As childhood Covid-19 hospitalizations continue to rise, some researchers are investigating children's immune systems to learn why so many young people successfully fend off coronavirus infections—and why others experience such severe symptoms.

How much worse will the 'delta surge' get? Watch these 7 factors.

More kids are falling ill with Covid-19

According to the American Academy of Pediatrics (AAP), children represented 28.9% of Covid-19 infections reported for the week ending Sept. 9—a 10% increase in just two weeks.

In the same week, doctors diagnosed more than 243,000 cases in children, bringing the total number of children under 18 infected with Covid-19 since the onset of the pandemic to 5.3 million, with at least 534 deaths.

Rates of childhood hospitalizations have also increased markedly, but it's not clear that children are especially vulnerable to the delta variant. Rather, experts say, more children are falling ill simply because the variant has spread so widely across the nation.

"If 10 times as many kids are infected with delta than previous variants, then, of course, we're going to see 10 times as many kids hospitalized," Dimitri Christakis, director of the Center for Child Health, Behavior, and Development at the Seattle Children's Research Institute, said. 

According to AAP, less than 1% of children who have been diagnosed with Covid-19 require hospitalization, and about 0.01% of infected children pass away. 

An additional factor in the surge of childhood hospitalizations is that children younger than 12 are not currently eligible for vaccination, so they don't benefit from a layer of protection that has reduced serious disease across the rest of the population.

Among older children, vaccines have proven powerfully protective: According to CDC, hospitalization rates are 10 times lower in vaccinated adolescents than among unvaccinated adolescents.

Why is Covid-19 usually mild in children?

Writing for Kaiser Health News, Liz Szabo reports that, despite the rise in childhood hospitalizations, children and young people are still significantly less likely than older people to become seriously ill. Most experience "little more than a sniffle."

Lael Yonker, an assistant professor of pediatrics at Massachusetts General Hospital, said evidence suggests that children's immune systems typically fend off the infection early on, which prevents the virus from "gaining a foothold and multiplying unchecked."

This may be because children have especially strong mucosal immunity. As Szabo writes, mucous membranes—such as those in the nose and throat—are lined with epithelial cells, which are covered in proteins called pattern recognition receptors.

These proteins act like guards to protect the body. If pattern recognition receptor proteins detect a virus or other foreign object, they trigger cells that release proteins called interferons, which activate the body's immune response.

In part due to the strength of the immune response in their airways, children may be able to mount an immune response to the coronavirus two days faster than adults, according to Roland Eils of the Berlin Institute of Health.Another factor may be that children have large supplies of a type of a "peacekeeper cell" called innate lymphoid cells, according to Jeremy Luban, a professor at the University of Massachusetts Medical School.

Children are born with a large amount of these cells, which help calm the immune system and repair lung damage. The numbers decline with age—and Luban's research has found that children and adults who contract Covid-19 tend to have fewer of these repair cells, Szabo writes.

Why do some children fall seriously ill?

Still, even though most childhood Covid-19 cases are mild, some can be serious or even fatal. And experts are currently investigating "red flags that could indicate greater disease severity in specific segments of the pediatric population," writes Mackenzie Bean for Becker's Hospital Review. 

For instance, especially young children might not have sufficiently experienced immune systems to fend off Covid-19 infection. Half of the Covid-19 cases at Children's Hospital New Orleans involve patients under the age of four.

Pre-existing conditions are also a factor. Between 30% and 70% of children hospitalized with Covid-19 have a serious underlying condition that could increase their risk of severe illness, such as Down syndrome or lung disease.

Still, according to Mary Taylor, chair of pediatrics at the University of Mississippi Medical Center, "There's really no way to know which child with Covid will get a cold and be just fine and which child will be critically ill."

She added, "It's just a very helpless sensation for families to feel like there is nothing they can do for their child."

In absence of any way to predict which children will fall especially ill, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, urged parents and children over 12 to get vaccinated—and said that for children too young for Covid-19 shots, "the best way to keep them safe is to surround them by people who are vaccinated." (Szabo, Kaiser Health News, 9/17; Bean, Becker's Hospital Review, 9/16)