A model developed by researchers at the US Centers for Disease Control and Prevention shows that people who don’t show symptoms for Covid-19 are responsible for over half of all the cases of SARS-CoV-2 transmission. The data, collected from multiple Chinese studies, was used to determine the proportion of presymptomatic people (who shed virus before developing symptoms), asymptomatic, and symptomatic people in the population. The findings from the model were published on 7 Jan in the journal JAMA Network Open. The model reiterates what earlier studies have found, indicating that asymptomatic individuals contribute to the rapid transmission of Covid-19.

According to the recent model, 59 percent of all transmissions of the novel coronavirus starts with asymptomatic individuals, of which 35 percent of new cases begin with presymptomatic people, who infect others before any symptoms of Covid-19 appear.

“30 percent of individuals with infection never develop symptoms and are 75 percent as infectious as those who do develop symptoms,” the authors wrote in the study. “These findings suggest that measures such as wearing masks, hand hygiene, social distancing, and strategic testing of people who are not ill will be foundational to slowing the spread of Covid-19 until safe and effective vaccines are available and widely used,” they added.

Earlier, the World Health Organisation and the CDC had noted that some people who have Covid-19 might not appear till 14 days after being infected.

The asymptomatic transmission has been the subject of studies from the early days of the Covid-19 pandemic. A study published on 12 March in the peer-reviewed medical journal Eurosurveillance found that 17.9 percent of the 3,711 passengers and crew members aboard the Diamond Princess cruise ship – the site of a major Covid-19 superspreader event – were asymptomatic. Even if the symptomatic cases were isolated — quarantined till they recovered or tested negative for the coronavirus — “a vast outbreak” would have unfolded anyway, the study concluded.

‘Silent transmission’ was the word used in a 6 July study in Proceedings of the National Academy of Sciences, referring to the phenomena of Covid-19 infections by asymptomatic and presymptomatic individuals. The recent CDC analysis adds to mounting evidence that testing and isolating symptomatic Covid-19 patients alone won’t curb the spread of SARS-CoV-2 in the community.

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