Nearly half of all SARS-CoV-2 infections may be asymptomatic

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Analysis of data from 16 groups of COVID-19 patients has allowed researchers at Scripps Research Institute (CA, USA) to estimate the percentage of people infected with the virus who don’t show symptoms. The results, published in Annals of Internal Medicine, suggest that asymptomatic infections may account for up to 45% of all COVID-19 cases, therefore playing a significant role in the spread of COVID-19. The report highlights the need for thorough testing and contact tracing strategies in order to reduce the spread of the virus.

“The silent spread of the virus makes it all the more challenging to control,” commented Eric Topol (Scripps Research Institute). “Our review really highlights the importance of testing. It’s clear that with such a high asymptomatic rate, we need to cast a very wide net, otherwise the virus will continue to evade us.”

In efforts to determine the approximate percentage of ‘silent’ carriers of the virus, Topol and his colleague, Daniel Oran, collected data from 16 diverse cohorts involved in COVID-19 studies around the world. These datasets were obtained from a variety of groups including nursing home residents, cruise ship passengers and prison inmates.

“What virtually all of them had in common was that a very large proportion of infected individuals had no symptoms,” explained Oran. “Among more than 3,000 prison inmates in four states who tested positive for the coronavirus, the figure was astronomical: 96 percent asymptomatic.”

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Further, the report suggests that asymptomatic individuals may be able to transmit the virus for longer than 14 days.

Surprisingly, the analysis also led the researchers to postulate that despite not showing symptoms, such individuals may still be damaged by the virus. CT scans from 54% of 76 asymptomatic individuals on the Diamond Princess cruise ship displayed significant subclinical lung abnormalities, suggesting that SARS-CoV-2 infection may impact lung function without it becoming immediately apparent.

Though the viral loads in those without symptoms are comparable to those with, it remains unclear whether their infectiousness is of the same magnitude. This suggests a need for large-scale studies to investigate this issue.

The authors also acknowledge that the lack of longitudinal data makes is difficult to distinguish between asymptomatic and presymptomatic.

“Our estimate of 40–45% asymptomatic means that, if you’re unlucky enough to get infected, the probability is almost a flip of a coin on whether you’re going to have symptoms. So to protect others, we think that wearing a mask makes a lot of sense,” Oran concluded.

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Sources: Oran DP & Topol EJ, Prevalence of Asymptomatic SARS-CoV-2 Infection. Ann. Intern. Med. doi:10.7326/M20-3012 (2020); www.scripps.edu/news-and-events/press-room/2020/20200609-oran-asymptomatic-infection.html

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