American schools can reopen safely without teachers being vaccinated as long as masks are worn and pupils stay six feet away from each other when possible, the Centers for Disease Control and Prevention has said.

The US public health authority revealed its new guidelines on Friday for how schools can safely reopen with in-person classes, even as coronavirus cases remain high across much of the country.

Schools in many parts of the US have remained closed either fully or partially since the beginning of the pandemic, as high levels of infection have discouraged school districts and teachers’ unions from allowing in-person instruction.

Joe Biden, the US president, has promised to have most state schools open within his first 100 days — although the White House clarified this week that a school would be considered open even if it only taught in-person one day a week.

Rochelle Walensky, the CDC director, said on Friday that US states should prioritise vaccinating teachers. But she added that safe reopening was possible even without vaccinations if schools took sufficient mitigation measures.

Walensky said: “The science also shows us that K-12 [primary and secondary] schools that have implemented strict mitigation strategies have been able to safely open for in-person instruction and remain open. And today’s operational strategy provides the road map to do this safely.”

The issue of whether schools should open has become one of the most politically fraught aspects of the US pandemic response. Teachers’ unions have argued that teaching in-person would expose them to unnecessary risk, while public health scientists have pointed to growing evidence that schools are not major contributors to the spread of the disease.

According to the most recent figures from Burbio, a school events app which has been gathering data on reopenings, only 40 per cent of US children are attending schools that teach in-person every day. Just over 35 per cent are at schools which are still operating entirely online.

Earlier this week Walensky said vaccinating staff was not a prerequisite to safe reopening. The White House insisted at the time she was speaking in a “personal capacity”, triggering speculation about rifts within the Biden administration over how to approach the issue.

The official guidance on Friday suggests those divisions may have now been healed.

The announcement was also welcomed by one of the country’s largest teaching unions, in a sign that the administration has its backing.

Randi Weingarten, the president of the American Federation of Teachers, said in a statement: “Today the CDC met fear of the pandemic with facts and evidence. For the first time since the start of this pandemic, we have a rigorous road map, based on science, that our members can use to fight for a safe reopening.”

According to the guidelines, schools can open safely in districts with low case numbers as long as everyone wears face masks. But if levels are as high as they currently are in 90 per cent of US counties, children should be kept six feet away from each other if possible, and school sports should be reduced.

The CDC is also recommending regular handwashing, good ventilation and rapid contact tracing should cases occur in schools. All of these remain recommendations, however, and the final decisions about school reopening will be made by state or local education boards, alongside individual schools themselves.