H3N2 Influenza Activity Increasing in the United States

HealthDay News — Influenza activity was low during October 2017 but started increasing in November, with influenza A, predominantly A(H3N2), most commonly identified, according to research published in an issue of the US Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

Vivien G. Dugan, PhD, from the CDC in Atlanta, and colleagues summarized US influenza activity during October1 to November 25, 2017.

The researchers note that influenza activity was low during October 2017 in the United States, but since the start of November, activity has been increasing. The most commonly identified viruses have been influenza A viruses, with influenza A(H3N2) predominating. Several indicators of influenza activity were reported to be higher than normally seen at this time of year. Most influenza viruses characterized during this period were genetically or antigenically similar to the Northern Hemisphere cell-grown vaccine reference viruses for 2017 to 2018. 

Related Articles

Currently circulating viruses seem not to have undergone significant antigenic drift; circulating A(H3N2) viruses are antigenically less similar to egg-grown A(H3N2) viruses used for most US influenza vaccine production. Hospitalizations and deaths were more common and vaccine effectiveness was lower in recent past seasons in which A(H3N2) viruses predominated; it is unclear which influenza viruses will predominate in the 2017 to 2018 season.

“Vaccination should continue to be offered as long as influenza viruses are circulating and unexpired vaccine is available,” the authors wrote.

Disclosures: One author disclosed holding 2 patents relating to influenza vaccines.


Dugan VG, Blanton L, Elal AIA, et al. Update: influenza activity United States, October 1 — November 25, 2017. MMWR. 2017;66(48):1318-1326.