As school and public health officials look for ways to reopen classrooms safely throughout the country, a potential road map emerges from the experience of four sleep-away camps and the extensive measures they adopted to prevent spread of the novel coronavirus among more than 1,000 campers and staff members.

Their experience, described in a federal study published Wednesday, shows the measures they took to keep the virus at bay. The four camps in Maine conducted virus testing before and after campers arrived and made them quarantine. Campers and counselors were kept in the same group at the camp. Face masks and physical distancing were employed, extensive cleaning and disinfection were frequent, and activities were conducted outdoors as much as possible, according to the study by the Centers for Disease Control and Prevention.

The Maine camps' experience contrasts with that of a Georgia sleep-away camp where 260 children and staff members - three-quarters of the 344 tested - contracted the virus less than a week after spending time together in close quarters. A CDC study of the Georgia camp found that asymptomatic infection was common and potentially contributed to undetected transmission.

Jeffrey Vergales, a pediatrician and senior author of the Maine study, said the key to limiting spread of the coronavirus and covid-19, the disease caused by the virus, at the four camps was keeping the 1,022 campers and staff members in separate bubbles, or cohorts, the entire time.

That way, "if we had a case, we wouldn't have to scramble to identify the contacts. We knew who they were, and we could very quickly quarantine those contacts," said Vergales, an associate professor of pediatrics at the University of Virginia. He added: "The fact that we had no known secondary spread is luck. The fact that we didn't have an exponential outbreak is planning."

Vergales and the other researchers said their findings have implications for the successful implementation of covid-19 mitigation strategies in other overnight camps, residential schools and colleges. But they and other experts acknowledge that the extensive measures taken by the camps may not be as feasible in K-12 schools, where it's harder to limit student interaction with the community.

Schools and universities that are reopening for in-person classes, or a hybrid approach of in-person and virtual learning, have reported rapidly rising coronavirus cases among students. Some universities have abandoned plans to hold in-person classes amid outbreaks in the first weeks of class.

"As schools reopen, I think it's really useful to have data or case reports from places that have successfully reopened high-risk settings or activities, so I'm glad for this report," said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security. "It sounds like they effectively created a bubble through a combination of serial testing and limiting outside contacts. We've seen other examples of the bubble approach being successful, and this is further evidence of that."

It will be difficult to get the same results in a K-12 school, she said. Students and families remain part of the larger community, "so they won't be able to effectively create a bubble," Rivers said in an email. Physical distancing, masks, hand-washing and keeping students in the same group can reduce transmission, and repeated testing can reduce risk, too, she said. "But it won't be possible to completely wall off new introductions of the virus like the camps were able to do."

The Maine camp sessions ranged from 44 to 62 days from mid-June to mid-August. The camps are not identified, Vergales said, for privacy reasons.

Campers had to quarantine with their families for 10 to 14 days before arriving. About five to seven days before arrival, all but a dozen campers and staff members were tested for the coronavirus. (Those 12 attendees had been diagnosed with coronavirus two months before the start of camp and had completed isolation protocols before arrival.)

Before the start of camp, four attendees tested positive and isolated at home for 10 days. They arrived at camp, remained asymptomatic and did not receive further testing for the duration of camp, according to the report.

After arrival, campers were quarantined in their groups for 14 days, regardless of test results. The groups - ranging from five to 44 people - were formed using bunk assignments or age. Campers and staff members were screened at least once a day for fever and covid-19-associated symptoms.

Daily symptom checks identified one staff member and 11 campers with potential signs of covid-19. They were immediately isolated and tested. Members of their group were quarantined until all 12 tested negative.

An additional three asymptomatic attendees - two staff members and one camper - tested positive after arrival and were isolated for 10 days, and their group members were quarantined for 14 days. After additional testing returned no positive results, everyone was released from quarantine. No secondary spread was identified, the report said.

The CDC said in a statement that the study "reinforces how powerful everyday preventive actions are in reducing and keeping COVID-19 transmission low. . . . Using a combination of proven public health strategies to slow the spread of COVID-19, campers and staff were able to enjoy a traditional summer pastime amid a global pandemic."

Because campers stayed in their bubbles, there was little need for masks unless the groups engaged in activities with other groups or were indoors, Vergales said. Staffers had to wear masks when food was being served.

Vergales, who was a volunteer at the two camps attended by his son and daughter, said he and camp leaders spent March, April and May "literally walking through every camp for every activity where there was risk and then figured out how to minimize risk."

Instead of serving meals to all campers at the same time, meals were staggered and each group of campers was called and dismissed at set times. Instead of family-style meals, buffet lines were set up where counselors served food to avoid campers getting near one another.