These low rates may stem, in part, from the fact that children under 10 seem to be less likely to transmit the virus than older children and adults are.
But another contributing factor is that schools are, or can be, controlled environments and may have stricter safety measures than the surrounding community, Dr. Benjamin said.
Additionally, research shows that the virus is much less likely to cause severe illness in children. They are not entirely protected; a small number of children may develop a rare but serious inflammatory condition, and some children with mild infections may experience long-term symptoms.
Initially, the C.D.C.’s guidelines recommended that unvaccinated people who were 2 or older wear masks in schools. And they strongly implied that vaccinated students did not need to be masked in the classroom.
But last week, because of concerns about Delta, the C.D.C. revised its guidelines, recommending that everyone, regardless of vaccination status, wear masks in schools this fall.
As the pandemic continues to evolve, schools and officials will need to make complicated decisions based on local conditions, including when to insist on certain precautions and when it is safe to lift them.
In schools or districts that do not have mask mandates, parents can provide some degree of protection by ensuring that their children, at least, wear masks to school, Dr. Maldonado said.
And adults can help protect younger children by getting vaccinated themselves. “The single most important thing any community can do in order to reduce the risk in schools is for the entire community to be vaccinated,” Dr. Schaffner said.