Temperature and COVID-19 symptom checks like the ones used at schools and doctor’s offices have again proved inadequate for spotting coronavirus infections and preventing outbreaks.

A study of Marine recruits found that despite these measures and strict quarantines before they started training, the recruits spread the virus to others even though hardly any of them had symptoms. None of the infections were caught through symptom screening.

The study, published Wednesday in the New England Journal of Medicine, has implications for colleges, prisons, meatpacking plants and other places that rely on this sort of screening to detect infections and prevent outbreaks.