By Matt Barnum January 4, 2021 at 8:13 p.m. EST

Opening school buildings does not increase the spread of COVID-19 in places where cases or hospital stays due to the virus are rare, according to two new studies.

However, reopening schools in areas with higher case numbers are spreading the virus, one found, while the other couldn’t rule out the possibility. It remains unclear when exactly the school reopening will be riskier.

The papers are the toughest effort yet to understand the connection between schools and COVID in the United States. This research could help school and health officials address the question of whether to reopen or keep school buildings open. However, there are no easy answers for places with higher numbers, at a time when COVID cases are peaking in some counties.

“It seems that with low hospital admission rates, it’s safe to reopen schools in person,” said Douglas Harris, a professor at Tulane University who co-authored one of the studies examining national data. “This is important given the side effects of the closure for students, such as restricted access to essential services, social isolation and loss of learning.”

At the same time, his co-author Engy Zieden warned that these conclusions do not apply everywhere. “Given the recent surge in hospital stays in recent months, policymakers should be cautious,” she said. “It may not be safe to reopen schools where the virus is already widespread.”

The national study, released on Monday, looked at whether opening schools would increase COVID hospitalization rates in surrounding communities. The answer was no, up to a point in the pre-existing COVID spread: between 36 and 44 hospitalizations per 100,000 people per week.

If the baseline hospitalization scores were lower, the opening of schools was more likely to be associated with fewer future hospitalizations. But with hospital stays above that level, researchers cannot say for sure whether the opening of schools has made the situation worse. (Different methods of finding the numbers lead to different conclusions.)

By mid-December, just over half of U.S. counties had hospital stay rates low enough to indicate schools could reopen without causing further spread, Harris said.

The second study, published in late December, looked at Michigan and Washington. It tested whether districts that offered face-to-face tuition this school year saw higher rates of COVID in the surrounding community as a result.

Where COVID numbers were already low, schools did not seem to be contributing to the spread of the virus. But where the pre-existing rates were higher, the risk of school opening was also higher. In Michigan, that threshold was about 20 new daily cases per 100,000 people. At this point, personal schooling seemed to increase the number of cases. In Washington, the bar was even lower: about five cases per 100,000.

“Once you hit a certain point in community infection rates, it looks like you will be personally linked to the spread of COVID in the community,” said Katharine Strunk, study author and professor at Michigan State University.

In both states, almost every county now has case numbers above these levels. Much of the country is also well above these values: Los Angeles has 141 cases per 100,000 people, Miami-Dade County 92, and Cook County, which includes Chicago, 41. The country as a whole has over 60 cases per population 100,000 inhabitants.

The results are not entirely surprising to public health experts. You have long said that fighting the virus more fully is the best way to ensure schools can reopen.

For much of the country, however, the question remains how the potential risks of school opening can be reconciled with the risks of a closed school given the benefits of having a personal school for students and an already documented loss of learning.

“We’re not coming down hard and saying that every school or district should or shouldn’t be reopened in person,” Harris said.

Rebecca Haffajee, a public health researcher at RAND, said local school officials should focus on whether they see it spread within schools and what safety measures they can take. “To a certain extent, you have to stick to what exactly is happening in your district,” she said.

Less than half of American students currently attend a district that currently has in-person tuition.

School and health officials making these calls face persistent staff shortages and the persistent reluctance of many teachers and parents to return to school buildings. Another problem could be the new strain of the virus that recently emerged in the United States and appears to be particularly contagious. England, where the variant originated and is rapidly spreading, announced plans on Monday to close all of its schools.

“This burden affects me particularly,” said Haffajee. “It can change the way we think about transmission in schools, simply because the burden is much more transferable in general.”

More positively, the vaccine is being rolled out across the country. Some states plan to prioritize access for teachers, which could make reopening schools more profitable.

“These are very difficult decisions for families, teachers, staff, and districts, especially given the dire levels of transmission, hospitalizations, and deaths we’ve seen in recent weeks,” said Whitney Robinson, an epidemiologist at the University of North Carolina.

This story was originally published by Chalkbeat, a non-profit news organization dedicated to public education. Sign up for your newsletter here.