New estimates of flu vaccine effectiveness during the United States’ 2018-19 season based on data from surveillance and information on ambulatory patients at five sites found that the predominance of H3N2 clade 3C.3a viruses late in the season decreased vaccine effectiveness  (VE).

A team led by US Centers for Disease Control and Prevention researchers published its findings today in the Journal of Infectious Diseases.

The 2018-19 season was unusual, because the 2009 H1N1 virus predominated during the first part of the season, with low levels of H3N2 that was similar to the vaccine component. During the latter part of the season, however, a drifted H3N2 variant emerged from the 3C.3a clade, which decreased protection from the vaccine. Also, very little influenza B circulated during the season.

94% of H3N2 viruses were variants

Credit CDC

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