For the first time in 27 years, influenza B viruses have predominated in the United States, accounting for more than 56% of samples tested in public health laboratories as of Jan. 18, according to CDC data.

Influenza B viruses are more common in children than adults, which potentially explains some incongruities in U.S surveillance data, according to Lynnette Brammer, MPH, who leads the CDC’s domestic influenza surveillance team.