Testing of throat swabs from two people with no symptoms of coronavirus illness revealed that they were nonetheless infected with the virus, according to a report from Germany published Tuesday in the New England Journal of Medicine.

The finding, though previously suspected, would make the job of containing the virus significantly more difficult.

“We discovered that shedding of potentially infectious virus may occur in persons who have no fever and no signs or only minor signs of infection,” concluded the paper, authored by 20 scientists, physicians, and public health officials in Frankfurt.

Purported evidence of asymptomatic transmission that was reported out of Munich — and also published in the New England Journal of Medicine — was debunked by further investigation. A seemingly healthy Chinese businesswoman who was linked to infections of several colleagues did have coronavirus, but her symptoms were mild and she was taking medicine to keep her fever down.

But asymptomatic transmission is theoretically possible — people can “shed” certain viruses they are carrying without showing symptoms — so experts have been vigilant for signs of that phenomenon with COVID-19.

The U.S. Centers for Disease Control and Prevention on Tuesday said it believes such transmission occurred among several hundred U.S. citizens who were evacuated on Sunday from the Diamond Princess cruise ship, quarantined in a Japanese harbor. Fourteen of the evacuees tested positive for the virus. All the passengers were flown to military bases in Texas and California, where they are quarantined.

“CDC believes the rate of new infections on board, especially among those without symptoms, represents an ongoing risk,” it said in a news release. “Therefore, to protect the health of the American public, all passengers and crew of the ship have been placed under travel restrictions, preventing them from returning to the United States for at least 14 days after they had left" the cruise ship.

Currently, more than 100 U.S. citizens are still on the ship or in hospitals in Japan, the CDC said.

The new report involved a group of 126 German citizens who had been staying in Hubei province, China, where the epidemic emerged and is concentrated. On Feb. 1, they were evacuated and flown to Frankfurt, where they were to be quarantined for 14 days, believed to be the virus’ maximum incubation period.

Ten passengers who had symptoms or were at high risk of infection were kept in a special isolation area on the plane. The remaining 116 patients — including 23 children — were evaluated by a team of doctors at the Frankfurt airport. Only one of them had a fever and was transferred to a hospital, and tested negative for COVID-19.

The doctors decided to offer throat swabs to test for the virus even in the 115 passengers with no symptoms, and all but one consented.

Two of those passengers without symptoms tested positive, and were transferred to the hospital for further evaluation and monitoring. Although one developed a faint rash and a mild sore throat, both remained well and fever-free seven days after hospital admission, wrote the authors from University Hospital Frankfurt and the city health protection agencies.

“In this effort to evacuate 126 people from Wuhan to Frankfurt, a symptom-based screening process was ineffective at detecting” coronavirus in two people later found to be carrying the virus, the authors wrote.

As of Feb. 13, the World Health Organization reported a total of more than 74,000 cases, most of them in China, and 1,386 deaths, two outside China.

The virus has spread to 24 countries. No cases have been confirmed in Pennsylvania or New Jersey, but more U.S. cases are expected among the travelers who have returned from China and remain under quarantine, the CDC has said.