Influenza activity in the United States was typically low over the summer months, with influenza A(H3N2) viruses predominating, according to the Centers for Disease Control and Prevention.
From May 19 to Sept. 28, 2019, weekly flu activity – measured by the percentage of outpatient visits to health care professionals for influenza-like illness (ILI) – was below the national baseline of 2.2%, ranging from 0.7% to 1.4%. Since mid-August, however, when the rate was last 0.7%, it has been climbing slowly but steadily and was up to 1.3% for the week ending Sept. 28, CDC data show.
The various public health laboratories of the U.S. Influenza Surveillance System tested over 7,600 respiratory samples from May 19 to Sept. 28, and 22.7% were positive for influenza viruses, Scott Epperson, DVM, and associates at the CDC’s influenza division said Oct. 10 in the MMWR.
Of the 1,737 samples found to be positive, 69.8% were influenza A and 30.2% were influenza B. The subtype split among specimens positive for Influenza A was 71.9% A(H3N2) and 28.1% A(H1N1)pdm09, while the samples positive for influenza B went 93.9% B/Victoria and 6.1% B/Yamagata, they reported.
Over the same time period in the Southern Hemisphere, “seasonal influenza viruses circulated widely, with influenza A(H3) predominating in many regions; however, influenza A(H1N1)pdm09 and influenza B viruses were predominant in some countries,” the CDC investigators noted.
They also reported the World Health Organization recommendations for the Southern Hemisphere’s 2020 flu vaccines. Components of the egg-based trivalent vaccine are an A/Brisbane/02/2018(H1N1)pdm09-like virus, an A/South Australia/34/2019(H3N2)-like virus, and a B/Washington/02/2019-like virus(B/Victoria lineage). The recommended quadrivalent vaccine adds a B/Phuket/3073/2013-like virus(B/Yamagata lineage), they wrote.
“It is too early in the season to know which viruses will circulate in the United States later this fall and winter or how severe the season might be; however, regardless of what is circulating, the best protection against influenza is an influenza vaccination,” Dr. Epperson and associates wrote.
SOURCE: Epperson S et al. MMWR. 2019 Oct 11;68(40):880-4.