Thirty US states have been placed in the highest tier for flu activity by the Centers for Disease Control and Prevention (CDC), with health officials warning that the severe season is likely to deteriorate further. New data released on Monday for the week ending Christmas Day showed that, by some measures, this season has already surpassed last winter's flu epidemic, one of the harshest in recent history.
Flu infections surged over the holiday period, with 45 states reporting high or very high flu activity, up from 30 the previous week. The dominant strain this season is influenza A H3N2, which historically causes the most hospitalisations and deaths among older people. More than 90% of H3N2 infections analysed were a new subclade K variant that differs from the strain included in this year's flu vaccine.
The CDC estimates that at least 11 million illnesses, 120,000 hospitalisations and 5,000 deaths from flu have already occurred this season. Nine paediatric flu deaths have been reported so far. For children, the percentage of emergency department visits due to flu has already surpassed the highest level seen during the 2024-2025 season.
COVID-19 infections have also been rising, though they remain less common than flu. The CDC recommends that everyone aged six months and older who has not yet been vaccinated this season receive an annual influenza vaccine. Flu seasons often do not peak until January or February, so it is too early to determine the full impact of the vaccine mismatch.
