Virus’ ‘silent spreaders’

People not displaying symptoms could be a major driver of the spread of coronavirus.

A study published in the journal Science found that before China’s lockdown on 23 January, nine out of 10 cases of the pandemic were passed on by people who showed no symptoms of the illness.

The study’s lead, Columbia University infectious diseases researcher Dr Jeffrey Shaman, says these “stealth transmissions” accelerate the spread of the disease and help explain why it is more difficult to combat than previous viral outbreaks such as SARS.

A Lancet study of transmissions in Hong Kong suggested that the virus can be transmitted easily, even when symptoms are relatively mild. This could account for the fast-spreading nature of this epidemic, the study found.

Another study, examining 91 cases in Singapore, found that 48 per cent of patients caught the virus from people showing mild or no symptoms.

And The New York Times has reported that the United States Centres for Disease Control and Prevention believes up to 25 per cent of people infected in the pandemic may not show symptoms.

This is important because carriers are believed to most contagious about one to three days before they begin to show symptoms. 

Dr Shaman says people with mild infections are taking the virus into the community and spreading it far and wide.

He is not alone.

“People think, ‘If I don’t feel bad, I don’t have it and can’t give it to anyone’, and that is now misguided thinking,” Chad Petit, PhD, told

Chad Petit, an assistant professor in the department of biochemistry and molecular genetics at the University of Alabama at Birmingham School of Medicine, says the onset of symptoms is often delayed.

“The virus is propagating in your body, but your immune system has not recognised that something is going on systemically,” says Prof. Petit. “That’s why you don’t get a fever right away. Just because you don’t have symptoms doesn’t mean you don’t have the virus.”

Unwitting carriers – becoming known as ‘silent spreaders’ – are rarely truly asymptomatic. They are likely to eventually feel the effects of the illness.

If carriers can transmit the virus to others, even when feeling healthy, keeping only sick people in isolation will not halt the contagion.

So, what should we do?

ProPublica’s Caroline Chen says increased testing is vital because it can let people know if they’re sick before symptoms emerge and prompt them to self-isolate.

“At a big picture level, testing helps public health officials know where the disease is spreading and better allow them to direct resources and response efforts.”

New York Times medical correspondent Apoorva Mandavilli concluded that social distancing is still the key policy.

“Rapid tests for infection might help detect people, especially healthcare workers, who are infected yet feel normal. Masks may help. But experts kept returning to social distancing as the single best tool for stopping the chain of transmission in the long term – not lockdowns, necessarily, but cancelling mass events, working from home when possible and closing schools.”

Jeremy Howard, research scientist at the University of San Francisco, believes getting as many people as possible to wear masks is vital.

“You might walk into stores over the next few days and sicken dozens without knowing it. Some might die. Others will think they are dying before they recover,” he wrote in The Guardian.

“Almost any kind of simple cloth covering over your mouth, such as a homemade mask, or even a bandanna, can stop the (COVID-19) assassin in its tracks.”

There is debate about the effectiveness of many masks as a protective device, and a need to prioritise appropriate protective gear for front-line medical workers.

But Jeremy Howard’s rallying cry is emblematic of the coronavirus crisis. It is aimed at helping those who are unaware they have the virus from spreading it.

We must think about others as much as ourselves.

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Written by Will Brodie


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