First, testing can estimate the prevalence of Covid-19 in a school, a key indicator of transmission risk. A positive test rate of 0.3%, for example, suggests that, in a school of 700 kids, about two are infected. Tracking prevalence allows a school to react swiftly to changes in the number of infected people who enter it. When these numbers rise, enhanced measures like strict cohorting of students within small-class groups or using higher-quality masks may be needed to reduce exposures and prevent outbreaks.

But it’s important not to directly compare the positivity rate in a school with that in the surrounding community because almost all those being tested in a school are asymptomatic while many who seek testing in the community do so because they have symptoms or have been exposed to someone with Covid-19.

Second, contact tracing can provide a better understanding of in-school transmission. When a case of Covid-19 is identified, a school should provide rapid antigen or PCR testing to the individual’s contacts. This can reduce the disruption associated with quarantines. In a “test to stay” strategy, for example, as implemented in Utah and the United Kingdom and planned in Massachusetts, exposed contacts can stay in school provided they use regular rapid tests and isolate only if positive.