NEW YORK • When the schools in Marietta, Georgia, opened their doors on Aug 3, the highly contagious Delta variant of Covid-19 was sweeping across the US South, and children were not being spared.

By Aug 20, 51 students in the city's small school district had tested positive for the coronavirus. Nearly 1,000 others were flagged as close contacts and had to quarantine at home for seven to 10 days.

"That's a lot of school, especially for children who are recovering from 18 months in a pandemic where they missed a lot of school or had to transition to virtual," said Dr Grant Rivera, the superintendent of Marietta City Schools.

Last week, the district changed tack. Students who are identified as close contacts can now continue attending school as long as they have no symptoms and test negative for the coronavirus every day for seven days.

An increasing number of US school districts are turning to testing to keep more children in the classroom and avoid disrupting the work lives of their parents.

The resource-intensive approach - sometimes known as "test to stay" or modified quarantine - allows students who have been exposed to the virus to stay in school as long as they take frequent Covid-19 tests, which are typically provided by the school, and adhere to other precautions.

Experts agree that children who are infected with the virus should isolate at home, but the question of what to do about their classmates poses a dilemma.

Allowing children who have been exposed to the virus to remain in school does pose a potential transmission risk, and the Centres for Disease Control and Prevention (CDC) says that it "does not have enough evidence" to support the approach.

Instead, it recommends that close contacts who have not been fully vaccinated quarantine for as long as 14 days. Vaccinated close contacts can remain in the classroom as long as they are asymptomatic and wear a mask, according to the agency's school guidance.

"At this time, we do not recommend or endorse a test-to-stay programme," the CDC said in a statement to The New York Times. The agency added: "However, we are working with multiple jurisdictions which have chosen to use these approaches to gather more information."

The CDC guidelines mean that in some cases, especially in classrooms where students are not vaccinated, masked or socially distanced, a single case of Covid-19 can force a dozen or more students out of school. New York City's school guidelines are even more stringent, stipulating that all unvaccinated students must quarantine for seven to 10 days if one of their classmates contracts the virus.

With the new academic year in the US barely under way, some districts in Florida, Louisiana, Missouri and other Covid-19 hot spots have already had to quarantine hundreds or even thousands of students. In mid-August, Mississippi had nearly 30,000 students in quarantine, according to data reported to the state.

A new study, which was published last week in The Lancet, suggests that the test-to-stay approach can be safe. The randomised controlled trial included more than 150 schools in Britain, and found that case rates were not significantly higher at schools that allowed close contacts of infected students or staff members to remain in class with daily testing than at those that required at-home quarantines.

Roughly 2 per cent of school-based close contacts ultimately tested positive for the virus, researchers found, which means that schools were keeping 49 uninfected students out of class every time one student tested positive.

"When you put that in the broader context of what we're doing in society, it's putting a pretty strong penalty on young people, I think," said Dr Bernadette Young, an infectious diseases expert at the University of Oxford and a lead author of the paper.

This summer, the United Kingdom announced that children identified as close contacts no longer needed to be quarantined, although it encouraged them to be tested for the virus.

As school officials embark on a third pandemic academic year, many say the time has come for a new approach.

"The philosophy of this is: How can we keep healthy kids in school and sick kids at home?" said Mr Isaac Seevers, the superintendent of Lebanon City Schools in Ohio, which is preparing to start a test-to-stay programme.

"I think there's some real optimism that this is a game-changer for how we learn to live with Covid-19."

NYTIMES