Researchers reported on Tuesday that a woman returning from the US to China carried the coronavirus back with her, causing an epidemic that infects at least 71 people.

The researchers started with an elevator and took place in the Emerging Infectious Diseases journal of the Centers for Disease Control and Prevention.

The woman returned to Heilongjiang Province from her trip to the United States on March 19, about a week after the last case of coronavirus was diagnosed in the province. He had no symptoms and tested negative for the coronavirus, but was asked to quarantine at home.

No one suspected anything wrong until a man with no obvious contact with the traveler suffered a stroke. It turns out that a neighbor who lives in the same building as the traveler is at a party with relatives.

When the researchers in China finally put the story together, they determined that the passenger must have somehow contaminated the elevator in his building. His sub-neighbor, who used the same elevator, was also infected and, in return, was also infected by his mother and boyfriend. They went to a party and infected the stroke patient and his sons.

The patient with stroke and his two sons went to two hospitals. At least 28 people were infected in the first hospital, and 20 more were infected when they moved to a second facility.

When the Chinese Center for Disease Control and Prevention investigated the genetic structure of the virus found in this new outbreak, they discovered it was different from the strains seen in China before. Viruses in patients were genetically the same or at least very similar, which made them believe that the virus came from overseas.

When the investigators found out that one of the cases had their neighbor recently returned from abroad, they tested it again. It was not currently infected, but had antibodies to the virus that showed a previous infection. They set it as A0, so it was the first case. “Therefore, we believe that A0 is an asymptomatic carrier, and B1.1 (its neighbor) both touch the surfaces in the elevator in the building where they live,” they wrote. All other residents in the building tested negatively.

“As of April 22, 2020, A0 remained asymptomatic and a total of 71 SARS-CoV-2 positive cases were detected in the cluster,” the researchers said.