At the start of the pandemic, kids made up 2% of new COVID infections. Now, they make up 24%. What happened?

Children make up a growing share of new coronavirus cases each week as overall infections decline with the rollout of COVID-19 vaccines, but infection and hospitalization rates among children remain stable.

In March 2020, children accounted for only about 2% of new infections. By the end of May, children made up more than 24% new weekly infections even though they account for only 16% of the population, according to data by the American Academy of Pediatrics.

Health experts say it’s a sign more adults and adolescents need to get vaccinated to avoid bringing the virus home and spreading it to children who aren’t yet eligible for a COVID-19 vaccine.

“The virus is an equal-opportunity infector. It doesn’t care if you’re young or old,” said Dr. Robert Frenck, professor of pediatrics and director of the Center for Vaccine Research at Cincinnati Children’s Hospital, who presented the data at a Johns Hopkins University-University of Washington symposium Wednesday called “COVID-19 and Kids: Impacts, Uncertainties and the Role of Vaccines.”

More than 4 million children have tested positive for the coronavirus in the U.S.; 18,500 have been hospitalized and 336 have died from the disease, according to the pediatrics academy. About 4,000 children have been hospitalized with multisystem inflammatory syndrome children, or MIS-C, a rare but dangerous condition the federal Centers for Disease Control and Prevention says is associated with COVID-19.

Although nearly 55% of Americans have received at least one vaccine dose as of Friday, health experts say vaccination rates are not uniform across the U.S., which leaves room for outbreaks and childhood infections.

“There are some places where vaccinations rates are as low as 20%,” Frenck said. “It’s not ‘if,’ but ‘when.’ (Outbreaks) will happen, and unfortunately, people will be surprised, and that’s what we’ve been trying to tell people.”

Health experts say it’s especially crucial to increase vaccination rates as the delta coronavirus variant spreads throughout the U.S. On Wednesday, Texas Children’s Hospital reported several of the country’s first pediatric infections – all in children under 12.

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It’s too early to tell if the delta variant causes more severe disease in children, but experts say the variant seems to be causing more symptomatic infections than the original virus.

“Because it’s the most highly contagious variant to date based on all the data we’ve accumulated so far, we expect to see more rapid transmission of this virus from adults and adolescents to children,” said Dr. James Versalovic, pathologist-in-chief and interim pediatrician-in-chief at Texas Children’s Hospital.

In the meantime, scientists are doubling their efforts to finish clinical trials so the Food and Drug Administration can authorize vaccines for younger children as soon as possible.

Pediatric hospitals are working closely with vaccine developers like Pfizer and Moderna to combine Phase 2 and 3 trials, expediting the process during the summer months to submit data by early fall.

“Early in the next school year, we hope to have emergency use authorization for these COVID vaccines for children under 12 by early to mid-fall,” Versalovic said. “That remains a top priority and has now added urgency with the rapid spread of the delta variant.”

Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.

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This article originally appeared on USA TODAY: COVID: Kids make up growing share of new weekly cases. What to know.