CNN  — 

Just as doctors feared, more children are getting hit hard by Covid-19 as the Delta variant tramples across the country.

And the school year just started.

“What we’re seeing now is extremely concerning,” said Dr. Edith Bracho-Sanchez, associate professor of pediatrics at Columbia University Irving Medical Center.

“This virus is really going for the people who are not vaccinated. And among those people are children who don’t qualify for the vaccine and children and teens who qualify but are choosing not to get it.”

Among the latest sobering statistics:

– A record-high 2,544 children were hospitalized with Covid-19 on September 10, according to data from the US Department of Health and Human Services.

– An average of 341 pediatric Covid-19 patients were admitted to hospitals every day during the week ending September 12, according to data from the US Centers for Disease Control and Prevention.

More than 57,000 children have been hospitalized with Covid-19 since August 2020, according to CDC data. Many of those children had no known preexisting conditions.

– While childhood Covid-19 deaths are still rare, that number is increasing. At least 529 children have died from Covid-19, according to CDC data.

Doctors say it’s critical to protect children against the Delta variant – not just for the sake of their health, but to preserve in-person learning and help prevent more aggressive variants from setting the entire country back.

243,373 new pediatric cases in one week

Since the last school year, a more contagious variant – Alpha – has been replaced by an even more contagious variant – Delta – as the dominant strain of coronavirus in the US.

At least 243,373 new pediatric Covid-19 cases were reported during the week ending September 9, the American Academy of Pediatrics said.

That’s an increase of about 240% from the weekly rate in early July.

And more pediatric cases has led to more kids hospitalized with Covid-19, said Dr. Jon McCullers, pediatrician-in-chief at Le Bonheur Children’s Hospital in Memphis.

“We have seen a significant surge in cases roughly for the last four weeks, roughly corresponding to the time when school got in,” McCullers said September 8.

“Interestingly, we’re seeing about three times the number of hospitalizations that we saw during our during our peak during the winter.”

It’s not just kids with preexisting conditions getting hospitalized

Almost half – 46.4% – of children hospitalized with Covid-19 between March 2020 and June 2021 had no known underlying condition, according to CDC data from almost 100 US counties.

And the Delta variant is further annihilating the myth that healthy kids can’t get hit hard.

Previously, “the majority of kids that I’ve seen get really sick (with Covid-19) have been kids with other illnesses or comorbid conditions,” said Dr. Susannah Hills, a pediatric airway surgeon at Columbia University Medical Center.

“But now, the difference with this Delta variant is that we’re seeing kids who may not necessarily have comorbid conditions also end up in the hospital.”

More children are getting MIS-C

In some cases, children who start with mild or no symptoms from Covid-19 end up hospitalized weeks or months later with a condition called MIS-C – multisystem inflammatory syndrome in children.

MIS-C is “a rare but serious condition associated with COVID-19 in which different body parts become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal organs,” the CDC says.

It happens when “the virus induces your body to make an immune response against your own blood vessels” – which can cause inflammation of the blood vessels, said pediatrician Dr. Paul Offit, director of the Vaccine Education Center at the Children’s Hospital in Philadelphia.

At least 4,661 cases of MIS-C had been reported, including 41 deaths, the CDC said.

Many of the children with MIS-C don’t start off very sick with Covid-19.

“Usually children are picked up incidentally as having (coronavirus). Someone in the family was infected, a friend was infected, so they got a PCR test. And they’re found to be positive. … Then they’re fine,” Offit said.

“Then a month goes by, and they develop a high fever. And evidence of lung, liver, kidney or heart damage. That’s when they come to our hospital.”

The CDC said 99% of MIS-C patients had tested positive for coronavirus, and the other 1% had contact with someone with Covid-19.

The median age of patients with MIS-C was 9 years old.

“CDC is working to learn more about why some children and adolescents develop MIS-C after having COVID-19 or contact with someone with COVID-19, while others do not,” the CDC says.

“Based on what we know now about MIS-C, the best way you can protect your child is by taking everyday actions to prevent your child and the entire household from getting the virus that causes COVID-19.”

The best steps parents can take include getting vaccinated and vaccinating children ages 12 and up, CDC Director Dr. Rochelle Walensky said.

And even if a parent is fully vaccinated, there’s a small chance they could catch an asymptomatic breakthrough infection and unknowingly pass the virus to their children.

That’s why it’s a good idea for all parents of young children to wear masks in public indoor settings, Walensky said.

For kids too young to be vaccinated, it’s important “to surround them with vaccinated people,” she said.

Long Covid can leave lasting impacts on kids

Long-term Covid-19 complications can be significant for children – even for some who initially had mild or no symptoms, the American Academy of Pediatrics said.

All pediatric patients who tested positive should have at least one follow-up exam with a pediatrician, the AAP said.

Pediatricians should watch out for residual or long-term Covid-19 problems such as respiratory symptoms, which can last three months or more; heart issues, including a type of heart inflammation known as myocarditis; cognitive problems such as “brain fog”; headache; fatigue and mental health issues, the AAP said.

Children who had moderate or severe Covid-19 may be at greater risk for subsequent heart disease, the pediatrician group said.

Protecting kids from Covid-19 is critical to keep them in schools

With the highly contagious Delta variant, the CDC recommends students from kindergarten through grade 12, along with teachers and visitors, wear masks in school.

The American Academy of Pediatrics recommends masks in schools for everyone older than 2.

“Our children deserve to have full-time, in person, safe learning with prevention measures in place. And that includes masking for everyone in schools,” Walensky said.

Some students are returning to schools for the first time in a year. But long-awaited classroom learning can be quickly derailed by an infection or outbreak.

In Mississippi and Florida, thousands of students just starting their school year have already had to quarantine.

And it doesn’t take much for Covid-19 to shut down a school again. Even one case can have a ripple effect on students, faculty and staff.

“We need adults to run schools, and if my adults are sick or needing to quarantine, I don’t have adults present to provide the education,” said Carlee Simon, superintendent of Alachua County Public Schools in Florida.

“When we have families that don’t want to have masks on their child, what they’re doing is not only (increasing the) chance they will have to be quarantined,” Simon said.

If a student gets infected, “they will also have other students who did have masks on who would also need to be quarantined.”

“Everybody wants to move forward. Nobody wants to have masks forever,” Simon said. But “we would like to be able to be safe and have instructional time with our students.”

In addition to masks in schools, the CDC recommends layering other strategies such as improved ventilation, physical distancing and testing on a screening basis.

Children can accidentally help spur new variants

Protecting children from getting Covid-19 can help everyone in the long run, doctors say.

As coronavirus keeps spreading, replicating itself in new people, the more chances it has to mutate – potentially leading to even more contagious variants or one that might evade vaccines.

“That’s, of course, the concern,” Walensky said.

Fully vaccinated people are less likely to get infected with the Delta variant.

But unvaccinated people – including unvaccinated children – are more susceptible to infection. And they can unknowingly help with the creation of new variants, Offit said.

“If we are going to continue to allow this virus to spread, we’re going to continue to allow these variants to be created,” he said.

“We’re not going to be able to stop this pandemic until we have a significant percentage of the population vaccinated.”

Covid-19 deaths in children shouldn’t be ignored, CDC chief says

While children are far less likely to die from Covid-19 than adults, the deaths are still significant, Walensky said.

At least 529 US children have died from Covid-19, according to CDC data. For the 2019-20 flu season, the CDC reported 199 confirmed pediatric flu deaths and an estimated 434 pediatric flu deaths.

One reason Covid-19 is deadlier for children than other infectious diseases is because many children are vaccinated against other diseases, said Dr. James Campbell, professor of pediatrics at the University of Maryland School of Medicine.

“Nobody’s dying of polio, nobody’s dying of measles in the United States. Nobody’s dying of diphtheria,” Campbell said.

But while children ages 12 to 17 can get a Covid-19 vaccine, many have not done so. And it’s not clear exactly when a vaccine might be authorized for children younger than 12.

Rebecca Calloway’s 7-year-old daughter, Georgia, is one of thousands of young children testing various doses of Covid-19 vaccines to make sure they’re safe and effective before they get authorized.

Part of the reason Calloway enrolled Georgia in the pediatric vaccine trial is because she recently lost her 3-year-old daughter to another unexpected disease – Type 1 diabetes – and doesn’t want any more families to lose a child to Covid-19.

While childhood deaths from Covid-19 and Type 1 diabetes are rare, Calloway said, “You don’t want to be that statistic.”

CNN’s Deidre McPhillips and Jen Christensen contributed to this report.