The 2014-2015 influenza vaccines offered little protection against the predominant influenza A/H3N2 virus, but were effective against influenza B, according to the vaccine effectiveness estimates provided by the U.S. Flu Vaccine Effectiveness Network.
Preferential use of the live attenuated influenza vaccine (LAIV) among young children, a recommendation previously published by the Advisory Committee on Immunization Practices, was not supported.
During the 2014-2015 influenza season, a total of 9,710 patients seeking outpatient medical treatment for acute respiratory infection with cough were enrolled into the U.S. Flu Vaccine Effectiveness study, reported Richard Zimmerman, MD, of the University of Pittsburgh, and his colleagues (Clin Infect Dis. 2016 Oct 4. doi: 10.1093/cid/ciw635).
Of these, 9,311 participants had complete data, and 7,078 (76%) tested negative for influenza. A total of 1,840 participants tested positive for influenza A – 99% of these cases were strain A/H3N2 – and 395 participants tested positive for influenza B.
Of the 4,360 vaccinated participants with known vaccine type, 39.7% received standard dose trivalent, 1.6% received high dose trivalent, 46.8% received standard dose quadrivalent, and 11.9% received quadrivalent live-attenuated vaccines.
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For influenza A and B combined, the overall adjusted vaccine effectiveness was 19% (95% Confidence Interval, 10-27%) against all medically attended influenza and was statistically significant in all age groups except 18-49 years.
Across all vaccine types, the vaccine effectiveness for the A/H3N2 strain was 6% (95% CI, -5-17%), estimates were similar across all age groups, and all vaccine types were similarly ineffective. These estimates were “consistent with a mismatch between the vaccine and circulating viruses,” the researchers noted.
Overall vaccine effectiveness for influenza B/Yamagata was 55% (95% CI, 43% to 65%) and was similarly significant in all age strata except 50-64 year olds. Trivalent vaccines were more effective at preventing influenza B and, of note, no cases of influenza B occurred among those who received a high dose trivalent flu vaccine.
The study was supported by the Centers for Disease Control and Prevention and the National Institutes of Health. Dr. Zimmerman and four other investigators reported receiving research funding from several pharmaceutical companies.
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