In some of those countries, the rate of community transmission is low enough to take the risk. But in others, including the United States, reopening schools may nudge the epidemic's reproduction number -- the number of new infections estimated to stem from a single case, commonly referred to as R0 -- to dangerous levels, epidemiologists warned after reviewing the results from the new studies.

In one study, published last week in the journal Science, a team analyzed data from two cities in China -- Wuhan, where the virus first emerged, and Shanghai -- and found that children were about a third as susceptible to coronavirus infection as adults were. But when schools were open, they found, children had about three times as many contacts as adults, and three times as many opportunities to become infected, essentially evening out their risk.