But Covid-19's most devastating weapon is unwitting asymptomatic carriers. It's a recurring story. An enthusiastic nightclubber single-handedly causing a fresh lockdown in Seoul.
An energetic partygoer in California infected dozens of his family and friends. Now, evidence shows even careful, conscientious unknowing carriers can be the source of serious contagion.
Superspreader
The CDC study centres on a deep-dive contact tracing effort in Heilongjiang Province, China in mid-April. The province had not reported a new Covid-19 diagnosis since March 11.
On April 2 a man in the area suffered a serious stroke – a known possible symptom of the disease – but he hadn't been diagnosed.
He was rushed to hospital and his three sons went with him and 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 stroke victim was taken to a second hospital to recuperate. There, he was the source of a further 20 infections. It was only then that the alarm was raised.
And it wasn't because of the stroke victim. Another man (Patient B) had presented with Covid-19 symptoms. All of his close contacts were tested. They also proved positive – one of them was the stroke victim.
Immediately, everyone the patient had been in contact with, or even close to while in the hospitals, was tested and put into lockdown. But for almost 50 of them, it was already too late. And some of these had passed the virus on even further.
Contagion chain
Contact tracers were racing against time to find the source. The pandemic's spread had to be halted. Was there a new, hidden outbreak already entrenched in the community?
Patient B had met the stroke victim and his sons at a party on March 29. But nobody else who attended tested positive. So where had he got it?
The city was not in lockdown. Small gatherings were allowed. Many businesses were open.
But contagion was usually through close contact. So they tested his girlfriend. She had it. Her daughter, who lived at home, also had it.
And he had visited them on March 26. Here the contact tracers hit a brick wall. None had travelled. None had contacts who had travelled. And no further cases of COVID could be linked to them. But the virus couldn't just materialise out of thin air.
Danger zone
The contact tracers had conducted standard genome sequencing of the virus. It was quickly evident it was different to that already known to be in circulation.
That indicated it had come from overseas.
But every chain of contacts and associations among the sufferers came up blank. Nobody had been overseas or even knew someone who had been.
By April 9, the contact tracers were becoming desperate. So they called up the profiles of everybody living in the same residential apartment tower as the earliest known sufferers.
One, they realised, had recently travelled from the United States – and she lived on the floor above the woman and her daughter.
A quick assessment revealed her to be well. And she's been following self-isolation protocol to the letter. But a swab revealed she had Covid-19 antibodies.
Detailed questioning and movement tracking showed the women had not come in direct contact with each other. Which left just one possible source of contagion.
The lift that portion of the apartment block shared – the asymptomatic traveller had used it to get to the apartment where she had self-isolated.
"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.
Complacency challenge
The study reveals how insidious Covid-19 can be. It also demonstrates how vital contact tracing remains in containing the pandemic. Lockdown alone is not enough.
Previous studies have demonstrated that Covid-19 can linger on some surfaces for days, particularly if that surface is stainless steel or plastic, and in an enclosed space.
The traveller had used the elevator alone. So there had been no direct contact. In this case, the researchers believe the daughter who lived on the floor below must have touched her face, eyes or nose after later using the same buttons or leaning against the same panels.
The virus had been contained in the city at the time. A measure of complacency had set in.
The researchers say people had begun to wash their hands less and take social distancing limitations less seriously. So once the virus slipped through the cracks, it found easy channels through which to spread.
And while the virus found a way to escape social isolation and self-quarantine measures, isolation remains the only tool available to choke its spread.
"Continued measures to protect, screen, and isolate infected persons are essential to mitigating and containing the Covid-19 pandemic," the study reads.
"Our results illustrate how a single asymptomatic SARS-CoV-2 infection could result in widespread community transmission," the study authors concluded.
