< Transmission

How does she get COVID-19 ?

Text updated on 2021-04-20

The coronavirus is mainly caught by breathing in the viral particles emitted by contagious people.

The SARS-CoV-2 coronavirus which is responsible for COVID-19 is transmitted:




To lead to an infection, viral particles must enter through one of our mucous membranes (nose, mouth, eyes). The virus cannot infect us simply by passing through the skin of our hands, even if we have a small cut on the skin.

Absorption of a single viral particle is not enough to trigger an infection. Researchers have estimated that 1,000 absorbed viral particles are needed to cause an infection.

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An excellent scientific synthesis published in June 2020 illustrates the existence and importance of aerosols in the transmission of COVID-19.

Morawska, L., & Cao, J. (2020). Airborne transmission of SARS-CoV-2: The world should face the reality. Environment International, 105730.

A good synthesis on the aerial transmission of the SARS-CoV-2, published in June 2020, strongly advises the wearing of masks on the basis of scientific evidence. Incidentally, it draws an analogy between the diffusion of cigarette smoke and that of viral aerosols.

Prather, K. A., Wang, C. C., & Schooley, R. T. (2020). Reducing transmission of SARS-CoV-2. Science.

The 60 members of a choir gathered in a room to sing for 2.5 hours on March 10, 2020 in Washington State, USA. They kept a safe distance, used hydro-alcoholic solutions, but did not put on masks. Three weeks later, 45 of them tested positive for COVID-19.

Los Angeles Times article of March 29, 2020.

In a windowless, air-conditioned restaurant on the 5th floor, a COVID-19 asymptomatic person contaminated at least two other people more than 2 meters away, who were eating at two adjacent tables.

Lu, J., Gu, J., Li, K., Xu, C., Su, W., Lai, Z., ... & Yang, Z. (2020). COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020. Emerging Infectious Diseases, 26(7).

Using a laser beam, researchers have observed that saying the words "Stay Healthy" generates thousands of droplets of saliva that are otherwise invisible to the naked eye. A homemade wet cloth mask significantly reduces droplet excretion.

Anfinrud, P., Bax, C. E., Stadnytskyi, V., & Bax, A. (2020). Could SARS-CoV-2 be transmitted via speech droplets?. medRxiv.

In Japan, many cases of "cluster" contamination have occurred in closed places: a sports hall, restaurant boat, hospital, festival where there were tents with minimal ventilation for eating.

Nishiura, H., Oshitani, H., Kobayashi, T., Saito, T., Sunagawa, T., Matsui, T., ... & Suzuki, M. (2020). Closed environments facilitate secondary transmission of coronavirus disease 2019 (COVID-19). medRxiv.

The analysis of 318 cases where one person contaminated at least 2 other people in China (for a total of 1,245 people tested COVID+) reveals that the majority of the contaminations took place in a closed space, mainly in the home and on transportation, but also in restaurants, and that only one contamination took place following an open-air discussion with a person returning from Wuhan.

Qian, H., Miao, T., Li, L. I. U., Zheng, X., Luo, D., & Li, Y. (2020). Indoor transmission of SARS-CoV-2. medRxiv.

This article describes non-contact cases of contamination: a person crossing the corridor in front of an open patient door (several times), and 4 technicians at the Wuhan hospital who did not come into contact with confirmed cases.

Wang, J., & Du, G. (2020). COVID-19 may transmit through aerosol. Irish Journal of Medical Science (1971-), 1-2.

An article showing that viral particles of the SARS-CoV-2 coronavirus agent from COVID-19 present in aerosols are infectious on cells. These particles are infectious up to 16 hours after aerosolization.

Fears, A. C., Klimstra, W. B., Duprex, P., Hartman, A., Weaver, S. C., Plante, K. S., ... & Nalca, A. (2020). Persistence of Severe Acute Respiratory Syndrome Coronavirus 2 in Aerosol Suspensions. Emerging infectious diseases, 26(9).

The authors of this opinion piece argue that aerosols are highly contaminating, comparing the slope of the epidemic in countries that have or have not imposed the wearing of masks in addition to distancing measures. They conclude that the use of masks is the most effective measure to limit the epidemic.

Zhang, R., Li, Y., Zhang, A. L., Wang, Y., & Molina, M. J. (2020). Identifying airborne transmission as the dominant route for the spread of COVID-19. Proceedings of the National Academy of Sciences.

A popular article, also available in French, which describes the turn taken by the WHO in July 2020 in the recognition of the "aerosol" pathway.

Erath B., Ferro, A. & Ahmadi, G. Aerosols are a bigger coronavirus threat than WHO guidelines suggest - here's what you need to know. The Conversation, 10 July 2020.

Two couples were infected because they lived in two apartments above an apartment where several people were infected with COVID-19. The three families did not know each other, and had not shared elevators during the contagion period. The investigation suggests that the contamination took place through flushing the toilets, which released aerosols that travelled through the drainage system and ended up in the bathrooms of the apartments. It is likely that the water from the U-shaped drainage traps under the bathtub had evaporated (as the bathtub is not used regularly) and therefore there was no longer a seal between the apartments. In addition, the two couples who were infected never opened their bathroom windows.

Kang, M., Wei, J., Yuan, J., Guo, J., Zhang, Y., Hang, J., ... & Peng, X. (2020). Probable evidence of fecal aerosol transmission of SARS-CoV-2 in a high-rise building. Annals of internal medicine.

Article summarizing the data showing that SARS-CoV-2 is transmitted via aerosols.

Lewis, D. (2021). COVID-19 rarely spreads through surfaces. So why are we still deep cleaning. Nature, 590(7844), 26-28.

Further reading

Is the SARS-CoV-2 coronavirus transmitted by aerosols?

Why put on a mask?

What is the purpose of social distancing?

What is close contact?

What is the recommended distance between two people?

Can a person without symptoms infect others?

What are the simple rules of hygiene to adopt during COVID-19?