Two weeks ago, the World Health Organisation issued a statement saying asymptomatic carriers are rare transmitters of Covid, only to retract the statement hours later. For all our faith in mankind’s scientific prowess, this pandemic has brought to fore the unfortunate truth that several months on from the discovery of the virus, our understanding of the virus and how it spreads is still patchy. This obviously restricts our ability to strategise to minimise infections as we let eocnomies open up. This piece in the WSJ gives some hope as it draws upon several studies and attempts to help us understand how we actually get infected. In summary, the authors suggest three key factors – the proximity with an infected person, the number of contacts with infected persons and the duration of contact with infected persons. The article goes on to elaborate these three factors and the concept of ‘superspreadders’ – “An estimated 10% of people with Covid-19 are responsible for about 80% of transmissions, according to a study…Some people with the virus may have a higher viral load, or produce more droplets when they breathe or speak, or be in a confined space with many people and bad ventilation when they’re at their most infectious point in their illness…”“It’s not common to contract Covid-19 from a contaminated surface, scientists say. And fleeting encounters with people outdoors are unlikely to spread the coronavirus.
Instead, the major culprit is close-up, person-to-person interactions for extended periods. Crowded events, poorly ventilated areas and places where people are talking loudly—or singing, in one famous case—maximize the risk.
These emerging findings are helping businesses and governments devise reopening strategies to protect public health while getting economies going again. That includes tactics like installing plexiglass barriers, requiring people to wear masks in stores and other venues, using good ventilation systems and keeping windows open when possible.
“We should not be thinking of a lockdown, but of ways to increase physical distance…This can include allowing outside activities, allowing walking or cycling to an office with people all physically distant, curbside pickup from stores, and other innovative methods that can facilitate resumption of economic activity without a rekindling of the outbreak.”
One important factor in transmission is that seemingly benign activities like speaking and breathing produce respiratory bits of varying sizes that can disperse along air currents and potentially infect people nearby.
Health agencies have so far identified respiratory-droplet contact as the major mode of Covid-19 transmission. These large fluid droplets can transfer virus from one person to another if they land on the eyes, nose or mouth. But they tend to fall to the ground or on other surfaces pretty quickly.
….Sufficient ventilation in the places people visit and work is very important, said Yuguo Li, one of the authors and an engineering professor at the University of Hong Kong. Proper ventilation—such as forcing air toward the ceiling and pumping it outside, or bringing fresh air into a room—dilutes the amount of virus in a space, lowering the risk of infection.
Another factor is prolonged exposure. That’s generally defined as 15 minutes or more of unprotected contact with someone less than 6 feet away, said John Brooks, the Centers for Disease Control and Prevention’s chief medical officer for the Covid-19 response. But that is only a rule of thumb, he cautioned. It could take much less time with a sneeze in the face or other intimate contact where a lot of respiratory droplets are emitted, he said.”
The article talks about a choir event where 87% of those present ended up getting infected attributing the cause to the heavier breathing of singers which increases the likelihood of inhaling infectious particles.
“Similar transmission dynamics could be at play in other settings where heavy breathing and loud talking are common over extended periods, like gyms, musical or theater performances, conferences, weddings and birthday parties. Of 61 clusters of cases in Japan between Jan. 15 and April 4, many involved heavy breathing in close proximity, such as karaoke parties, cheering at clubs, talking in bars and exercising in gyms, according to a recent study in the journal Emerging Infectious Diseases.
The so-called attack rate—the percentage of people who were infected in a specific place or time—can be very high in crowded events, homes and other spaces where lots of people are in close, prolonged contact.”
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