COLUMBUS, Ohio (AP) — The Ohio Department of Health has recommended that local health departments shift their practice of COVID-19 contact tracing to a model that follows clusters of cases and not individual diagnoses.

Schools can also discontinue universal contract tracing but should still plan to help health departments trace clusters of a coronavirus outbreak. according to a memo sent to all districts and health departments earlier this week.

The memo instructs local health departments to focus on outbreaks in crowded settings such as homeless shelters, correctional facilities and nursing homes. What constitutes a school outbreak will likely be a judgment call by experts, Dr. Bruce Vanderhoff, the Health Department director, said Thursday.

“When you have 10 cases in a very small school, that looks a whole lot different than 10 cases in a very, very large school, or large district,” he said. “There’s no hard and fast number.”

Schools are still encouraged to follow state guidelines that allow students to stay in class or participate in extracurricular activities after possible coronavirus contact as long as they follow certain masking and testing protocols.