review is to examine how effective current influenza-vaccine strategies are in aged adults, by analysing which are the most important factors modulating the interpretation of study results in this population. Furthermore, consideration will be given to how immune factors influence the measurement of vaccine efficacy/effectiveness, where advancing age leads to deleterious changes in the adaptive immune system, resulting in less than optimal responses to infectious agents and vaccination. Finally this review concludes with possible strategies to improve the ability of the senescent immune system to respond to vaccination.

Research highlights▶ Vaccine preventable diseases remain a severe burden in aged population, and foremost amongst them is influenza. ▶ While it is widely believed that influenza vaccination safes many lives, the exact magnitude of its benefits is still debate in the aged population, because: ▶ the accurate assessment of vaccine benefits is still fraught with considerable methodological and epidemiological challenges, ▶ Pre-immunization antibody levels severely influence the interpretation of vaccine response, HAI-antibody titres cannot be used as gold standard against which to measure vaccine efficacy/effectiveness ▶ while immunosenescence is undoubtedly one of the most important phenomenon adversely influencing vaccine response, methods identifying and measuring it and for understanding its implications for vaccine response still require further evaluation.