Mild Flu Season Credited to Well-Matched Influenza Vaccine, Less Severe Predominant Strain

The 2015-2016 influenza season was remarkably mild compared to the previous 3 flu seasons.

The most recent flu season generated fewer outpatient visits for influenza-like illness, hospitalizations, and percentage of deaths attributed to pneumonia and influenza. Influenza activity also peaked later than usual (in March), and case incidence remained lower than expected.

Influenza A(H1N1)pdm09 was the predominant strain overall. Influenza A(H3N2) viruses were more common early in the season, and monitoring identified a surge of influenza B virus activity from mid-April to mid-May. In addition, the 2015-2016 vaccine was well-matched to the prevailing strains in circulation.

The FDA’s Vaccines and Related Biological Products Advisory Committee has recommended a change to the influenza A(H3N2) and influenza B components for the coming flu season. The 2016-2017 trivalent vaccines will contain A/California/7/2009 (H1N1)pdm09-like virus, A/Hong Kong/4801/2014 (H3N2)-like virus, and B/Brisbane/60/2008-like virus (B/Victoria lineage).

The quadrivalent vaccines will include B/Phuket/3073/2013-like virus (B/Yamagata lineage), as well. The 2016-2017 vaccine will match the current Southern Hemisphere seasonal influenza vaccine.

H3N2-predominant seasons tend to be more severe than influenza A(H1N1)pdm09—predominant seasons. This season created even fewer cases and mortality than the 2013-2014 H1N1pdm09 outbreak.

Influenza A(H1N1)pdm09, such as the strain seen during the swine flu epidemic, is associated with higher mortality and morbidity in young adults. During the most recent flu season, the distribution of mortality in young adults and older cohorts (>50 years) was comparable.

Public health officials tested all influenza A(H3N2) and influenza B virus samples for antiviral resistance, and all were susceptible to oseltamivir, zanamivir, and peramivir. The amantadines are still largely ineffective against an extensive array of the circulating influenza A and B strains.

Researchers identified 2 novel strains in 2 patients who had direct contact with swine. A third patient with a novel strain refused an interview. All H1N1 strains except a single one were antigenically similar to the A/California/7/2009 vaccine strain.

The combination of a well-matched vaccine and a less severe predominant strain produced a mild 2015-2016 influenza season. The FDA predicts that the 2016-2017 influenza vaccine will protect against the prevailing strains of next season based on the Southern Hemisphere’s circulating strains.