(CNN) A new report from the US Centers for Disease Control and Prevention found that Covid-19 cases among younger children remained low even after schools restarted for in-person learning. To safely reopen schools, however, transmission in communities must be kept in check.
The report, published Wednesday in the CDC's Morbidity and Mortality Weekly Report, considered more than 2.8 million laboratory-confirmed Covid-19 cases in people ages zero to 24 from March 1 through December 12. More than 57% of those cases occurred among people ages 18 to 24. Cases among children and teens paralleled cases among adults through the summer and fall -- including spikes in cases in early summer followed by a decline, and then a steep increase in October through December.
By early December, 62% of US K-12 school districts had opened for full or partial in-person learning. Despite that, reports of school outbreaks were "limited," CDC researchers wrote. Covid-19 incidence among the general population was similar in counties with in-person learning -- 401.2 cases per 100,000 people -- and those that were online only -- 418.2 per 100,000 people.
There was no sign that increased cases among school-age children and teens preceded increases in other age groups; however, there were increases among other age groups after cases increased among people ages 18 to 24.
To prevent coronavirus transmission in schools, transmission in the community must be controlled, the report said. Communities and schools should implement mitigation strategies such as wearing masks, and people must adhere to them. CDC recommendations already say K-12 schools should be the last to close after all other mitigation measures have been tried, and the first to reopen once it's safe to do so, the report noted.
"When community transmission is high, cases in schools should be expected, and as with any group setting, schools can contribute to COVID-19 transmission, especially when mitigation measures, such as universal and proper masking, are not implemented or followed," the report said.
There are limitations to the report, including that Covid-19 cases are likely underestimated among young people, and case trends among teachers and school staff members are not available.
President-elect Joe Biden has pledged that his administration will reopen most schools within 100 days of taking office.