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Experts have long debated the effect of previous-season influenza vaccination on current-season flu risk. To address the question more systematically, researchers tallied vaccination history with laboratory-confirmed cases of influenza from 2004 through 2013 (a total of 8 flu seasons) among residents of a single Wisconsin community.
Circulating influenza strains varied (as did vaccine components), but only two strains that made repeat appearances during the study period were counted. For each strain, participants who were vaccinated during the current flu season only, during the previous season only, or during both seasons had similar rates of protection (all rates were better than that in unvaccinated controls). These patterns applied both to unadjusted rates of illness — estimated vaccine effectiveness was ≈40% for influenza A (H3N2) and ≈70% for influenza B — and to rates after adjustment for factors such as age and comorbid conditions.
McLean HQ et al. Impact of repeated vaccination on vaccine effectiveness against influenza A (H3N2) and B during 8 seasons. Clin Infect Dis 2014 Nov 15; 59:1375. (http://dx.doi.org/10.1093/cid/ciu680)
Comment
These data can be interpreted in several ways. Perhaps previous-season vaccination does confer some enduring protection against the flu. Or perhaps those who choose to get vaccinated, whether routinely or just occasionally, have other health-savvy behaviors that put them at lower risk.