States with lower numbers of confirmed infections at the time of school closure likely saw the biggest positive impact of children remaining at home, cutting cases by an estimated 72%, researchers at Cincinnati Children's Hospital Medical Center said.
Still, states with more COVID-19 cases may have reduced their total number of infections by 49% by shutting schools, they said.
"States that closed schools earlier, when cumulative incidence of COVID-19 was low, had the largest relative reduction in incidence and mortality," the researchers wrote.
"However, it remains possible that some of the reduction may have been related to other concurrent non-pharmaceutical interventions," they said.
The findings come at a time when many school districts across the country are debating reopening plans for the fall.
Some, including New York City, have announced that schools will offer a mix of online and in-person learning, while others, like Los Angeles, plan to provide online classes only.
Although children are generally thought to be at lower risk for severe illness from COVID-19, data on how much they can spread the virus remains limited.
Evidence from several European countries -- including Denmark and Germany, which closed schools in March and began phased reopenings later in the spring, and Sweden, which kept schools open-- suggests the virus can spread to some degree among children, a University of Washington report said.
However, just two of 34 households with multiple lab-confirmed COVID-19 infections in Chicago involved cases of child-to-child transmission, and only two others had instances of child-to-adult transmission, a study published last month in the Journal of the Pediatric Infectious Diseases Society found.
For the JAMA analysis, the Cincinnati Children's Hospital researchers modeled disease spread in 23 states that closed schools this year because of the pandemic between March 9 and May 7.
The analysis included 11 states with the lowest incidence of COVID-19 cases and the 12 with the highest at the time of school closure, the researchers said.
The researchers compared virus spread in these states before and after school closure and factored in a number of variables, including the general age of the population, they said.
Overall, closing schools in these states cut infections by an estimated 62% and deaths by an estimated 58%, according to the researchers.
Dr. Sarah Fortune, chair of the Department of Immunology and Infectious Diseases at Harvard T.H. Chan School of Public Health, said on a conference call with reporters Wednesday that findings such as these suggest that states considering a full school reopening in the fall could see a spike in COVID-19 cases as a result.
Fortune, who was not involved in the JAMA analysis, described schools as "like little bars" because large numbers of people are confined in tight spaces, and said that states that opt to reopen could see "equivalent" spread to what was seen in areas that allowed bars and restaurants to open.
"Reopening schools is going to be a major challenge for our [public health] system," she said.
"We need to be able to test students all the time [in order] to reopen schools in a more normal way, and that means increasing testing capacity by a couple of orders of magnitude."