As summer ended, experts expressed concern regarding a “twindemic,” the spread of both influenza and COVID-19 at the same time in the United States. As winter approaches, however, some doctors are warning of a “tripledemic,” a combination of the flu, COVID and pediatric respiratory viruses, such as respiratory syncytial virus (RSV).

Seniors are more likely to develop severe COVID, flu and RSV, while young children are at greater risk for severe flu and RSV but at low risk for severe COVID.

According to the Centers for Disease Control and Prevention (CDC), respiratory illnesses are occurring earlier than in recent years and are affecting more people.

What’s more, flu cases have appeared earlier across most of the country, signaling the possibility of a more severe flu season than the last two.

In past years, COVID cases have spiked around Thanksgiving. Experts warn that indoor gatherings paired with fading immunity to COVID and other viruses could leave people vulnerable to illnesses. Add to that the fact that 75% of the estimated 40,000 pediatric hospital beds are occupied amid hospital staff shortages and the United States may be facing what John Brownstein, PhD, an epidemiologist and chief innovation officer at Boston Children’s Hospital, called “an unfortunate perfect storm” in an ABC News article.

“Mostly the issue is there’s low population immunity and kids are, once again, gathered again, and this is facilitating rapid spread of viruses like RSV,” Brownstein said. “And because of the sheer volume of infection, when you have that larger denominator, you have a situation where a portion of those kids are going to require hospital treatment. And because of that, our hospitals are spread thin, not only for bed capacity but also for critical staffing of those beds.”

Although restrictions put in place at the start of the pandemic—such as masking, school closures, social distancing and strict capacity limits—helped keep virus cases relatively low, they also left young children, particularly those born since March 2020, vulnerable to RSV and other viruses because their immune systems have never encountered them.

Experts advise that adults should get an annual flu vaccine and the latest recommended COVID boosters. Parents are advised to have their children vaccinated against influenza and to send their kids to school with sanitizer for their hands and for wiping surfaces.