Where and how are you most likely to get COVID-19? 

By Trish Greenhalgh, Jose-Luis Jimenez, Shely Miller, and Zhe Peng -

After two years of the pandemic, most of us are fed up.

COVID-19 infection numbers are higher than ever, and in many countries hospitalization rates are rising rapidly again.

But despite this dark horizon, our wish is to get back to normal.

We would like to meet friends in a bar, or invite them to dinner at home;

we would like our battered companies to grow again as they did before the pandemic;

We would like our children to return to what was once their routines in schools (which we want them to attend in person) and in extracurricular activities.

We would like to get on a bus, sing in a choir, go back to the gym or dance in a disco without fear of catching coronavirus.

But how many of these activities are safe, and to what extent?

These have been the questions we have tried to answer in our latest research.

SARS-CoV-2, the virus that causes COVID-19, is transmitted primarily through the air.

Hence, the key to avoiding getting infected is to understand how these particles that circulate in the air behave, which requires knowledge of both physics and chemistry.

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Air is a fluid made up of invisible molecules that move quickly and randomly.

and end up being dispersed throughout interior spaces, such as cigarette smoke in a room or on a bus.

An infected person can exhale aerosols containing the virus, and the closer we are to them, the more likely we are to inhale them.

Also, the longer we are in the same room, the more viruses we will inhale.

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If we are outdoors, the space is almost infinite and the virus does not behave in the same way, although we can still get infected if we talk near an infected person.

An infected person can emit viral particles every time they breathe, but these are especially abundant when it comes to deep breaths (such as when exercising) or when vocalization (talking or singing) is performed simultaneously with breathing. 

A well-fitting mask reduces transmission because it decreases the number of viral particles emitted.

We are much less likely to be infected by an infected person who is not wearing a mask and who is sitting quietly in a corner, than by another who walks up to us and starts a heated argument.

All variants of SARS-CoV-2 are airborne, but the chances of contracting the virus depend on the transmissibility or contagiousness of each variant (delta was more contagious than previous variants, but omicron is even more so) as well as the number of people who are infected at a given time (the prevalence of the disease).

Study published in the American Chemical Society, on January 5, 2022. American Chemical Society

Although omicron seems more contagious, it also seems to cause less severe illness, especially among vaccinated people.

But the high speed of contagion can saturate health systems.

Likelihood of getting infected

In our study we have calculated the way in which the different transmission situations cause the probability of becoming infected to vary: viral factors (transmissibility, prevalence), personal factors (wearing the mask / not wearing it, exercising / being at rest, doing some vocalization activity/being silent), air quality factors (indoors/outdoors, large rooms/small rooms, crowded/uncrowded, ventilated/unventilated).

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To make this calculation, we have carefully analyzed the empirical data regarding the number of people who had been infected in supercontagious events in which the key parameters (the size of the room, the number of people in it, the levels of ventilation , etc.) were well documented, and later we have represented how this transmission took place through a mathematical model.

Study published in the American Chemical Society, on January 5, 2022.American Chemical Society.

The graphs, which are based on our article and shown below, give a percentage chance of getting infected in different situations (they can be enlarged by clicking on them).

One way to make it easier to spread COVID-19 is to do several of the things that appear in the red cells of the table, such as:

• Meeting with many people in closed spaces with poor air quality, such as poorly ventilated gyms, nightclubs or school classes.

• Doing something strenuous or hectic like playing sports, singing, or yelling.

• Go without a mask.

• Staying in one place for a long time.

And to avoid getting COVID, try to do the things that appear in the green or amber cells, such as:

• If you must meet with others, do so outdoors or in a well-ventilated space, or

  indoors where ventilation is good

  and the air quality is known.

• Minimize the number of people you meet with.

• Stay together the minimum necessary time.

• Do not yell, sing, or exercise vigorously indoors.

• Wear good-quality, well-fitting masks from the moment you enter a building until you leave.

Although the graph gives an approximate probability for each situation, the actual risk will depend on specific parameters such as exactly how many people are in the room or exactly how big the room is.

If you want to calculate your own numbers for a certain meeting or activity, try our COVID-19 Aerosol Transmission Estimator.