The Centers for Disease Control and Prevention (CDC) advised clinicians yesterday that the 2014-2015 seasonal flu shot may not be as effective as originally thought as the major circulating strain this year has mutated to the point reduced protection is being provided.

generic influenza virion
3D influenza/CDC

This happened as flu season in the US starts to pick up.

The predominant strain seen thus far this flu season, as in past flu seasons, has been H3N2 influenza A. However, federal health officials say influenza viral characterization data indicates that 48% of the influenza A (H3N2) viruses collected and analyzed in the United States from October 1 through November 22, 2014 were antigenically “like” the 2014-2015 influenza A (H3N2) vaccine component, but that 52% were antigenically different (drifted) from the H3N2 vaccine virus.

In past seasons during which predominant circulating influenza viruses have been antigenically drifted, decreased vaccine effectiveness has been observed. However, vaccination has been found to provide some protection against drifted viruses. Though reduced, this cross-protection might reduce the likelihood of severe outcomes such as hospitalization and death. In addition, vaccination will offer protection against circulating influenza strains that have not undergone significant antigenic drift from the vaccine viruses (such as influenza A (H1N1) and B viruses).

The 2014-2015 Influenza vaccine composition:  Trivalent influenza vaccines will contain hemagglutinin (HA) derived from an A/California/7/2009 (H1N1)-like virus, an A/Texas/50/2012 (H3N2)-like virus, and a B/Massachusetts/2/2012-like (Yamagata lineage) virus. Quadrivalent influenza vaccines will contain these antigens, and also a B/Brisbane/60/2008-like (Victoria lineage) virus.

The CDC reports the drifted H3N2 viruses were first detected in late March 2014, after World Health Organization (WHO) recommendations for the 2014-2015 Northern Hemisphere vaccine had been made in mid-February.

CDC officials still advise physicians to encourage all patients 6 months and older who have not yet received an influenza vaccine this season to be vaccinated against influenza. In addition, they re-emphasize the importance of the use of neuraminidase inhibitor antiviral medications, oseltamivir (Tamiflu®) and zanamivir (Relenza®, when indicated for treatment and prevention of influenza, as an adjunct to vaccination.

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