An asymptomatic carrier of coronavirus rode an elevator alone, had no symptoms and self-quarantined after travel but managed to infect 71 people.

Intensive contact tracing is revealing how infectious the virus can be, with the US Centres for Disease Control (CDC) analysing the impact of this single traveller.

Coronavirus can spread through droplets, be carried through the air and linger on surfaces as well as be transferred through asymptomatic carriers.

The CDC study did a deep-dive contact tracing effort in Heilongjiang Province, China, which had not reported a new coronavirus diagnosis since March 11.

On the second of April, a man in the area suffered a serious stroke but had not tested positive for coronavirus.

He was rushed to the hospital and his three sons took turns staying by his bedside, furthering infecting 28 people, including a nurse and a doctor.

Before being diagnosed with coronavirus, the stroke victim was taken to a second hospital where he was the source of another 20 infections.

Another man had also presented with coronavirus symptoms and his close contacts were tested. They were also all positive tests.

Everyone the second patient had been in contact with was tested and put into lockdown, but for 50 of them, it was too late.

Contact tracers were urgently trying to find out the source of the outbreak, as there were fears it could have started within the community. 

After tracing it back through the community, they tested the man's girlfriend and her daughter who lived with them, who both tested positive for coronavirus.

The contact tracers then hit a wall, as no one had travelled or had contacts who had travelled. Due to standard genome sequencing of the virus, the tests indicated it had come from overseas, but every chain of contacts had come up blank as no one had travelled.

It was only until they extended their search to people living in the same residential apartment tower that they figured out where the coronavirus cluster had started.

A woman who had recently travelled from the United States and intensely followed self-isolation protocol still tested positive for coronavirus antibodies.

After detailed questioning from the contact tracers, which showed that the women had not come into direct contact with each other, there was only one possible source of contagion.

The lift that was shared by that portion of the apartment block.

The traveller had used it to get into the apartment where she self-isolated for 14 days and had food delivered.

“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.

The researchers are urging people to not become complacent, as the virus is still wreaking havoc. It appears that isolation is the only tool available to stop the spread.

“Continued measures to protect, screen, and isolate infected persons are essential to mitigating and containing the COVID-19 pandemic,” the study reads.

This article originally appeared on Over60.