The US Centers for Disease Control and Prevention on Friday issued new guidelines urging schools to reopen safely and as soon as possible, while offering a detailed plan for limiting the spread of Covid-19.

The strategy emphasizes universal masking, handwashing, and disinfection, as well as contact tracing, but spent relatively little time on the subject of ventilation, despite this being the virus' main transmission route.

While it recommends vaccination for teachers and staff as soon as supply is available, it stops short of saying it is necessary -- a divisive issue among teachers' unions.

"CDC is releasing an operational strategy for K - 12 schools, through phased mitigation that provides a pathway to support schools in opening for in person instruction and remaining open," the agency's director Rochelle Walensky told reporters.

The document recommends different approaches depending on the level of community transmission in a given area, defined by the number of new cases per hundred thousand in the past seven days.

Because younger children are at lower risk than older children, it recommends that in areas of substantial transmission (50 to 99 new cases per 100k), middle and high schools should switch to hybrid learning.

Where transmission is defined as high (more than 100 new cases per 100k), middle and high schools should move to virtual instruction only unless the schools are already open, have few cases and are implementing mitigation strategies.

In-person learning is emphasized over sports and school events, and schools are strongly encouraged to use cohorting or podding to facilitate contact tracing.

In developing the plan, the CDC said equity considerations were a key driver.

"The absence of in-person educational options may disadvantage children from low-resourced communities, which may include large representation of racial and ethnic minority groups, English learners, and students with disabilities," the plan said.

During a briefing call, Walensky said that while children can become infected with the coronavirus and contract Covid-19, "less than 10 percent of Covid-19 cases in the US have been among children and adolescents between the ages of five and 17."

What's more, children and adolescents are thought to be less commonly infected with the virus than adults, and in-person learning in schools has not been associated with substantial community transmission.

"Students are not the primary sources of exposure to SARS-CoV-2 among adults in school settings," she said.

"Evidence suggests that staff to staff transmission is more common than transmissions from students to staff, staff to students, or students to students."

It remains to be seen how powerful teachers unions in major cities respond to the guidelines.

Schools in politically liberal parts of the country have been more reluctant to open than in areas that are politically conservative.