Hosting a few friends at home could be riskier than going grocery shopping or having a jogger whisk past you when it comes to catching the new coronavirus (Covid-19), according to an immunology expert's viral blog post.
In a post which has received over 10 million views, Dr Erin Bromage, a biology professor specialising in infectious diseases and immunology, laid out a detailed explanation of the settings and situations where people are most at risk of contracting the virus.
Bromage, who teaches at the University of Massachusetts Dartmouth, wrote the explainer as some US states began to ease lockdown restrictions, warning that such a move risked "giving the virus more fuel."
Like the US and several other countries, Belgium on Wednesday gave the go-ahead to the reopening of schools, hairdressers and even some social events in an effort to relieve a reeling economy.
As countries begin to reopen, Bromage said his blog post aimed to provide the necessary information to guide people away from "situations of high risk."
"I regularly hear people worrying about grocery stores, bike rides, inconsiderate runners who are not wearing masks.... are these places of concern? Well, not really. Let me explain."
To do so, Bromage drew from data established for other infectious diseases, including coronaviruses, to explain two concepts key for understanding how viruses spread among populations: viral load and viral exposure.
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"In order to get infected you need to get exposed to an infectious dose of the virus," Bromage writes, noting that some scientific estimates said a Covid-19 infection could take hold through exposure to as little as 1,000 viral particles.
A single cough or sneeze releases thousands more respiratory droplets than breathing or talking, meaning they have higher viral loads which also travel at higher speeds and therefore, carry higher risks of transmitting an infection.
But Bromage said that a simple conversation or interaction with an infected individual carried no less risk, especially in closed or poorly ventilated areas.
"Infection could occur, through 1,000 viral particles you receive in one breath or from one eye-rub, or 100 viral particles inhaled with each breath over 10 breaths, or 10 viral particles with 100 breaths," he wrote. "Each of these situations can lead to an infection."
Citing real-life scenarios identified as the source of large infection clusters in the US, Bromage highlighted five places presenting particularly high risks: one's own home, the workplace, public transportation, social gatherings and restaurants.
Dinners, social gatherings, the workplace: 'Know the risks'
"Any environment that is enclosed, with poor air circulation and high density of people, spells trouble," Bromage wrote, adding that, as work and social activities picked back up, social-distancing rules touted by governments across the world only solved half the equation.
"Social distancing rules are really to protect you [from] brief exposures or outdoor exposures. In these situations, there is not enough time to achieve the infectious viral load."
By contrast, Bromage warned that working in a small and poorly ventilated office or attending a social gathering could open the flood gates for a new wave of infections.
Additionally, Bromage underscored that one of the most common places where infection spread was not on cruise ships, but in peoples' own homes: "A household member contracts the virus in the community and brings it into the house where sustained contact between household members leads to infection."
The attendance of a single infected (but asymptomatic) person to two family gatherings in Chicago, including a birthday party with only 9 people, was found to be at the root of the city's wider Covid-19 outbreak.
In another case, Bromage explains how airflow and a 1.5-hour-long dinner between nine friends at a restaurant resulted in transmission to half of the people at their table but also to 75% of those at the adjacent table.
Additional examples include a curling match in Canada attended by 76 people, a church choir performance and an outbreak linked to a single call-centre employee who went to work at a 216-employee strong office.
In the case of the choir, Bromage said that the aerosolising of respiratory droplets that occurs when singing played a significant role in infecting 45 of the 60 choir members, making similar observations for the screaming and excitement of an indoor sports game.
"Social distancing guidelines don't hold in indoor spaces where you spend a lot of time," he stressed, doubling down on the fact that the key principle to grasp for assessing risk was viral exposure over time.
"In all these cases, people were exposed to the virus in the air for a prolonged period (hours). Even if they were 50 feet away (choir or call centre), even a low dose of the virus in the air reaching them, over a sustained period, was enough to cause infection and in some cases, death."
"As the work closures are loosened, and we start to venture out more, possibly even resuming in-office activities, you need to look at your environment and make judgments," he said, adding that frequent hand-washing also remained key as infection could equally happen through contact with an infected surface.
The Brussels Times