Overall, flu activity in the U.S. is low, according to the Centers for Disease Control and Prevention (CDC). But experts warn that Americans may be in for another severe season driven by a new version of the virus.
One flu strain, influenza A or H3N2, mutated over the summer as the virus spread through the southern hemisphere. The new version, called subclade K, is already causing a surge of flu cases in Japan, Canada and the U.K.
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In October, the Japanese government declared a nationwide epidemic and closed more than 100 schools after health officials recorded an unusually high number of cases — more than 6,000 at the time — so early in the year. U.K. health officials warn that this flu season may be the worst in a decade.
Although flu activity in other countries does not strictly predict what will happen in the U.S., it is often a good indicator. Of the thousands of flu cases already documented in the U.S., about 12% were caused by subclade K.
Since the formula for this year’s flu vaccine was finalized in February, vaccine developers did not account for this new viral mutation. As a result, this year’s vaccine may be less effective. Similarly, subclade K may be so significantly different from earlier versions that existing immunity — developed after a patient becomes sick and recovers from the illness — may not help patients infected with the new H3N2 version. Reduced vaccine effectiveness and diminished existing immunity may together create a perfect storm for a severe flu season marked by a high number of people infected and possibly becoming severely sick.
The CDC has not released any information about the effectiveness of this year’s flu vaccine. British researchers reported that their vaccine, which is different from the one used in the U.S., reduced hospitalizations by about 70-75% in kids and by about 30-40% in adults.
Declining vaccine rates
Even if the flu vaccine in the U.S. does prove effective, vaccination rates have been declining since the COVID-19 pandemic. During the 2023-24 flu season, about 55% of children and 45% of adults received at least one dose of the flu vaccine, down from 64% and 48% during the 2019-20 season.
Vaccination rates are on track to be lower this flu season than last. So far this year, just over 26 million flu shots have been administered at retail pharmacies across the U.S., about 2 million fewer than this time last year, according to analysis by IQVIA, a health and science research company.
Declining vaccination rates may be one of several reasons that the number of flu-related cases, hospitalizations and deaths has increased steadily for the past four years. Last year, the U.S. recorded its highest flu-related hospitalization rate in 15 years.
The CDC recommends that most people aged 6 months and older receive an annual flu vaccine. However, vaccine manufacturers projected they would supply only about 154 million doses to the U.S. for the 2025-2026 flu season, which is not enough to cover the nation’s entire adult population.
Other respiratory diseases
Cases of respiratory syncytial virus, or RSV, another respiratory disease that is more common in the winter, are also increasing, particularly in the southern and southeastern states, the CDC reported. The CDC has not reported exact numbers because its disease surveillance and reporting activities were interrupted by the federal government shutdown that ended last week.
Fortunately, the number of COVID-19 cases has been “very low” and is further decreasing.
