A woman with coronavirus who had no symptoms infected 71 people when she took a 60-second trip in a lift on her own, according to a new study.

Contact tracing experts were left baffled when trying to locate the source of an outbreak that spread across two hospitals in Heilongjiang Province, China, in mid-April.

They discovered a woman, who had recently travelled to the US, was patient zero of a cluster in the province which had not reported a new Covid-19 infection in a month.

The study, released by the US Centers for Disease Control (CDC), reveals the impact a single person with no symptoms can have on the spread of the deadly bug.

Coronavirus contact tracing experts were left baffled when trying to locate the source of an outbreak that spread across two hospitals

The origins of the case can be traced back to when a man suffered a severe stroke who had coronavirus but hadn’t been diagnosed, the Daily Star reports.

The man was rushed to the hospital, and his three sons took turns staying by the man’s side.

Between them, they unknowingly infected another 28 people, including five nurses and a doctor.

Before being diagnosed, the man was taken to another hospital to recover from his stroke, where 20 more people became infected.

By April 9, the contact tracers were becoming desperate, so they called up his girlfriend and her daughter who lived at home who had the virus.

Desperate contact tracers reached out to everybody living in the same residential apartment tower as the earliest known sufferers.

One person had recently travelled from the US lived on the floor above the woman and her daughter.

The woman had used the lift of the apartment block where the virus had been picked up.

“Therefore, we believe A0 (the traveller from the US) was an asymptomatic carrier and that B1.1 (the daughter) was infected by contact with surfaces in the elevator in the building where they both lived,” the researchers wrote.