The delta variant is sending more children to the hospital. Are they sicker, too?

Pilar Villarraga had spent much of the summer counting down the days until her daughter Sophia's birthday. In early August, Sophia would turn 12 — and become officially eligible for a COVID-19 vaccine. "I didn't want her to start school without the vaccine," said Villarraga, who lives in Doral, Florida.

And then, in late July, just two weeks before the milestone birthday, Sophia caught the coronavirus. At first, she just had a fever, but on July 25, after four quiet days convalescing at home, her ribs began to hurt. The next day, Villarraga took her to the emergency room, where chest X-rays revealed that Sophia had developed pneumonia. She soon began coughing up blood.

Sophia was promptly admitted to Nicklaus Children's Hospital, in Miami. Her parents, and their friends, were in shock. "I didn't think that kids could get that sick," Villarraga said.

But Sophia was one of roughly 130 children with COVID-19 who were admitted to a U.S. hospital that day, according to the Centers for Disease Control and Prevention. That number has been climbing since early July. From July 31 to Aug. 6, 216 children with COVID were being hospitalized every day, on average, nearly matching the 217 daily admissions during the pandemic's peak in early January.

Hospitals in coronavirus hot spots have been particularly hard hit. On a single day last week, Arkansas Children's Hospital, in Little Rock, had 19 hospitalized children with COVID; Johns Hopkins All Children's Hospital, in St. Petersburg, Florida, had 15; and Children's Mercy Kansas City, in Missouri, had 12. All had multiple children in the intensive care unit.

These numbers have sparked concerns that what had once seemed like the smallest of silver linings — that COVID-19 mostly spared children — might be changing. Some doctors on the front lines say they are seeing more critically ill children than they have at any previous point of the pandemic and that the highly contagious delta variant is likely to blame.

"Everybody is a little bit nervous about the possibility that the delta variant could in fact be, in some way, more dangerous in kids," said Dr. Richard Malley, a pediatric infectious disease specialist at Boston Children's Hospital.

Most children with COVID-19 have mild symptoms, and there is not yet enough evidence to conclude that delta causes more severe disease in children than other variants do, scientists said.

What is clear is that a confluence of factors — including delta's contagiousness and the fact that people under 12 are not yet eligible to be vaccinated — is sending more children to the hospital, especially in areas of the country where the virus is surging.

"If you have more cases, then at some point, of course it trickles down to children," Malley said.

Climbing cases

Many children's hospitals had been hoping for a quiet summer. Several run-of-the-mill childhood viruses are less common during the warmer months, and national COVID rates had been declining through the spring.

But last month, as delta spread, that began to change.

"The number of positive COVID tests started to climb in early July," said Marcy Doderer, president and CEO of Arkansas Children's Hospital. "And then that's when we really started to see the kids get sick."

The vaccines are effective against delta — and provide powerful protection against severe disease and death — but children under 12 are not yet eligible for them. So as more and more adults get vaccinated, children make up an increasing share of COVID cases; between July 22 and July 29, they accounted for 19% of reported new cases, according to the American Academy of Pediatrics.

"They're the unvaccinated," said Dr. Yvonne Maldonado, a pediatric infectious-disease specialist at Stanford Medicine and chair of the AAP Committee on Infectious Diseases. "That's where we're seeing all the new infections."

From July 22-29, nearly 72,000 new pediatric COVID cases were reported, almost twice as many as in the previous week, according to the association. At Johns Hopkins All Children's Hospital, 181 children tested positive for the virus in July, up from just 12 in June.

Most of those children have relatively mild symptoms, such as runny noses, congestion, coughs or fevers, said Dr. Wassam Rahman, the medical director of the pediatric emergency center at All Children's. "Most of the kids are not very sick," he said. "Most will go home and be treated with preventive care at home. But as you might imagine, families are scared."

A small share of children do develop severe disease, showing up at the hospital with pneumonia or in respiratory distress.

Of the 15 children with COVID-19 who were inpatients at Children's Hospital New Orleans late last week, four — including a 3-month-old baby — were in intensive care, said Dr. Mark Kline, the hospital's physician in chief. None of the children, including the eight who were old enough to be eligible, had been vaccinated.

"This delta variant of COVID-19 is an infectious disease specialist's worst nightmare," Kline said. "And there's just no sign that it has started to plateau."

Some hospitalized children have other chronic conditions, such as diabetes or asthma, that may make them more vulnerable to COVID, but doctors said that they also have seriously ill patients without any obvious risk factors.

Sophia, who was on her school's track and cross-country teams, was healthy and active before getting COVID, her mother said. Her parents were surprised by how quickly she deteriorated.

"From one minute to another, she got super bad," Villarraga said. "I said, 'You know, I could lose my child.'"

After Sophia was admitted, doctors began treating her with the antiviral drug remdesivir, as well as antibiotics, steroids and a blood thinner.

"From there, it was a day-by-day thing," Villaraga said. "Little by little, she got better."

Sophia, like most children with COVID-19, is expected to make a full recovery, her mother said. (A small percentage of children may experience lingering, long-term symptoms often known as long COVID.) She was discharged on July 31 and celebrated her birthday several days later — at home, with an ice cream cake.

Delta differences

Villaraga was not told whether Sophia had the delta variant, but more than 80% of new cases in the United States are caused by delta, the CDC estimates, and doctors said that it is clear that delta is behind the surge in childhood infections.

What remains unknown is whether children who are infected with delta are actually getting sicker than they would have if they had caught a different variant — or if delta, which is roughly twice as transmissible as the original virus, is just so infectious that many more children are getting sick.

There is some emerging evidence — mostly from data on adults — that delta may cause more severe disease. Studies in Canada, Scotland and Singapore, for instance, have suggested, variously, that delta may be more likely to lead to hospitalization, ICU admission or death.

But the research is preliminary, experts said, and there is not yet enough good data on the severity of delta cases in children.

"There's no firm evidence that the disease is more severe," said Dr. Jim Versalovic, the pathologist in chief and interim pediatrician in chief at Texas Children's Hospital, in Houston, where about 10% of children now test positive for the virus, up from less than 3% in June. "We certainly are seeing severe cases, but we've seen severe cases throughout the pandemic."

Although not all states report their pediatric hospitalization rates, the data that is available suggests that they have remained essentially steady for months. Nationally, roughly 1% of children who are infected with the virus end up hospitalized, and 0.01% die, according to the AAP data. Both hospitalization and death rates have declined since last summer.

It is still possible, of course, that delta could turn out to cause more severe disease in children. Hospitalization rates, which are a lagging indicator, could rise in the weeks and months ahead. And the rare but serious inflammatory syndrome that develops in some children with COVID-19 can take weeks to appear.

"I think time will tell, really," Rahman said. "We need at least a month, maybe two months before we get a sense of trends."

But in the U.K., where delta swept through the population before the variant became widespread in the United States, experts say they have not seen clear evidence that the variant is making children sicker.

"There was a wave, there were children who became unwell," said Dr. Elizabeth Whittaker, a pediatric infectious disease and immunology specialist at Imperial College London. "But not in the kind of, 'Oh, my gosh, this is very different, this is worrying.'"

Dual surges

Whether delta turns out to be more severe, the variant is clearly driving a surge of new infections in children and adults, especially in areas where vaccine coverage is low.

"The rates among children are going up because the rates among unvaccinated family members in their homes are going up," Maldonado said.

And more infected children means more hospitalized children.

"It's a numbers game at this point," Versalovic said.

Making matters worse, many hospitals are also reporting a highly unusual spike in children with respiratory syncytial virus, a contagious flulike illness that typically strikes in the fall and winter. RSV cases were abnormally low last winter, likely because of lockdowns and pandemic precautions, but cases have been rising as officials lift restrictions and children begin to mingle.

Children's Mercy Kansas City had nearly three times as many RSV patients as COVID patients late last week, while Texas Children's had nearly 1,500 positive RSV tests in the last 90 days, hospital officials said.

"That has created a dual surge," Versalovic said. "Because both viruses are widely circulating, we're seeing a much greater impact."

The combination of RSV and COVID has pushed Children's Hospital New Orleans to capacity.

"We haven't had an empty bed in any of our intensive care units in six weeks," Kline said.

It is not yet clear when children under 12 may be eligible for vaccination, but in the meantime, experts said, the best way to reduce the danger to children, and relieve the stress on hospitals, is for older children and adults to get vaccinated, which will help curb delta's spread.

"The safest way to never find out whether delta is more aggressive to children than the original strain is to really enhance vaccination," Malley said.

This article originally appeared in The New York Times.