The delta variant causing coronavirus spikes in the U.S. and across the globe could be more contagious and more severe in children, according to a Harvard expert, and cases are expected to increase as schools open and the weather gets colder.

“There’s every reason to believe that (the delta variant) is more contagious to children and from children than the older variants, and that means that at a societal level, we’re seeing higher numbers of cases in all age groups, including in children,” said Marc Lipsitch, infectious disease epidemiologist at the Harvard T.H. Chan School of Public Health.

Children are still much less likely to get severe coronavirus compared to adults even with the delta variant circulating in the community, said Lipsitch.

But as schools reopen and the weather gets colder, cases are expected to increase.

“We will have this problem of a large number of infections in children, the large majority of them mild in their effects, but a proportion of them more serious,” Lipsitch said during a Tuesday virtual briefing.

As the number of coronavirus cases grows, the number of severe cases will grow as well.

More data is needed to learn about the exact contagiousness and the severity of the delta variant compared to other strains of the virus, but Lipsitch said it’s “reasonable to believe” the delta may be somewhat more severe in kids.

Lipsitch said it’s clear that kids will be exposed to coronavirus in school and in the household. Even vaccinated adults who are infected with the delta variant could pass the virus onto an unvaccinated child, he said.

“It’s clearly possible for a vaccinated adult infected with delta to infect a child, whether it happens with high frequency is going to be an important research question for the coming weeks,” Lipsitch said.

Children age 12 and older are currently eligible to get vaccinated, and several back-to-school vaccination efforts have been underway in the U.S. However many parents of younger children are anxiously awaiting clearance to get a shot for their little one.

Lipsitch said children age 5 to 11 might become eligible around the end of this year or early 2022.

In the meantime, wearing a mask inside and having proper hygiene and ventilation in schools will help to keep class in person and minimize valuable time that could be lost due to illness or quarantine, according to Lipsitch.

He added that being flexible and open to change on masking policies is important though. “Wearing masks is no one’s idea of fun.”