How Well the 2025–26 Flu Vaccine Worked: CDC's Interim Estimates

CDC estimates that from October 1, 2025, to February 28, 2026, influenza caused at least 26 million illnesses, 340,000 hospitalizations and 21,000 deaths in the United States. Annual flu vaccination is recommended for everyone aged 6 months and older, yet as of February 21, 2026, only about 48% of children and teens and 47% of adults had been vaccinated for the season. In the previous 2024–25 season, vaccination prevented an estimated 5 million medical visits, 180,000 hospitalizations and 12,000 deaths.

This season posed a particular challenge. Most influenza A viruses were A(H3N2), and most of those belonged to subclade K — an antigenically "drifted" virus first identified by CDC in June 2025, after the season's vaccine strains had been chosen, and different from the vaccine's H3N2 component. Most influenza B viruses (clade C.3.1) also differed from the vaccine strain; circulating H1N1 viruses were similar to it.

How the estimates were made

CDC combined data from three networks for 142,494 patients with acute respiratory illness from September 2025 to February 2026:

• the New Vaccine Surveillance Network (NVSN), children and teens, outpatient and hospitalized;
• the U.S. Flu Vaccine Effectiveness Network, outpatients only;
• VISION, which reviews medical records of outpatients and inpatients.

Using a test-negative design, researchers compared vaccination among patients who tested positive for flu with those who tested negative. Vaccine effectiveness (VE) is the reduction in risk among vaccinated people; a person counted as vaccinated if they had received at least one dose of a 2025–26 flu vaccine at least 14 days before getting sick.

What they found

By virus type:

• Influenza A: 37%–42% in children and teens across settings, and 30%–34% in adults.
• A(H3N2): 35% against outpatient visits and 38% against hospitalization in children and teens.
• Influenza B: 45%–71% against outpatient visits in children and teens, and 63% in adults — notably high.

Some estimates weren't statistically significant or couldn't be reported because data were sparse.

Putting it in context

Effectiveness was lower than in recent seasons, but the vaccine still protected against flu illness and hospitalization — even against the drifted H3N2 virus. Protection was highest in children and lower in older adults, a pattern seen in past seasons, especially against H3N2. The U.S. estimates were similar to interim estimates from China (24%), Canada (38% against influenza A) and Europe (37%–40% against outpatient visits; 21%–42% against hospitalization), and U.S. protection against influenza B was higher than Europe's.

Lower-effectiveness seasons still save lives. In 2018–19, when a drifted H3N2 virus circulated, overall VE was 29%. In 2022–23, VE was just 30%, yet vaccines prevented an estimated 71,000 hospitalizations and 4,300 deaths.

Limitations

• These are interim estimates and may change by season's end.
• Factors such as underlying conditions and prior-season vaccination weren't modeled.
• Vaccination status may be misclassified — for example, doses received outside the medical system, or children aged 6 months–8 years who need two doses the first time — which would bias estimates downward.
• Small numbers ruled out some estimates, including VE against H1N1, and differences between networks may reflect their different definitions, settings and populations.

What CDC recommends

• Get vaccinated if you haven't yet: vaccines remain available for everyone 6 months and older, and while flu keeps circulating, vaccination can still prevent illness and death.
• Antiviral medicines matter especially in lower-effectiveness seasons. They are recommended as soon as possible for anyone with suspected or confirmed flu who is hospitalized, has severe, complicated or worsening illness, or is at higher risk of complications.

Sources

• Maloney P, Reeves EL, Wielgosz K, et al. "Interim Estimates of 2025–26 Seasonal Influenza Vaccine Effectiveness — United States, September 2025–February 2026," MMWR Vol. 75, No. 9, CDC; a work of the United States government in the public domain. The report gives subclade K's share of H3N2 viruses as 93% in one place and 83% in another (from different samples), so neither is stated here.