(NBC News) - The number of very sick children admitted to Children's Hospital New Orleans with Covid-19 has exploded over the past two weeks — from zero to 20.
"I've never seen anything like it," said Dr. Mark Kline, the hospital's physician-in-chief. "We are seeing children fall ill that we just simply didn't see in the first year of the pandemic, before the delta variant came along."
Doctors at Orlando Health Arnold Palmer Hospital for Children Infectious Diseases in Florida have seen similar surges recently. "The last two weeks, cases have continued to increase," said Dr. Federico Laham, medical director for the hospital. "I don't think we have reached our peak."
Despite the dramatic increase in cases, Laham and other pediatric infectious disease experts nationwide tell NBC News that there is no hard evidence yet that the delta variant has transformed the virus into something more dangerous in kids.
"It's too early to tell," said Dr. Bernhard Wiedermann, an infectious diseases specialist at Children's National Hospital in Washington, D.C. "It's going to take time and a collection of data from multiple sites to know" whether the delta variant is, in fact, more virulent in kids than previous versions of the virus.
The Centers for Disease Control and Prevention is working to determine whether the delta variant can, in fact, cause more severe disease in children, Dr. Rochelle Walensky, the CDC's director, said during a briefing Thursday.
She added that such research is complicated by the combination of increased cases and relaxed restrictions on masking and physical distancing. "The mitigation strategies that were used last summer, even in the winter, have not been employed in many of these areas that we are having surges right now," Walenksy said.
Delta is a doctor's 'worst nightmare'
What is obvious now, experts say, is that surges in pediatric cases are due to the variant's hypertranmissibility, circulating in a population left unvaccinated and therefore vulnerable to the virus.
"If more children get sicker, that's just because more kids are getting sick," said Dr. Paul Offit, a vaccine researcher at the Children's Hospital of Philadelphia. "It's clearly increased contagiousness, not increased virulence."
Children have been susceptible to Covid-19 all along. More than 4.1 million children have been diagnosed with Covid-19 since the beginning of the pandemic, accounting for 14.3 percent of all cases, according to the latest data from the American Academy of Pediatrics.
From July 15 to July 29, that percentage rose to 19 percent of weekly reported cases.
"Over the course of the first year of this pandemic, the myth existed that children never got very ill from Covid," Kline said. That was mainly because the number of pediatric cases was relatively low.
Because the delta variant is so contagious, he said, the increase in cases clearly shows the virus's potential, even in young, otherwise healthy children.
"This delta variant is an infectious disease specialist's worst nightmare," Kline said.
The uptick in pediatric Covid cases comes at a time when young people are preparing to go back to school in person — and in many areas, without the added protection of masks. What's more, children's hospitals have also been inundated with kids sick with other respiratory viruses, such as severe colds, croup and respiratory syncytial virus, or RSV.
Dr. Evan Anderson, a pediatric infectious diseases expert at Children's Healthcare of Atlanta, warned that the convergence of viruses may overwhelm already stretched pediatric hospital wards.
"Many children's hospitals get quite full during the usual winter months with influenza, RSV and other viral respiratory pathogens," Anderson said. With an added surge of Covid-19, "there would be major concerns about hospital capacity."
Kline agreed. "Children don't have very many options when it comes to finding care if they have severe disease or complex medical conditions," he said. "Covid is filling beds that otherwise might be occupied by children with other medical problems."
The impact of rising Covid-19 cases, coupled with the other viruses, worries pediatricians like Dr. Natasha Burgert, a national spokesperson for the American Academy of Pediatrics and practicing physician in Overland Park, Kansas.
"We're starting to see kids that have both Covid and RSV that are not doing well," Burgert said. "I have no idea what's going to happen if they have Covid and influenza."
While the flu can cause severe illness in kids, and even lead to death, Burgert said that the potential effects of Covid-19 are "beyond what flu would ever do."
"Even though Covid has flu-like symptoms, Covid is not the flu," she said.
Burgert and colleagues all point to the potential for children with Covid-19 to develop multisystem inflammatory syndrome in children, or MIS-C.
That's when children develop dangerous inflammation around the heart and other organs, most often weeks after their initial infection. Many times, patients are unaware they were ever infected because their symptoms were either nonexistent or extremely mild.
When a child is diagnosed with influenza, they clearly have symptoms, and physicians know to be on the lookout for complications. Covid's potential to result in MIS-C is different, because it blindsides doctors and their young patients.
"This could be a kid that looks totally fine, and then three weeks later is in the ICU with organ failure," said Dr. Nicole Baldwin, a pediatrician in Cincinnati.
Laham, in Orlando, is bracing for such cases. "Once you have a wave of Covid activity in your community, we know that three to four weeks later, we start seeing children coming into the hospital presenting with MIS-C."