Flu season has started weeks early in the United States. The CDC estimates that cases are likely rising in 44 states.
National activity is still low overall, but some cities and states are reporting numbers they would normally see in November or December.
Influenza A H1N1 has driven most early cases. Flu test positivity hit 5.7 percent in the week ending October 17, up from under 1 percent a year earlier.
Researchers are watching a G155E mutation in an H1N1 strain, which was first flagged in Seattle in September.
Preliminary CDC data indicate that antibodies from the current vaccine still recognise the circulating virus.
In New York, flu cases rose 19 percent in one week in mid-October, and flu hospitalisations went from 199 to 233.
Los Angeles County has reported its first two flu deaths of the season. Officials there urge everyone six months and older to get vaccinated.
The last two flu seasons saw an unusually high number of child deaths, and most were in unvaccinated children.
The CDC estimated 45,000 flu deaths and 710,000 flu hospitalisations in the 2024-2025 season.
Health officials say a flu shot can reduce the risk of hospitalisation and severe illness, even though it does not stop every infection.