Covid-19: Asymptomatic cases are less infectious, says study

Hemani Sheth Mumbai | Updated on November 29, 2020 Published on November 29, 2020

Asymptomatic Covid-19 patients are four times less likely to spread the infection than symptomatic ones, according to a study by the Imperial College London Covid-19 Response Team conducted in collaboration with multiple partners.

The researchers identified a total of 97 studies based around contact transmission, out of which 45 of met inclusion criteria for meta-analysis.

These reports were studied to “estimate the secondary attack rate (SAR) and observe the reproduction number (Robs) in different settings and to explore differences by age, symptom status, duration of exposure and household size,” as per the report summary.

According to the study, the chance of an asymptomatic patient infecting a close contact was 3.5 per cent (95 per cent CI: 0.0-6.4 per cent) as compared to a 12.8 per cent (95 per cent CI: 8.9-16.7 per cent) chance of a symptomatic patient infecting a close contact.

“This analysis provides some of the first evidence that asymptomatic infections are substantially less infectious than symptomatic cases. It also reinforces growing evidence of the importance of household transmission, especially in the context where symptomatic cases are not isolated outside the home,” Professor Neil Ferguson, the author of the report author was cited saying in an article published on the Imperial College of London’s website.

Higher risk in households

The study also focussed on identifying Covid-19 transmission rates in various indoor and outdoor settings. It found that when it comes to indoor settings, households have the highest transmission rates as compared to other settings such as social gatherings, travel, healthcare, workplace and casual close contacts.

The SAR in households was estimated to be 21.1 per cent (95 per cent CI: 17.4-24.8 per cent). This is the chance to pass on the infection in a household. The chance of a member infecting others in the household is significantly higher when the duration of household exposure is over five days, according to the report.

Furthermore, the report also noted that prolonged exposure in settings with familiar close contacts, including households, increases the risk of Covid-19 transmission.

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Importance of ‘staying home’

According to the study, SAR related to familiar and prolonged close contacts in settings such as social events with family and friends were higher at 5.9 per cent (95 per cent CI: 3.8-8.1 per cent) than those related to low-risk casual contacts such as strangers. SAR related to strangers was observed to be 1.2 per cent (95 per cent CI: 0.3-2.1 per cent).

“With many countries continuing to recommend ‘stay-at-home’ measures, the upcoming festive season and with cases isolating inside households, our results show it is likely that this location will continue to be important in sustaining transmission,” said Hayley Thompson, another author of the study, as quoted by the article.

However, the researchers also noted that data reporting transmission in workplaces, schools, and care-homes was limited, highlighting the need for further research in these settings.

“Understanding the conditions where transmission is more likely to occur is essential in guiding policy interventions to reduce transmission whilst balancing the economic impact,” Imperial College researcher Andria Mousa said.

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Published on November 29, 2020
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