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.
The super-spreader, who had not shown any symptoms, had gone straight into isolation upon return from travelling abroad.
But the woman had used the lift of the apartment block before going into quarantine for just a minute - which was enough for the virus to be spread to a cluster of 71 different people in a domino effect.
Contact tracers tried to work backwards to discover how patients had contracted the disease.
Experts had been trying to find the source after a man was rushed to hospital when he suffered a severe stroke - a rare symptom of Covid-19, the Daily Star reports.
He ended up having undiagnosed coronavirus and his three sons took turns staying by the man’s side.
They unknowingly infected another 28 people between them, 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.
One of the man's friends, known as Patient B, reported Covid-19 symptoms and scientists discovered the strain had come from overseas.
Patient B had met the stroke victim and his sons at a party on March 29, but no-one else who attended tested positive for the virus.
By April 9, the contact tracers were becoming desperate, so they called up the man's girlfriend who lived at home with her daughter - but neither had the virus.
Frantic contact tracers then reached out to everybody living in the same residential apartment tower as the two women.
They finally discovered an asymptomatic woman who had recently travelled from the US and lived on the floor above the woman and her daughter.
It is thought she had contracted the virus before and contaminating the elevator on her return home.
“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.
“Our results illustrate how a single asymptomatic SARS-CoV-2 infection could result in widespread community transmission,” the study authors concluded.
