A woman unwittingly became the source of at least 71 Covid-19 patients after using the lift in her tower block soon after her return to China from a trip abroad, researchers in China believe.

The traveller returned to her home in Heilongjiang province, China, from a trip to the United States on 19 March – eight days after the area last reported any new Covid-19 cases.

She did not have any symptoms of the disease and tested negative for the virus, but self-isolated at home alone anyway, researchers wrote in the Chinese Centres for Disease Control and Prevention’s journal Emerging Infectious Diseases.

The woman had not taken the lift with anyone else – but her downstairs neighbour used it at some point after her.

On 29 March the downstairs neighbour’s mother and her boyfriend, who had visited her flat, attended a party with another group of people. Then, on 2 April, one of that group suffered a stroke and was taken to hospital; however, at this point there was no obvious connection between him and the traveller and he was not tested for coronavirus.

Researchers later concluded that the traveller must have contaminated the lift in her building. Her downstairs neighbour is thought to have contracted the virus while using it, before infecting her mother and mother’s boyfriend when they visited her home. The couple then infected the stroke patient and his two sons at the party.

The two sons took it in turns caring for their father in hospital, who was then moved to a different hospital on 6 April. The stroke patient and his sons all later tested positive and researchers discovered they had infected 28 people in the first hospital, including five nurses and one doctor. They went on to infect another 20 people in the second hospital.

The boyfriend of the downstairs neighbour’s mother displayed Covid-19 symptoms and became the first case in the cluster to test positive for the disease.

When investigators learned someone in the block had recently travelled, they tested the traveller again and this time she was found to have antibodies, suggesting she had previously had the disease.

Researchers from the Chinese Centres for Disease Control and Prevention wrote that they believed that the traveller was an asymptomatic carrier and that the downstairs neighbour was infected after coming into contact with surfaces in the building’s lift.

“Our results illustrate how a single asymptomatic Sars-CoV-2 infection could result in widespread community transmission.”