While rare, serious illness linked to Covid-19 in children has researchers studying variants.

Pediatric researchers are investigating whether Covid-19 is becoming more severe for children now that variants are causing localized flare-ups, even as U.S. cases overall decline.

By early April, the rate of Covid-19 cases in young kids and early teens began surpassing that of those 65 and older, according to the Centers for Disease Control and Prevention. The latest CDC data suggest the trend is continuing. At the same time, hospitalizations for children with Covid-19 aren’t falling as much as for those 18 and up. That has researchers concerned that variants may be affecting youths in new ways, including a rare inflammatory disease that has been linked to Covid-19 infection.

“The big concern is that we’ve left a whole population of children unprotected,” said Adrienne Randolph, a critical-care doctor at Boston Children’s Hospital who is leading the CDC-funded research.

Cases of the rare disease, called multisystem inflammatory syndrome in children, or MIS-C, totaled more than 2,000 in February and surpassed 3,000 by April 1, according to the CDC, which will soon update its tally.

Vaccine manufacturers’ clinical trials have mostly focused on adults, and now almost 72% of those 65 and older are fully inoculated, according to the CDC. Meanwhile, young children don’t yet have access to shots, and companies are still studying the effects on them. Pfizer Inc.’s vaccine has been available for teens 16 and older, and won authorization Monday for ages 12 to 15.

The vaccines have been found to work well in adults against the current virus mutations, including the B.1.1.7 variant identified in the U.K. Still, pockets of unvaccinated people are giving the variant room to spread, which may be especially worrisome for children. That variant, which is more contagious than the original virus, became the most dominant strain in the U.S. in early April and now makes up almost 60% of cases, according to the CDC.

In Colorado, which has one of the country’s highest infection rates, Sam Dominguez, a pediatric infectious disease specialist at Children’s Hospital Colorado, started to see an increase in cases of the rare inflammatory disease at the end of April.