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The possibility of a mismatch between vaccine and virus threatens to undercut the success of every winter influenza-vaccination campaign. To evaluate the bottom line on the 2015–2016 flu season, researchers enrolled a sample of children and adults with acute respiratory symptoms who presented to U.S. ambulatory care clinics from November 2015 through mid-April 2016. Of 6879 participants, 50% had received some form of flu vaccine at least 2 weeks prior to illness, and 19% tested positive for influenza A or B. Among those with confirmed flu, 494 had been vaccinated.
Overall vaccine efficacy was estimated at 48%, ranging from 26% in 50- to 64-year-olds to 59% in 9- to 17-year-olds. When data were parsed by vaccine type, overall efficacy of the …