The term super flu gets tossed around periodically, generally referring to a more severe strain in a particular year that spreads more rapidly and/or produces more serious illness. This time, it’s because there is a worrisome novel variant — called subclade K — of the decades-old H3N2 subtype of the influenza A virus. The variant has already fueled nasty outbreaks in the U.K., Japan, Australia, and Canada, and now it’s here and spreading across the U.S. In the most recent CDC tracking data, nearly 90 percent of the new H3N2 samples it collected were subclade K.
The new variant’s mutations help it evade our existing immunity to infection. Scientists in the U.K. have estimated that it has a slightly higher reproduction number than normal seasonal flu strains, which means it spreads faster.
Flu experts at Johns Hopkins Bloomberg School of Public Health said earlier this month that it wasn’t yet clear whether or not subclade K also causes more serious illness:
“There have been reports that the disease is particularly strong in children, and certainly there are reports of the elderly having higher hospitalization rates,” [virologist Andrew] Pekosz explains. “Those are two things we expect with any influenza season surge.” It’s not clear yet, he says, whether the high rates of illness and hospitalization are proportional to the higher number of total cases, or whether subclade K is more likely to cause severe symptoms.