The same pattern could play out again this year, experts warn. 

Just when and how high levels of each virus will spike remains to be seen. But “the addition of COVID-19 into flu and RSV season exacerbates the burden on individual patients, as well as the health care system as a whole,” Tom Cotter—executive director of nonprofit Healthcare Ready, which connects government, nonprofits, and medical supply chains to prepare for disasters—tells Fortune. 

Flu and RSV season usually begins in the fall, around October, and ends in the spring, around May. “Last winter, both flu and RSV hospitalizations started to grow weeks earlier than expected, and then a new, extremely contagious variant of COVID-19 piled on top,” putting hospitals in a predicament, Cotter says.

While it’s too early to say how things will play out this fall and winter, there are factors working in the country’s favor. The Southern Hemisphere didn’t experience an early start to their flu season, which bodes well for the Northern Hemisphere. Doctors have tools in their RSV toolbox that weren’t available last cold and flu season, including a vaccine for older adults and a monoclonal antibody treatment for babies. And updated COVID vaccines are expected this fall. Even if they’re no longer a perfect match to the dominating variants, experts say they should still offer renewed protection against hospitalization or death from the virus.

“I don’t think there is an expectation of a ‘tripledemic,’ rather a caution to the American public to be mindful it could happen,” Doug Laher, chief operating officer of the American Association for Respiratory Care, tells Fortune. “Will it happen for certain? That is impossible to say. But as they say, an ounce of prevention is worth a pound of cure.”

COVID levels, hospitalizations trending upward in U.S.

Not all signs, however, point toward a smooth respiratory illness season. Wastewater levels of COVID are already on the rise in the U.S., according to Biobot Analytics, which collects such data for the federal government. Hospitalizations, too, are on an upward trend, having risen 12% in the past week, according to the CDC. And the World Health Organization is tracking a new variant of interest that is rising as other variants recede: EG.5, a descendant of XBB variants currently dominating the U.S. and the world.

Already, variant trackers have dubbed a descendant EG.5.1, or Eris, after the second-largest known dwarf planet in the solar system. The variant doesn’t have any extremely notable mutations and isn’t even the fastest-growing XBB-related variant out there, according to Ryan Gregory, a biology professor at the University of Guelph in Ontario. He’s been assigning “street names” to high-flying variants since the WHO stopped assigning new Greek letters to them.

“But it is increasing quickly in frequency and is one to watch even if it’s not expected to cause a large wave,” Gregory tweeted this week.

Looks like we're going to be talking about EG.5.1 a fair bit as it is increasing significantly in many places. To aid communication about it, we're going with the nickname "Eris" for EG.5.1*.

Note that nickname ≠ expected to cause a big wave by itself.https://t.co/LwMHPoyqX2

— T. Ryan Gregory (@TRyanGregory) August 1, 2023

Lately, COVID waves haven’t been much of an issue. “But immune-escaping variants like Eris…may return us to a high baseline” of cases, he adds.

‘The new “flatten the curve” moment’

As kids return to school and weather drives more people indoors, Laher emphasizes the importance of good hand hygiene. (“Some people recommend singing ‘Happy Birthday’ while washing their hands as a reminder to keep the lather going for a full 20 seconds,” he said.) Social distancing and wearing an N-95 mask, when appropriate, can also help keep illness at bay.

“The most important thing,” however, “is to get vaccinated. Full. Stop,” Cotter says. “The tripledemic is likely here to stay, but its impacts on communities can be greatly reduced by preventing the spread.”

Contributing to last year’s tripledemic was the fact that a majority of U.S. kids hadn’t received a flu or COVID vaccine. As of December, nearly 60% of children ages 6 months through 17 years hadn’t received the flu vaccine. And 90% of children ages 6 months through 4 years hadn’t received a COVID vaccine, according to the CDC.

This fall, many Americans will be eligible for three vaccines to help protect them from severe illness: the COVID vaccine; the flu vaccine; and new RSV vaccines, available for those age 60 and older, and infants.

On Thursday, newly appointed CDC director Dr. Mandy Cohen adopted the recommendations of the agency’s Advisory Committee on Immunizations Practices, recommending new injectable, long-acting monoclonal antibody treatment nirsevimab—trade name Beyfortus—for infants 8 months and younger, and for some higher-risk children through age 19 months.

The new treatment has been shown to reduce the risk of hospitalizations and doctor visits for RSV in infants by about 80%, according to the agency, and should reduce the impact of the virus on the health care system.

While no vaccine is perfect, “they can lessen the symptoms and keep you and your loved ones out of the hospital,” Cotter said. “There were an enormous number of sick children around the country who suffered greatly along with their families last year. But now we have new tools and awareness to help. 

“This is the new ‘flatten the curve’ moment. Get vaccinated.”