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Although the seasonal influenza vaccine is the darling of preventive medicine these days, the data behind its efficacy in older age groups is problematic. In many observational studies, vaccination is associated with less all-cause mortality during influenza season, but bias is always a problem in this kind of study. Vaccine researchers sought to examine this bias by studying flu vaccination patterns and mortality during seasons without circulating influenza. They used a Kaiser Permanente health maintenance organization database of older adults (age, ≥65), which yielded almost 1.5 million person-years of follow-up during 4 years.
Overall, both the sickest individuals (as measured by a risk score that predicted future medical costs) and those…