Background: Elderly populations are particularly vulnerable to influenza and often require extensive clinical support. In Japan, nationwide passive sentinel surveillance monitors seasonal influenza but does not capture the full disease burden. We synthesized existing evidence on epidemiology, vaccine effectiveness (VE) and costs of seasonal influenza in the elderly population aged over 50yrs.
Methods: PubMed, EMBASE, and ICHUSHI search for articles describing seasonal influenza in Japan, published between 1997 - 2018, in English or Japanese. Grey literature was also assessed. A random-effects meta-analysis characterized VE of standard dose vaccines among studies reporting this information.
Results: Of 1,147 identified articles, 143 met inclusion criteria. Reported incidence rates varied considerably depending on study design, season, study setting and, most importantly, case definition. In nursing homes, influenza attack rates were as high as 55.2% and aged persons suffered significant influenza disease burden, hospitalizations, and mortality. Renal diseases were the most commonly-reported comorbidity. Most hospitalizations were in people aged >60. In metaanalysis, overall VE was 19.1% (95% CI: 2.3% - 33.0%) with high heterogeneity (I2: 89.1%). There was a trend of lower VE in older people (40.1% in the <65 group; 12.9% in those ≥65; p = 0.21).