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This award-winning blog supplements the articles in Hospital Infection Control & Prevention.

CDC: Current Flu Shot Efficacy at 48%

By Gary Evans, Medical Writer

In interim efficacy data for the 2016-2017 flu season, the current vaccine is proving 48% effective overall, but only 43% for the A strain causing the most infections, the Centers for Disease Control and Prevention reports.

The overall efficacy was based on effectiveness against influenza A and influenza B virus infection associated with acute respiratory illness requiring medical attention. As of Feb 17, 2017, most influenza infections were being caused by A (H3N2) viruses. Vaccine efficacy specifically against H3N2 was estimated to be 43%, but efficacy rose to 73% against the circulating B virus.

“These interim vaccine efficacy estimates indicate that influenza vaccination reduced the risk for outpatient medical visits by almost half,” the CDC reported.

Historically, a comprehensive review of the literature estimated an overall flu shot efficacy of 59% in healthy adults ages 18 to 64 years.1 Evidence was inconsistent or lacking to determine vaccine efficacy in those ages 2-17 and those over 65.

Of course, the vagaries of antigenic “drift” or more dramatic “shift” of circulating, mutable influenza viruses create an ongoing challenge to making an annual vaccine with a good match. For example, last year’s overall flu vaccine efficacy was 41% against the predominate H1N1 influenza A strain and 55% against circulating B strains.


1. Osterholm MT, Kelley NS, Sommer A, et al. Efficacy and effectiveness of 3. influenza vaccines: a systematic review and meta-analysis. Lancet Infect Dis 2012;12:36-44.

For more on this story see the April 2017 issue of Hospital Employee Health