The Centers for Disease Control and Prevention reported that this season’s flu vaccine delivered lower-than-expected effectiveness, citing the emergence of a strain that was not well matched to the shot.

According to federal health data, vaccine effectiveness among adults age 18 and older ranged from 22% to 34%, while effectiveness among children was slightly higher at 38% to 41%. Health officials typically aim for effectiveness between 40% and 60%, placing this season among the lowest in recent years.

The CDC said the mismatch contributed to an intense early wave of influenza, with the strain spreading more easily, though not necessarily causing more severe symptoms.

Between October 1, 2025, and February 28, 2026, the CDC estimates there were at least 26 million flu illnesses, 340,000 hospitalizations, and 21,000 deaths in the United States. At least 101 pediatric deaths have been reported, with most occurring among children who were not fully vaccinated, based on available data.

Despite the reduced effectiveness, the CDC continues to recommend vaccination, stating that flu shots can still reduce the risk of severe illness, hospital visits, and death. Officials noted that even in seasons with lower effectiveness, vaccines have prevented thousands of hospitalizations and fatalities.

Federal data also shows that fewer than half of U.S. adults and children had received the flu vaccine as of late February, raising concerns about broader population-level protection during the current season.