NEW YORK – The onset of the flu season in the United States is progressing at a gradual pace, yet the forecast remains uncertain when compared to the harshness of last winter. However, a new strain of the virus has emerged, causing concern among health experts, according to data released by the U.S. Centers for Disease Control and Prevention on Friday.

Preliminary assessments indicate that the current flu vaccines might retain some effectiveness against this new variant, which is largely responsible for the latest wave of infections, as evidenced by CDC findings.

The greater concern for scientists and healthcare professionals, however, lies in the low vaccination uptake. This shortfall significantly contributed to the severe flu-related hospitalizations and fatalities experienced last season—one of the deadliest in recent history.

“I anticipate we’ll face a very severe season,” stated Asefeh Faraz Covelli from the George Washington University School of Nursing.

Last year, the flu hospitalization rate soared to levels not seen since the H1N1 pandemic 15 years ago. The flu was a primary or contributing factor in over 18,000 deaths, with one particularly devastating week recording more than 1,800 fatalities—the highest in a decade. The number of deaths among children was also unusually high.

CDC data posted Friday showed low flu activity so far, with only one state — Louisiana — reporting moderate activity. Most of the reported infections have been in children, said the CDC’s Alicia Budd, who tracks flu infections for the Atlanta-based agency.

Most also have been a new version of the type A H3N2 virus that historically has caused the most hospitalizations and deaths in older people. That type is responsible for most flu infections so far this year, and more than half have been a new subclade K variant that is different from the strain this year’s flu shots were built to fight.

A preliminary analysis from the United Kingdom suggests the shots do provide at least partial protection, although it will take some time for scientists to know exactly how effective they are. Experts say any protection that softens the blow of a flu infection is important to get.

Flu seasons tend to get bad between December and February, and illnesses likely will accelerate as people travel and gather for Thanksgiving, Covelli said.

“I think it’s going to start picking up here,” she said. “This is the ideal time to get vaccinated.”

Researchers this year have been facing an unusual struggle to get a handle on how respiratory infection and vaccination rates have been trending.

They usually rely on the CDC for data, but a recent government shutdown halted data collection and reporting just as respiratory infections started ramping up.

Meanwhile, government efforts to promote disease-preventing shots have been more limited since U.S. Health Secretary Robert F. Kennedy Jr. was put in charge of the CDC and other federal health agencies. Kennedy, a leading anti-vaccine activist, has fostered uncertainty about the safety of vaccines, including flu shots that contained a preservative called thimerosal.

Indeed, vaccination rates against COVID-19 clearly continue to plummet, with about 6% of children and 14% of adults up to date on their shots, according to other CDC data posted Friday. Each figure is about 3 percentage points less than it was at this point last fall.

For the flu, vaccination trends are a little muddier. Some sources have suggested flu vaccinations are down. Over two million fewer flu shots were given at U.S. pharmacies through the end of October compared to last year, according to data from IQVIA, a health information and research company.

But the latest CDC data indicates that for children, the vaccination rate this year is about the same as it was at this point last fall, at 34%. And the vaccination rate for adults is up a few percentage points to about 37%, according to the CDC data, which relies on survey information.

It is early in the season and too early to know if the increase will be sustained or what is causing it, CDC officials said.

As of the beginning of November, the U.S. flu hospitalization rate is about the same as it was at the same point in 2024. Hospitalization rates for COVID-19 and another respiratory virus, RSV, are lower so far this season, according to another set of CDC data.

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