2014-15 Flu Vaccine Not a Perfect Match to Circulating Viruses

By Kelly Young

Edited by André Sofair, MD, MPH, and William E. Chavey, MD, MS

Roughly half of the circulating influenza A (H3N2) viruses collected in the U.S. early this flu season are antigenically different from the H3N2 virus included in this year's vaccine, prompting CDC officials to remind healthcare providers about using neuraminidase inhibitors to treat and prevent influenza.

H3N2 has been present in about 90% of influenza-positive tests this flu season. Years with high H3N2 activity tend to see higher flu morbidity and mortality.

The World Health Organization recommended components for the Northern Hemisphere vaccine in February. Antigenically drifted H3N2 viruses were detected in March and became more prevalent in September, too late to change the vaccine.

"They're different enough that we're concerned that protection from vaccination ... may be lower than we usually see," CDC Director Tom Frieden told reporters on Thursday.

The CDC is still recommending that people get vaccinated against the flu because it provides partial protection and the B strains are well matched. But Frieden said that if clinicians suspect influenza in high-risk patients, they should start neuraminidase inhibitor treatment without waiting for confirmatory test results.