Silent coronavirus carriers ‘unlikely to set off another wave of infections’

Laura Zhou

Asymptomatic patients are unlikely to trigger another major outbreak of a pandemic coronavirus given their low transmission rates, according to a Chinese epidemiologist.

In Beijing on Thursday, Wu Zunyou, from the Chinese Centre for Disease Control and Prevention, said research indicated that while one confirmed coronavirus patient could infect three people, an asymptomatic patient passed the virus on to “less than one person”.

Wu’s assessment was based on work by researchers at the disease control centre in Ningbo, in eastern China’s Zhejiang province, who found that only 4.4 per cent of confirmed cases were caused by asymptomatic carriers.

As well trying to rein in imported cases of the virus, China is shifting attention to tackling “silent” carriers. By Wednesday, China had 1,075 asymptomatic patients still under medical observation, with 55 new asymptomatic cases.

Wu said that in a separate study disease control officials in “a southern Chinese province” tracked more than 1,500 close contacts of asymptomatic patients, and found that only seven were infected.

But further investigation also showed the seven also had contact with other symptomatic coronavirus patients.

“So we cannot fully conclude that they were infected by asymptomatic patients,” Wu said.

It is still not clear what role asymptomatic transmission is playing in the global pandemic. But the South China Morning Post reported earlier that as many as one-third of those who test positive could be “silent carriers” with delayed or no symptoms.

Classified government data seen by the Post showed that by the end of February more than 43,000 people in China had been confirmed with Covid-19, the disease caused by the coronavirus, but had no immediate symptoms.

Wu admitted that more investigation was needed to better understand the transmission of the pathogen.

Fears over people who contract the virus but show no immediate symptoms – a condition typically known as asymptomatic – have been growing in recent weeks, with concerns that they could set off a second outbreak in China.

On Tuesday, a county in central China’s Hunan province announced a total lockdown after reports of a group of asymptomatic infections in the area. The county’s roughly 600,000 residents were told to stay home and all businesses, except utilities, medical suppliers, logistics companies and food processing firms, were shut down.

While China does not include asymptomatic cases in its official tally of confirmed cases, it did start releasing numbers for the category on Wednesday for the first time since the coronavirus outbreak was reported in late December.

Meanwhile, China has stepped up efforts to contain imported cases, including a sharp reduction in the number of international flights and a mandatory 14-day quarantine for arrivals from other countries.

China has said that patients who test positive but show delayed or no symptoms are treated in the same way as confirmed cases – they cannot be released from quarantine until they test negative twice.

Wu said such measures were expected to cut asymptomatic spread, given that an asymptomatic patient would have either had close contact with another infected person or had a history of travel to an outbreak area.

“Nobody is infected without a cause,” Wu said. “What we are doing will help us fast track and control symptom-free patients so the possibility of them causing an outbreak will be small.”

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