Flu Season Arrives: Rising Cases & Epidemic Warning

Flu activity is currently increasing in the United States, marking a potential second peak in the 2024-25 influenza season. While cases initially peaked in December 2024, recent data indicates a rebound in infections, particularly among school-aged children.

A Second Peak in Flu Activity

According to epidemiologists, the current increase is driven largely by influenza B viruses, which are distinct from the subclade K strain that dominated earlier in the season. Dr. Caitlin Rivers, an epidemiologist, notes that this pattern mirrors what the US experienced last year, with a second peak emerging in late February. “We are seeing a second peak, a second rebound from flu,” she stated. This resurgence is currently most pronounced in children aged 5 through 17, but experts anticipate it will spread to adults as well.

The winter storm that impacted roughly half of the US last week may also be contributing to the current trends, potentially influencing transmission rates.

Severity of the 2024-25 Influenza Season

The 2024-25 influenza season was classified as a high severity season, representing the most significant influenza impact since the 2017-18 season. Preliminary national estimates suggest at least 43 million symptomatic illnesses, 19 million medical visits, 560,000 hospitalizations, and 38,000 deaths occurred during the season, which ran from September 29, 2024, to August 30, 2025.

Influenza A viruses were the predominant strain throughout the season, with both A(H1N1)pdm09 and A(H3N2) viruses circulating at roughly equal levels. While influenza B activity remained relatively low for much of the season, it has shown a slight increase more recently.

Recent Increases and Geographic Distribution

As of December 31, 2025, the Centers for Disease Control and Prevention (CDC) reported that flu cases were on the rise across the country. A new variant, subclade K, is driving this increase, and cases are expected to continue growing, particularly with the onset of holiday travel. The CDC reported high or very high levels of illness in more than half of the states.

By December 20, 2025, the CDC estimated at least 7.5 million illnesses, 81,000 hospitalizations, and 3,100 deaths from flu. This figure includes at least eight pediatric deaths. Some states are experiencing particularly high levels of activity. for example, New York reported 71,000 cases during the week ending December 20, 2025, the highest weekly total recorded since 2004.

Vaccine Effectiveness and Recommendations

While it is still not too late to receive a flu vaccination, it’s important to understand the current situation regarding vaccine effectiveness. Although this year’s vaccine isn’t a perfect match for the circulating subclade K strain, preliminary analysis from the United Kingdom suggests it offers at least partial protection, potentially reducing the risk of hospitalization. As of this report, approximately 42% of adults and children have been vaccinated against the flu this season.

Health experts continue to emphasize the importance of vaccination as a key preventative measure, even if it doesn’t provide complete protection against all strains. The vaccine can still help to mitigate the severity of illness and reduce the risk of complications.

Looking Ahead

It remains to be seen whether the current flu season will be as severe as last winter’s. However, the recent rebound in cases and the emergence of the subclade K variant warrant continued vigilance. Public health officials are closely monitoring the situation and providing updates as new data becomes available. Individuals are encouraged to practice good hygiene, such as frequent handwashing, and to stay home if they are experiencing flu-like symptoms to help prevent further spread.