U.S. clinical laboratory flu test positivity climbed to 5.0% in the week ended Oct. 10, the Centers for Disease Control and Prevention (CDC) said, while national flu activity remained low.
Clinical labs reported 2,270 positive specimens among 45,331 tested, compared with 2,070 among 53,137 a week earlier. The positive share rose from 3.9%, even as the total number of tests fell. These counts describe specimens and test results, not the number of people infected.
The CDC reported increased activity in several regions but said the national rise remained within normal seasonal variation. Public health laboratories subtyped 251 influenza A viruses during the week: 242 were A(H1N1)pdm09 and nine were A(H3N2).
In a separate analysis, the CDC tested selected viruses collected since May 24 against ferret antibodies raised against vaccine reference viruses for cell, recombinant and nucleic acid vaccines. The antibodies recognized 70 of 74 A(H1N1)pdm09 samples and 20 of 20 A(H3N2) samples. The antigenic results cover selected viruses collected since May 24, while the 251 subtyped viruses were reported by public health laboratories during the latest week.
The comparisons show how the selected viruses reacted to the antibodies. They do not establish how well vaccination prevents illness or hospitalization in people. The CDC estimates vaccine effectiveness separately through observational studies that use laboratory-confirmed flu as an outcome.
The Food and Drug Administration (FDA) recommended the 2026-27 vaccine composition March 13. All three components changed from the prior season, the CDC said. The H3N2 update followed widespread circulation of subclade K in 2025-26.
The CDC recommends annual flu vaccination for people aged 6 months and older without contraindications, with September or October the preferred timing for most. CDC survey estimates show 41.9% of U.S. adults were vaccinated in 2024-25, down from 44.9% in the previous season and the lowest rate in seven seasons.
The agency’s continuing surveillance will track flu test positivity and other indicators of activity as the season progresses.
