While flu activity remains low nationally, cases have risen in recent weeks, especially among young adults, the CDC said in a Nov. 24 health advisory.
The majority of cases have been the H3N2 strain, which is associated with more severe flu seasons, health officials say. The CDC said the recent increase in flu activity "could mark the beginning of the 2021-2022 influenza season."
Of 628 H3N2 cases with known age reported to the CDC so far this season, nearly 90 percent were among people aged 5 to 24. Earlier this month, the CDC was investigating a flu outbreak at the University of Michigan in Ann Arbor, where at least 528 students had contracted the flu since Oct. 6. The majority of those cases were among unvaccinated students.
Flu outbreaks have hit other college campuses as well, including the University of Florida in Gainesville and the University of Rhode Island in Kingston.
Since flu activity was so low last season amid widespread COVID-19 mitigation measures such as mask-wearing and social distancing, "we are anticipating a lower level of community protection that we rely on year after year to reduce the risk of a severe influenza season," the CDC advisory said, adding that national flu vaccination coverage is still low and that there is still time to benefit from a flu shot.
As of Nov. 19, 166.9 million doses of flu vaccine have been distributed in the U.S.