The U.S. flu season is underway, with cases surging across much of the country, health officials said Friday.

The Centers for Disease Control and Prevention noted sharp increases in several measures, including lab tests and emergency room visits.

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“It's been increasing at a pretty steady pace now for the past several weeks. So yeah, we are certainly in flu season now,” said the CDC's Alicia Budd.

Thirteen states reported high or very high levels of flu-like illness last week, about double from the week before. One is Tennessee, where a sickness spike is hitting the Nashville area, said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University.

“Flu has been increasing, but just this last week has exploded,” Schaffner said. He noted that in a local clinic that serves as an indicator of illness trends, as many as a quarter of the patients have flu symptoms.

Louisiana is another early hot spot.

“Just this week is really that turning point where people are out because of the flu," said Dr. Catherine O’Neal, an infectious diseases doctor at the largest private hospital in the state, Our Lady of the Lake Regional Medical Center in Baton Rouge. "You hear parents saying, ‘I can’t come to work because of the flu’ and ‘Where can I get a flu test?’”

Of course, there are a number of bugs that cause fever, cough, sore throat and other flu-like symptoms. One is COVID-19. Another is RSV, or respiratory syncytial virus, which is a common cause of cold-like symptoms but can be dangerous for infants and the elderly.

The most recent CDC data show COVID-19 hospitalizations have been declining since summer. COVID-19 activity is low nationally, but high in the Midwest, according to CDC wastewater data.

RSV hospitalizations started rising before flu did and now show signs of possibly leveling off, but they remain a little more common than admissions for flu. Overall, RSV activity is low nationally, but high in the South, the wastewater data show.

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The CDC called the start of flu season based on several indicators, include lab results for patients in hospitals and doctor's offices, and the percentage of emergency department visits that had a discharge diagnosis of flu.

No flu strain seems to be dominant, and it's too early in the season to know how good a match the flu vaccine will be, Budd said.

Last winter's flu season was considered “moderate” overall, but it was long — 21 weeks — and the CDC estimated there were 28,000 flu-related deaths. It was unusually dangerous for children, with 205 pediatric deaths reported. That was the highest number ever reported for a conventional flu season.

The long season was likely a factor, Budd said. Another factor was a lack of flu vaccinations. Among the children who died who were old enough for flu vaccinations — and for whom their vaccination status was known — 80% were not fully vaccinated, according to the CDC.

Vaccination rates for children are even lower this year. As of Dec. 7, about 41% of adults had received a flu vaccination, similar to the rate at the same point last year. The percentage is the same for kids, but for them that’s a drop from a year ago, when 44% were vaccinated against the flu, according to CDC data.

Vaccination rates are lower still against COVID-19, with about 21% of adults and 11% of children up to date.

Flu experts suggest everyone get vaccinated, especially as people prepare to attend holiday gatherings where respiratory viruses can spread widely.

“All those gatherings that are so heartwarming and fun and joyous are also an opportunity for this virus to spread person to person,” Schaffner said. “It's not too late to get vaccinated.”