The Centers for Disease Control and Prevention (CDC) issued a Health Alert Network (HAN) Health Advisory about increased influenza A(H3N2) activity that could mark the beginning of the 2021-2022 influenza season.

3D print of influenza virus. The virus surface (yellow) is covered with proteins called hemagglutinin (blue) and neuraminidase (red). NIH

Recent increases in influenza activity in many places in the United States could mark the beginning of the 2021-2022 influenza season in the United States. While influenza activity is still low overall nationally, an increase of influenza A(H3N2) viruses has been detected in recent weeks, with most of these infections occurring in young adults.

CDC also is aware of influenza outbreaks in colleges and universities in several states. Influenza vaccination coverage is still low and there is still time this season to benefit from getting an annual influenza vaccine.

The influenza A(H3N2) component of this season’s vaccines was recently updated in response to the evolution of a new group of viruses called 2a (i.e., 3C.2a1b.2a) that did not circulate widely last year and were not included in last season’s H3N2 vaccine component.

Most H3N2 viruses that have been analyzed in the United States so far are genetically closely related to the current vaccine’s H3N2 component.

CDC recommends that healthcare providers continue to recommend and offer influenza vaccination to persons aged six months and older because influenza activity is ongoing. Vaccination protects against four different viruses and is likely to reduce hospitalization and death associated with currently circulating influenza viruses and other influenza viruses that might circulate later in the season.