New research confirms that temperature and symptom checks miss many coronavirus infections. A study published Wednesday found that these measures failed to detect infections in new Marine recruits before they started training, even after several weeks of quarantine.

Many recruits had no symptoms yet still spread the virus. Genetic testing revealed six separate clusters of cases among the recruits.

The work has implications for colleges, prisons, meatpacking plants and and other places that rely on symptom screening. Doctors say more COVID-19 testing is needed, especially in younger people who often don't develop symptoms.

An estimated 20% of coronavirus infections do not result in symptoms. The good news is those who are asymptomatic generally do not spread the virus as easily as those who have symptoms, according to a University of Minnesota study.