Though the "tripledemic" - COVID-19, RSV and influenza - remains a problem in many places, experts say the flu is beginning to hit the country hard.
Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center in Nashville, Tennessee, said he has seen the first signs that RSV infections may be stabilizing after an early jump, while COVID-19 is "smoldering."
But "influenza is fierce," he said.
About 20,000 people were hospitalized with the flu last week, according to the Centers for Disease Control and Prevention, nearly double the number from the week before.
"Hospitalizations for flu continue to be the highest we have seen at this time of year in a decade," CDC director Dr. Rochelle Walensky said in a briefing Tuesday.
Here's what to know:
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COVID-19: Experts say coronavirus cases no longer give an accurate picture of the pandemic as Americans test at home and results go unreported. But health officials are seeing an increase in cases and hospitalizations since Thanksgiving, Walensky said. In the last week of November, the CDC reported:
RSV: Transmission remains high but appears to have "peaked" in the South and Southeast, and is "leveling off" in the mid-Atlantic, New England and Midwest, Walensky said
Influenza: Thirty-one jurisdictions in the United States, which includes states, major cities and territories, report a "very high" level of flu activity, and 16 report "high" levels, according to the CDC. So far this season, the agency has reported:
8.7 million cases
Flu symptoms: What does flu feel like?
Flu symptoms: Fever, chills, muscle aches, cough, congestion, runny nose, headache and fatigue.
Flu strains: There are four types of influenza viruses - A, B, C and D - but the strains that typically cause seasonal flu illness are influenza A and B. The CDC says influenza C primarily causes mild disease and influenza D is normally seen in cattle, not people.
How long does the flu last?
Signs and symptoms of the flu typically resolve after three to seven days, according to the CDC, but general weakness and fatigue can last up to two weeks.
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This year's flu vaccine is a good match for the three strains Schaffner said he's seeing most often in Nashville. The prevalence of all three strains at the same time is what experts believe "accounts, at least in part, (for) why it is we're having so much influenza," he said.
But not enough people are getting vaccinated against the flu. About 40% of children were vaccinated as of Nov. 19 and about 36% of adults at the end of October, the latest data available from the CDC shows.
"Here, as everywhere, the acceptance of the influenza vaccine has been disappointing," he said. "People really have vaccine fatigue."
Experts say it's never too late to get a flu shot. Here's where to get one:
Doctor's office: Experts advise people to call ahead before making an appointment to make sure the vaccine they want or need is available.
Pharmacy: Major pharmacy retailers, such as Walgreens and CVS, offer flu vaccines and accept walk-ins or appointments online.
What is the difference between a cold and flu?
Cold symptoms are generally less severe than the flu, but many symptoms overlap: fever, runny nose, sore throat, coughing and general fatigue.
The only definitive way to know the difference between a cold or the flu is through testing, experts say. Patients can be tested simultaneously for COVID-19 and influenza with a single swab.
How to treat the flu
Experts say most flu infections can be treated at home with rest and plenty of fluids. Doctors can, however, prescribe an antiviral to patients who are at risk of severe disease.
Four antivirals are authorized for flu in the United States, but the most widely used is known by its brand name Tamiflu.
But antivirals work best when taken early in the disease, so it's important to discuss options with a doctor before getting sick, said Dr. Pedro Piedra, professor of molecular virology and microbiology at Baylor College of Medicine.
"We're into the flu season now without a question, and there's a lot of it all around us," he said. "Planning ahead is important."
Contributing: Karen Weintraub, USA TODAY. Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.
Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.
This article originally appeared on USA TODAY: Flu worsens amid COVID, RSV: What to know about symptoms, vaccine