COVID-19 : how to evaluate the risk factors for infection

If you are worried about the virus, it's important to know the risk of contagion depending on where and how long you may have been in contact with it. How do we estimate whether a situation is risky or not? Here's an overview.
How much of the virus do we need to absorb?

The coronavirus is a large-sized virus that penetrates through the nostrils. It doesn't migrate immediately in the lungs. In theory, a single virus could make you sick if it infects a nasal cell and reproduces itself, but the chances of that happening and thus provoking a severe infection are extremely low, if not inexistent. To penetrate a cell, the virus has to reach the right cell receptor and hook itself there, which is unusual. The odds increase with the number of viruses in the air.

We don’t know what the minimal infectious dose for SARS-Cov2 is, but based on the other coronaviruses associated to pulmonary distress, we estimate that it could be between a few hundreds and a few thousands. Moreover, low dose infections can remain local (in the nose) and go unnoticed. The first defense mechanism of the organism (protection due to epithelial cells in nasal passages, mechanical elimination by ciliated cells and innate immunity) would then act to counter the illness               



Establishing the level of risk

To establish the level of risk in various environments (public bathrooms, workshops, weddings, funerals and other family reunions, work meetings, restaurants, public transit, etc.), Erin Bromage, biologist at the University of Massachussets Darmouth, suggests a calculation, starting with the plausible hypothesis that we have to breathe about 1000 viruses to exceed the natural abilities of our immune defense to combat the virus. He writes that this dose can be reached if someone breathes in a burst of air of more than 1000 viruses or if one takes 10 inspirations transmitting100 viruses each.

A person can disperse about 30 000 droplets just by sneezing. Those droplets could contain up to 200 million viruses, which is more than the minimal infectious dose. Luckily, the biggest droplets fall right away on the ground, within about 1 to 2 metres of the person sneezing, which is why the recommendations to stay 2 metres away from other people, coughing in the elbow, and wearing a mask are encouraged.

There are also droplets created simply by breathing. As opposed to those splashed by coughing or sneezing, these droplets are issued at low speed, on very short distances. We do not know what is the average viral load of SARS-Cov2 transmitted by breathing, but we do know that in the case of influenza, a much smaller virus, the average load doesn't exceed 33 viruses per minute. For COVID-19, it would probably be inferior to 20. To reach 1000, it would then require someone to breathe this contaminated air for 50 minutes!



If a person talks aloud, the viral density can be multiplied by 10. In this case, you are at risk if you stay close to her in a non-ventilated area for more than 5 minutes.

What are the high-risk situations?

Some places are more likely at risk to the transmission of COVID-19, like senior homes, where the tenants live in small rooms where the air barely circulates. However, many of these patients are already immunocompromised, so they have a very high viral load. We also note jails and slaughterhouses (which were identified as important transmission places in Quebec and abroad).

There has been important contagion outbreaks at indoor activities where people would have discussions or would yell or sing at close range. For example, we remember a wedding where more than 250 guests gathered at the Shar Hashomayim synagogue in Westmount. It was at the beginning of the pandemic in Montreal and was found to be a hotbed of contagion. ''Every closed environment without a lot of air circulation and many people indoors will attract problems,'' writes Erin Bromage. The more one spends time in such a space, the higher the risk becomes.

Many other examples of ''super contagions'' were deconstructed by epidemiologists in the last few months. There has been the case of a call centre in Korea, where 216 workers were packed on the same floor. 94 of them got infected. Or the one of a restaurant in Guangzhou where nine people got infected, many of which didn't share the table of the person carrying the virus. Yet, they spent an hour and a half in the restaurant, and their table was located in the air conditioning trajectory.

What about streets and parks?

In conclusion, everything is about the viral charge, the distance, and the exposure time. The risk increases or lowers according to these three factors. If you are in a well-ventilated place with a relatively low number of people, the risk is pretty low. And if you run into someone on the street or in a park, even at close range, don't panic! The contact time is very short. Outside, you would need to be in the air flow of someone for more than 5 minutes (and at less than 2 metres) to become infected... and the person would have to talk. Otherwise, the time required to become infected would be up to 10 times longer!

This equation also applies to large retail stores, where the air circulates and where each encounter doesn't last longer than a few seconds... except for employees who spend their whole day in the store, specifies Erin Bromage.

To know more about the propagation of viral particles in closed spaces, see the experience conducted by Japanese television.


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