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(NewsNation) — Swirling cases of a trio of respiratory illnesses have some hospitals swamped, and at least one infectious disease expert says COVID-19 may not be the biggest culprit this year.

“This was always the worst time for hospitals,” the University of California, San Francisco’s Dr. Monica Gandhi said on “Rush Hour” Friday. Doctors are used to seeing the flu spike this time of year. But this time, COVID-19 and RSV have joined the fray.

“I’m more worried about influenza this year,” Gandhi said. “Luckily, we have a way to fix it or at least bring severe disease down, but our vaccination rates are lower than you would think.”

Data from the Centers for Disease Control and Prevention shows 20% fewer flu vaccine doses have been distributed this season compared to the same week in 2020.

Gandhi believes COVID-19’s prevalence the past three years forced other respiratory illnesses like the flu and RSV to the sidelines. She credits immunity to COVID-19 from vaccines and infections for curbing its impact, but the reverse may be true for the other diseases.

“We may have had less immunity in our own bodies for those two viruses,” Gandhi said.

There is no vaccine for RSV, but one is in development. Gandhi is optimistic it could be available for pregnant women by the fall of 2023.

Until then, if you or your children are having difficulty breathing or have a high fever, you should head to a hospital.

Dr. Vincent Hsu, who oversees infection control for AdventHealth, said the system’s pediatric hospital in Orlando is nearly full with kids sickened by these three viruses. Dr. Greg Martin, past president of the Society of Critical Care Medicine, sees a similar trend elsewhere.

Pediatric hospitals’ emergency departments and urgent care clinics are busier than ever, said Martin, who practices mostly at Grady Memorial Hospital in Atlanta. “This is a record compared to any month, any week, any day in the past,” he said.

The Associated Press contributed to this report.