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Surprising story of Infection #2

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Tabitha111 View Drop Down
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    Posted: July 09 2020 at 11:26am 

Coronavirus: The mystery of asymptomatic 'silent spreaders'

By David Shukman

Science editor

As the crisis has unfolded, scientists have discovered more evidence about a strange and worrying feature of the coronavirus. While many people who become infected develop a cough, fever and loss of taste and smell, others have no symptoms at all and never realise they're carrying Covid-19.

Researchers say it's vital to understand how many are affected this way and whether "silent spreaders" are fueling the pandemic.

When people gathered at a church in Singapore on 19 January, no-one could have realized that the event would have global implications for the spread of coronavirus.

It was a Sunday and, as usual, one of the services was being conducted in Mandarin. Among the congregation at The Life Church and Missions, on the ground floor of an office building, was a couple, both aged 56, who'd arrived that morning from China.

As they took their seats, they seemed perfectly healthy so there was no reason to think they might be carrying the virus. At that time, a persistent cough was understood to be the most distinctive feature of Covid-19 and it was seen as the most likely way to transmit it. Having no symptoms of the disease should have meant having no chance of spreading it.

The couple left as soon as the service was over. But shortly afterwards, things took a turn for the worse, and in a wholly confusing way. The wife started to become ill on 22 January, followed by her husband two days later. Because they had flown in from Wuhan, the epicenter of the outbreak, that was no big surprise.

But over the following week, three local people also came down with the disease for no obvious reason, leading to one of Singapore's first and most baffling coronavirus cases. Working out what had happened would lead to a new and disturbing insight into how the virus was so successfully finding new victims.

Mobilizing 'disease detectives'

"We were extremely perplexed," says Dr Vernon Lee, head of communicable diseases at Singapore's Ministry of Health. "People who didn't know one another somehow infected each other," while showing no sign of illness. This new batch of cases simply did not make sense, according to what was known about Covid-19 back then.

So Dr Lee and his fellow scientists, along with police officers and specialist disease trackers, launched an investigation, generating detailed maps showing who was where and when. This involved the very best of the process known as contact tracing - a version of which is getting under way now in the UK. It's seen as a vital system for tracking down everyone involved in an outbreak and helping to stamp it out, and Singapore is renowned for the skill and speed with which this is carried out.

Amazingly, within a few days, investigators had spoken to no fewer than 191 members of the church and had found out that 142 of them had been there that Sunday. They quickly established that two of the Singaporeans who became infected had been at the same service as the Chinese couple.

"They could have spoken to each other, greeted each other, during the usual activities of a church service," says Dr Lee.

That was a useful start and would explain in theory how the infection could have been passed on, apart from one key factor. It did not answer the crucial question of how the virus could have been transmitted by the two Chinese people when at that stage they had shown no indication of having the disease.

And on top of that was an even greater puzzle. It was confirmed that the third Singaporean to become infected, a 52-year-old woman, had not been at the same service as the others. Instead she had attended another event in the same church later that day, so how could she have picked up the virus?

Evidence no-one expected

Investigators resorted to going through the CCTV recordings made at the church that Sunday to search for clues. And they stumbled across something completely unexpected - the woman who'd attended the later service, after the Chinese couple had left, had sat in the seats they had used several hours earlier.

Somehow, despite having no symptoms and not feeling ill, the Chinese husband and wife had managed to spread the virus. Maybe they'd had it on their hands and touched the seats, maybe their breath carried the infection and it landed on a surface, it's not clear, but the implications were huge.

For Dr Lee, piecing everything together, there was only one possible explanation - that the virus was being passed by people who had it without even realizing. This was a revelation that would be relevant the world over because the central message of all public health advice on coronavirus has always been to look out for symptoms in yourself and others.

But if the virus was also being spread by people without symptoms, silently and invisibly, how could the disease be stopped? He remembers the moment, while working in his office, when the reality dawned on him. "Every time you make a scientific discovery, it is like a 'eureka' moment when you realize that this is something important that you've uncovered, through the hard work of many individuals and teams."

Spread before symptoms show

What was revealed was what's known as "pre-symptomatic transmission" where someone is unaware of being infected because the cough, fever and other classic symptoms have yet to begin.

Along with many others, this study highlighted a critical period of 24-to-48 hours before the visible onset of the disease in which people can be highly infectious, perhaps even their most infectious.

Being aware of that is potentially invaluable, because as soon as you realize you're ill then everyone you've been in close contact with can be warned to stay at home.

That would mean that they would be isolating during the key phase of infection before their own symptoms start. But exactly how the disease can be transmitted without a cough to project droplets containing the virus is still open to debate.

One option is that simply breathing or talking to someone can do the job. If the virus is reproducing in the upper respiratory tract at that time then it's possible that some of it will emerge with each exhalation. Anyone close enough, especially indoors, could easily pick it up.

And another potential form of transmission is by touch - the virus gets onto someone's hands and they touch another person or a door handle - or a seat in a church. Whatever the route, the virus is clearly exploiting the fact that people are bound to be less vigilant if they're not aware that they might be infected.

Some people never show symptoms

This is an even more mysterious scenario, and one that scientists simply have no definitive answer to. It's one thing to know that people can be infectious before their symptoms show, quite another when they become infected but never have any sign of it.

This is what's known as being "asymptomatic" because you are a carrier of the disease but do not suffer in any way yourself. The most famous case is that of an Irish woman who was working as a cook in New York at the beginning of the last century.

Wherever Mary Mallon was employed, in house after house, people became ill with typhoid and at least three, maybe many more, died of it, but she was completely unaffected. Eventually a connection was established and it was confirmed that she was the unwitting spreader of the disease.

Reporters dubbed her "Typhoid Mary", a label she always resented, but the authorities took no chances and kept her in confinement for 23 years until her death in 1938.

[more at link]

'A man who does not think and plan long ahead will find trouble right at his door.'

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