US influenza hospital admissions have hit the highest rate in a decade as vaccinations sag, US officials say.
They said adults have received five million fewer influenza jabs this year compared with the same time last year.
Health experts are worried a so-called tripledemic of flu, respiratory syncytial virus (RSV) and coronavirus could swamp hospitals this winter.
At least 730 people have died of flu this year, according to the Centers for Disease Control and Prevention (CDC).
While older people are the most vulnerable age group, at least four children are among the dead.
CDC data shows there have been at least 1.6 million flu cases overall and some 13,000 people have been taken to hospital.
This season's severity has not been matched at this point in the year since the H1N1 swine flu pandemic hit the US in 2009.
"There's no doubt we will face some challenges this winter," Dawn O'Connell, the US Department of Health and Human Services' (HHS) assistant secretary for preparedness and response, said at a media briefing on Friday.
Flu season in the US typically peaks in December and January but has surged early and rapidly this year.
Health officials said the coronavirus pandemic had sharply curbed the spread of flu and other respiratory viruses,
So many young children are now encountering them for the first time, but may have little to no immunity, said the officials.
The early arrival of flu season has been compounded by a rise in Covid hospital admissions, fuelled by the spread of different Omicron subvariants.
It has also coincided with a country-wide surge in RSV, a cold-like infection that is estimated to claim more than 14,000 lives annually in the US, mostly among older Americans.