Most areas of North America are in the throes of a flu season marked by an unusual early dominance of influenza B, a strain not typically seen in large numbers until the later months of the flu season.

“We had a paucity of influenza B last year, so we may have anticipated community immunity would be low,” said Danuta Skowronski, MD, MHSc, an epidemiologist with the British Columbia Centre for Disease Control in Vancouver. In fact, a few months ago, Skowronski said she gave a Grand Rounds talk about the upcoming flu season and the risk of influenza B.

“My concern was we had not seen influenza B/Victoria make a strong showing since the 2015-2016 season,” Skowronski told CIDRAP News. “So immunity to that virus would be low.”

And that low immunity may be behind the season’s more severe illness in children.

Influenza B comes in two primary strains—Victoria and Yamagata. So far this year 98% of flu B viruses subtyped have been from the Victoria lineage. And B strains have outpaced A strains 58% to 42%. Quadrivalent (four-strain) vaccines contain both B strains, whereas trivalent (three-strain) versions this year have only the Victoria strain.

Severity in children

Read more at CIDRAP…