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ATLANTA — Seasonal influenza activity remained elevated and rose the final week of December, U.S. Centers for Disease Control and Prevention data showed on Monday, as the agency for the first time classified the 2025-26 season as "moderately severe."
The season's toll, so far, is estimated to have caused at least 11 million illnesses, 120,000 hospitalizations and 5,000 deaths.
In the 2024–25 season, CDC estimated at least 5.3 million illnesses, 63,000 hospitalizations and 2,700 deaths in the week ended Dec. 28, 2024.
Flu cases are always "under-reported" because not everyone tests for the flu, said Eric Ascher, a family medicine physician at Northwell's Lenox Hill Hospital, adding that "we're not going to see the peak until at least another week or two."
Public health experts told Reuters last week that holiday travel, low vaccination rates and misinformation contributed to the surge in cases.
Fewer people getting vaccinated and a partial vaccine mismatch are key reasons for the surge this year, Ascher said.
"It's premature to try to make that conclusion yet," said Aaron Milstone, professor of pediatrics at the Johns Hopkins University School of Medicine.
The CDC reported one flu-related pediatric death last week, bringing the 2025-26 season total to nine, and said flu activity will likely continue for several weeks. Influenza A(H3N2) remains the dominant strain, accounting for more than 91% of subtyped influenza A cases, according to the CDC. Among 389 influenza A(H3N2) viruses collected since Sept. 28, 2025, and genetically characterized at CDC, 90.5% belonged to subclade K, a variant of A(H3N2).
Hospitals admitted about 33,301 influenza patients in the week ended Dec. 27, up sharply from earlier weeks, CDC data showed.
Outpatient visits for respiratory illness rose to 8.2% of health care visits, above seasonal baselines.
