The Delta variant may be making COVID-19 more common in kids - but severe infections are still rare

• COVID-19 vaccines haven't been authorized for children under age 12.
• That means kids are at higher risk of contracting the Delta variant, the most transmissible to date.
• But experts worry more about kids spreading the variant than about children getting severely ill.

The best way to protect yourself from the Delta variant? Get vaccinated.

But that's not an option for the roughly 48 million children under 12 in the US. So for the time being, children are at high risk of acquiring a Delta infection.

"The fact that we're seeing outbreaks in certain parts of the country specifically in children is because, at this moment, those are the most vulnerable hosts because they're not vaccinated," Erlinda Ulloa, a pediatric infectious-disease specialist at the University of California, Irvine, told Insider.

The Delta coronavirus variant is the most transmissible to date. An analysis from Public Health England found that it was associated with a 60% increased risk of household transmission compared with the Alpha variant, which was discovered in the UK. The Alpha variant is already about 50% more transmissible than the original strain, according to the Centers for Disease Control and Prevention.

That means the variant can more easily spread among kids, too. Indeed, several countries have recently recorded a higher share of coronavirus cases among children.

In Israel, half of the 125 new infections reported on Monday were among children, according to the country's health ministry. About 70% of Monday's new infections, the ministry added, were caused by the Delta variant.

Researchers in Scotland also found that Delta cases were present mostly in younger age groups. In the UK overall, a study still awaiting peer review found that coronavirus infections were now five times more prevalent among children ages 5 to 12 and young adults 18 to 24 than among people older than 65. (The Delta variant now accounts for up to 99% of the UK's coronavirus cases, according to Public Health England.) Most young adults who recently got infected were unvaccinated, according to that study.

In the US, meanwhile, kids represented nearly 25% of new weekly cases for the week ending June 17, despite them making up about 22% of the population. That's higher than the overall share since the start of the pandemic: 14%.

The US's Delta cases appear to have tripled in just 11 days, from 10% of all coronavirus cases sequenced earlier this month to 31% of all cases last week, according to an estimate from the Financial Times. At that rate, experts predict the Delta variant will become the nation's dominant strain in a matter of weeks.

"With these new, more contagious variants, I think we're going to see that children and schools do become more of a focal point of spread," Scott Gottlieb, a former commissioner of the Food and Drug Administration, told CNBC on Monday.

Kids could spread the virus to unvaccinated adults

Disease experts say kids don't seem to be developing more severe COVID-19 cases as a result of the Delta variant - and don't seem to be somehow more biologically susceptible to contracting the variant than adults are.

"There could be increased transmissibility of this virus, but it's increased transmissibility to all people, not just because you're a child," Ulloa said.

The Scotland researchers found that getting infected with the Delta variant doubled the risk of hospital admission overall relative to the Alpha variant. (Previous studies have suggested that the Alpha variant may be 30 to 70% deadlier than the original strain.) But even if you doubled a child's risk of being hospitalized from a Delta infection, it would still be "minuscule," according to Eyal Leshem, an infectious-disease specialist at Israel's Sheba Medical Center.

That's because severe COVID-19 is extremely rare among children: In the US, kids account for just 1.4 to 3.3% of all COVID-19 hospitalizations and less than 0.23% of all COVID-19 deaths.

But the variant's transmissibility means that kids who get infected with the Delta variant can easily spread it to unvaccinated adults, or perhaps to people who have a less-robust immune response to vaccines - like older people or people who are immunocompromised.

Some kids may get severely ill in rare cases

Pfizer and Moderna expect to release trial data about the safety and efficacy of their vaccines in young children in the fall, then apply for FDA authorization. Until then, disease experts continue to recommend masks for unvaccinated kids.

In very rare circumstances, some kids may get severely ill from the Delta variant. Ulloa said she had seen a few pediatric patients who were hospitalized after contracting the virus from an unvaccinated family member.

"If we've had a few cases here and there, then that's the story that we're getting: that basically a lot of the family members are vaccinated, but then they're exposed to an unvaccinated infected person," she said.

Gottlieb told CNBC that any rise in pediatric hospitalizations could be attributable the virus' transmissibility.

"It's just math that if more kids get infected, even if the rate of bad outcomes in kids is very low, more kids are going to have bad outcomes," he said.

Ulloa said she'd also seen a few kids develop a persistent cough or fatigue similar to adult long-haulers - patients those whose symptoms last at least three weeks but can drag on for months.

"I wonder if these different variants are more likely or less likely to cause these long-haul syndromes in kids," Ulloa said. "That's something else that we're going to be investigating."