The U.S. winter virus season is in full force, and by one measure is the most intense in 15 years.

One indicator of flu activity is the percentage of doctor's office visits driven by flu-like symptoms. Last week, that number was clearly higher than the peak of any winter flu season since 2009-2010, according to data posted Friday by the Centers for Disease Control and Prevention (CDC).

Of course, other viral infections can be mistaken for flu. But COVID-19 appears to be on the decline, according to hospital data and to CDC modelling projections. Available data also suggests another respiratory illness, RSV, has been fading nationally in the U.S. and Canada.

The latest Canadian respiratory virus surveillance report, posted Friday, suggest flu is on the rise. Test positivity for influenza stood at 21 per cent and is increasing, according to the report. 

The flu has forced schools to shut down in some American states. The Godley Independent School District, a 3,200-student system near Fort Worth, Texas, last week closed for three days after 650 students and 60 staff were out Tuesday.

Jeff Meador, a district spokesperson, said the vast majority of illnesses there have been flu, plus some strep throat. He called it the worst flu season he could remember.

So far this season, the CDC estimates, there have been at least 24 million flu illnesses, 310,000 hospitalizations and 13,000 deaths — including at least 57 children. Traditionally, flu season peaks around February in the U.S.

Overall, 43 states reported high or very high flu activity last week. Flu was most intense in the south, southwest and western states.

Respiratory illness around 'with a vengeance': ER doctor 

In Rochester, N.Y., the flu season has been intense but not necessarily worse than at the peak of other years, according to Dr. Elizabeth Murray, a pediatric emergency medicine doctor at the University of Rochester Medical Center.

She said there's a lot of flu, but there's also still a lot of RSV and a surprising number of babies with COVID-19.

"All of the respiratory illnesses are around, with a vengeance," Murray said.

The CDC declined to let an Associated Press reporter speak to an agency flu expert about recent trends. The Trump administration ordered a temporary "pause" on health agency communications and has continued to refuse interview requests that were routinely granted in the past.

U.S. health officials recommend that everyone six months and older get an annual flu vaccination.

Testing results from patients indicate that two strains of seasonal flu are causing most illnesses — a Type A H1N1 and a Type A H3N2. Health officials are closely watching a third strain — a bird flu known as Type A H5N1 — that has sickened tens of millions of animals but is known to have infected only 67 people in the U.S.

To avoid seasonal viruses, doctors say you should avoid touching your eyes, nose and mouth because germs can spread that way. You should also wash your hands with soap and water, clean frequently touched surfaces and avoid close contact with people who are sick.