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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.

“We spent a lot of time putting measures like that in place and they’re probably not worth the time as we had hoped,” said Jodie Guest, a public health researcher at Atlanta’s Emory University who had no role in the research.

“Routine testing seems to be better in this age group” because younger adults often have no symptoms, she said.

The study was led by researchers from the Icahn School of Medicine at Mount Sinai in New York and the Naval Medical Research Center.

It involved 1,848 Marine recruits, about 90% of them men, who were told to isolate themselves for two weeks at home, then in a supervised military quarantine at a closed college campus, The Citadel in Charleston, South Carolina, for two more weeks. That included having a single roommate, wearing masks, keeping at least 6 feet apart and doing most training outdoors. They also had daily fever and symptom checks.

The recruits were tested for coronavirus when they arrived for the military quarantine and 7 and 14 days afterward. Sixteen, or about 1%, tested positive on arrival and only one had any symptoms. Another 35 — an additional 2% — tested positive during the two-week military quarantine and only four had symptoms.

Only recruits who tested negative at the end of both quarantine periods were allowed to go on to Parris Island for basic training.

Genetic testing revealed six separate clusters of cases among the recruits.

“A lot of the infection that occurs, we don’t even realize it is occurring,” said one study leader, Navy Cmdr. Andrew Letizia, a doctor at the Naval Medical Research Center.

The quarantine measures and adherence to them were far more strict than what would occur at a typical college campus, said another study leader, Mount Sinai’s Dr. Stuart Sealfon.

“This is a really infectious virus. You really need to use a combination of good public health measures, temperature checks, mask wearing, social distancing, hand washing … and comprehensive testing” to prevent spread, he said.

Previously, federal officials said a screening project to check temperatures and symptoms at U.S. airports caught fewer than 15 cases out of 675,000 travelers. It’s not known how many cases were missed, just that very few were detected.

A separate study published Wednesday in the New England journal reports on an outbreak last spring on the aircraft carrier USS Theodore Roosevelt. Among the crew of 4,779, mostly young people, 1,271 became infected; 77% did not show symptoms when diagnosed and 55% never developed any.

The case shows that “young, healthy persons can contribute to community spread of infection, often silently,” Dr. Nelson Michael of the Walter Reed Army Institute of Research wrote in a commentary.