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Two In Five People With COVID-19 Have No Symptoms, Huge Study Finds

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Maddy Chapman

Maddy is a Editor and Writer at IFLScience, with a degree in biochemistry from the University of York.

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Regular testing is essential, even for those showing no symptoms, the study authors say. Image credit: SeventyFour/Shutterstock.com

A shocking two in five people who have COVID-19 have no symptoms. That’s according to a huge new study of over 29 million people.

Published today in the journal JAMA, the study, which collated data from 95 papers, found that 40.5 percent of positive cases showed no signs of illness. Meaning just shy of half of us could have COVID and be completely in the dark.

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“The high percentage of asymptomatic infections highlights the potential transmission risk of asymptomatic infections in communities,” the authors write. 

While so many COVID cases are asymptomatic, just 0.25 percent of the 29 million people tested were, reflecting the discrepancy in tests being taken by those with and without symptoms.

“Screening for asymptomatic infection is required, especially for countries and regions that have successfully controlled SARS-CoV-2. Asymptomatic infections should be under management similar to that for confirmed infections, including isolating and contact tracing.”

The studies used in the meta-analysis were largely conducted in Europe, North America, and Asia, but also include data from Africa and South America. Amongst the total tested population, most asymptomatic infections were in Europe, while Asia had the lowest percentage. The authors suggest this could be linked to the large SARS-CoV-2 screening program in China.

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In the population that tested positive for COVID-19, asymptomatic infections were most common in pregnant people and those traveling by plane or on a cruise. Screening and quarantine for travelers, therefore, is important in minimizing community transmission, the authors argue. 

Around one-third of asymptomatic cases in those with confirmed COVID-19 were health care workers or hospital patients, highlighting the importance of measures to prevent spread in this setting. Testing of asymptomatic individuals is key, the authors once again stress, as is personal protective equipment (PPE) and hand hygiene.

Asymptomatic infections were also more common in younger people – under 39 years – perhaps because they are less likely to experience more severe symptoms.

The study itself has a few notable limitations. Namely, the number of asymptomatic infections includes cases where symptoms were yet to develop. There is also a potential publication bias, as studies that did not find evidence of asymptomatic infections were less likely to have been published. Both mean that the number of symptomless infections could be overestimated.

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Still, the authors hope that their findings will help influence public health measures and, ultimately, protect people by highlighting just how much of the population may be infected and transmissible even when not showing any symptoms.

“Our results could raise awareness among the public and policy makers and provide evidence for prevention strategies,” they conclude.


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