During the 2020-21 school year, a study from Salt Lake County, Utah, published in the CDC’s Morbidity and Mortality Weekly Report, found only four positive cases among 735 students tested while in quarantine, a transmission rate of 0.7%. A study from St. Louis County, Mo., also published in MMWR, found no positive cases among quarantined students at a time when the total countywide two-week case count was 711 per 100,000 people (the CDC classifies a one-week rate of 100 per 100,000 as “high”).

In mid-September 2021, the Los Angeles Unified School district reported that, since the school year began, 30,000 students had been quarantined and only 63 tested positive—a 0.2% positivity rate. That’s about half the rate at which the district’s students turned up positive in large-scale surveillance testing—meaning that random quarantines would have been expected to turn up more cases. The district changed its policy to test-to-stay: Asymptomatic students suspected of exposure may remain in school, subject to testing every few days for a week, along with other restrictions. Parents are still directed to keep them at home outside school hours.

Test-to-stay dramatically reduces the educational costs of quarantine. Utah reported that it saved 109,752 school days across 13 high schools from November 2020 to March 2021. But many states are sticking with the CDC guidelines, and in September the agency looked at numerous studies on quarantined students and sidestepped the discussion. At an Oct. 13 White House press briefing, Director Rochelle Walensky concluded only that “masks are working.”