Researchers say that SARS-CoV-2’s rapid spread is likely due to the movements of people with no or very mild symptoms — namely, those who are unaware that they even have the virus. That is why social distancing is such an important containment measure, they explain.
One of the main questions on scientists’ and public health advisors’ minds at this time is: Why is the new coronavirus, SARS-CoV-2, spreading so quickly in countries around the globe?
And why is this happening despite the ever-increasing volume of travel bans and restrictions?
More and more researchers are getting behind the hypothesis that this is likely due to the movement of people who are unaware of the fact that they have contracted the virus — either because they had mild symptoms or because they had no symptoms at all.
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In a new preliminary study, Prof. Chaolong Wang and colleagues — from Huazhong University of Science and Technology in Wuhan, China — argue that during the aggressive outbreak in their city, over 50% of cases were non-confirmed, and that these may have included people with no or limited symptoms who had the potential to remain socially active.
The study has not yet appeared in a peer reviewed journal, but it is currently available online.
Asymptomatic cases may transmit the virus
In the new study, the researchers developed a method capable of predicting virus transmission patterns. It took into account population movement, non-confirmed cases, and those in quarantine.
The estimates also took into account 25,961 confirmed COVID-19 cases that Chinese officials had recorded through February 18, 2020 in the Hubei province.
However, the researchers calculate that the real number of COVID-19 cases by that date was actually closer to 36,798.
This could explain the virus’s aggressive spread in the region. Many people, the researchers suggest, must have contracted the virus but remained unaware of it. They therefore continued to maintain social contact.
“By our most conservative estimate, at least 59% of the infected individuals were out and about, without being tested and potentially infecting others,” says lead study author Prof. Wu Tangchun.
“This may explain why the virus spread so quickly in Hubei and is now circulating around the world,” he adds.
The researchers also suggest that upward of 90% of infections were prevented in the time period after officials brought in social distancing interventions.
Only way to break the chain of contagion
Other studies seem to point in a similar direction. For example, a statistical modeling analysis that appeared in the journal Eurosurveillance indicates that a significant number of people who contracted the virus aboard the Diamond Princess cruise ship remained asymptomatic.
In February, several passengers aboard the cruise ship tested positive for COVID-19, which led to an outbreak of coronavirus infections on the vessel.
Almost immediately, the relevant authorities declared that the ship would be entering a state of quarantine for 14 days.
The authors of the Eurosurveillance analysis suggest that the high proportion of COVID-19 cases on the cruise ship may have had something to do with what they estimate was a significant number of asymptomatic cases. In fact, they say that as many as 17.9% of the cases may have produced no symptoms.
However, the investigators also point out that the ship was a favorable “breeding ground” for the virus because a large number of passengers were older and in the most at-risk category.
“You have to keep in mind that this was a special population,” explains study co-author Prof. Gerardo Chowell, from Georgia State University in Atlanta.
Although it remains unclear to what extent people with no or very mild symptoms really contribute to the spread of the virus, emerging evidence suggests that they are at least able to pass it on.
One preliminary study from Germany suggests that people had high viral loads in swabs taken from their noses and throats during their first week of symptoms after contracting the virus.
This means that although they may initially experience few to no symptoms, people who have contracted SARS-CoV-2 may still be able to pass it on without realizing.
Another study, this time in the journal JAMA, showed that one person who had no symptoms of infection actually passed the virus on to five other people.
For these reasons, researchers maintain that social distancing is the most effective measure of containing the spread of the new coronavirus: Every person who limits their social contact can become a broken link in the chain of contagion.
“Implementing strong social distancing measures is the only way to stop the virus from spreading.”
– Prof. Gerardo Chowell
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