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How can we reduce the contagion in enclosed spaces: schools, colleges, places of business, and public transport?

Text updated on 2021-04-15

Current knowledge of coronavirus transmission by aerosols and sputum and salivary tests allow for effective measures to be taken to protect oneself from coronavirus and not to contaminate others.

Barrier to coronavirus: mask and ventilation

To limit the transmission of SARS-CoV-2, the virus of the COVID-19virus, it is necessary to know that this coronavirus is transmitted mainly by aerosols and droplets (see the question Is the coronavirus SARS-CoV-2 transmitted by aerosols?). The aerosols are micro-droplets containing particles of SARS-CoV-2 and they are produced when we breathe, speak, eat, sing, etc. Depending on their size, these micro-droplets will disperse more or less. Droplets with a diameter greater than 5 µm, called sputters, will fall to the ground fairly quickly around the person who produced them (1 or 2 m around the person). Aerosols with a diameter of less than 5 µm can remain suspended in the air for hours. They "float" in the air and can be found within seconds of the person who produced them, much like cigarette smoke. To avoid being contaminated by viral aerosols, you must wear a well-fitted mask (see the question Why wear a mask?) that will filter them. Another way to protect yourself is to limit the quantity of viral aerosols in a closed space by airing the room with outside air (see the question What precautions should be taken in the workplace to limit airborne transmission of the virus?).

Because of this transmission by viral aerosols, closed places are much more at risk. To limit the spread of the coronavirus, windows should be opened in classrooms, offices, canteens, but also in public transport (metros, buses, RER, trams). And you should prefer to meet outside and not eat with your friends or colleagues in poorly ventilated areas.

Detecting infected people before symptoms: saliva tests

To limit the transmission of the coronavirus, the other important point to know is that one can be contagious without having symptoms. On average, 50% of people will not have symptoms and, therefore, will never know that they are infected. On the other hand, even without symptoms, they can be contagious and transmit the coronavirus to other people (see the question Can a person without symptoms contaminate other people?). And for the 50% of the persons who are going to have symptoms, they are contagious 2 to 3 days before the appearance of the symptoms, i.e.,they will begin to be able to transmit the coronavirus 2 to 3 days before taking a test.

One way to detect contagious people, whether or not they have symptoms, is to test systematically and frequently: for example, once a week among all students in a school or college, or among all staff in a company. This requires that individuals adhere to the protocol and are willing to be tested frequently. Nasopharyngeal testing is unpleasant and no one wants to be tested every week. On the other hand, saliva tests or nasal swabs are not painful and can easily be done by all age groups, even the youngest (see the question Which swab to test for COVID-19 : nasopharyngeal or oral?).

Pooling methods can reduce the costs of these tests without reducing their effectiveness (see the question What approaches could accelerate large-scale screening?). Saliva pooling methods are now routinely used in the United States, notably in New York State. Frequent salivary testing is an effective way to stop the spread of coronavirus and to detect infected people before they infect others.

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Study showing aerosol transmission in a quarantine hotel in New Zealand. PCR testing took place in the poorly ventilated corridor of the hotel. Some travellers were contaminated by aerosols from infected persons stagnating in the corridor.

Eichler, N., Thornley, C., Swadi, T., Devine, T., McElnay, C., Sherwood, J., ... & Geoghegan, J. L. (2021). Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 during Border Quarantine and Air Travel, New Zealand (Aotearoa). Emerging Infectious Diseases, 27(5).

Transmission of SARS-CoV-2 can occur without symptoms.

Rasmussen, A. L., & Popescu, S. V. (2021). SARS-CoV-2 transmission without symptoms. Science, 371(6535), 1206-1207.

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.

Several people were infected after a 1 hour and 40 minute bus ride when the contagious individual with COVID-19 was more than 6 meters away.

Shen, Y., Li, C., Dong, H., Wang, Z., Martinez, L., Sun, Z., ... & Xu, G. (2020). Airborne transmission of COVID-19: epidemiologic evidence from two outbreak investigations.

A child was infected after a bus ride that also included two people from Wuhan who were found to have the disease. COVID-19.

Jiehao, C., Jin, X., Daojiong, L., Zhi, Y., Lei, X., Zhenghai, Q., ... & Mei, Z. (2020). A case series of children with 2019 novel coronavirus infection: clinical and epidemiological features. Clinical Infectious Diseases, 71(6), 1547-1551.

A detailed study of people who travelled by train between December 19, 2019 and March 6, 2020 in China revealed that 234 passengers were likely contaminated on the train. In at least 13 cases, the contamination occurred while the contagious individual was off the train. Those who were in the same row as the contagious individual were 10 times more likely to get the COVID-19 than those placed one or two rows down.

Hu, M., Lin, H., Wang, J., Xu, C., Tatem, A. J., Meng, B., ... & Lai, S. (2020). The risk of COVID-19 transmission in train passengers: an epidemiological and modelling study. Clinical Infectious Diseases.

A study of 103 cases of COVID-19 contamination possibly work-related in Asia up to April 2020 shows that the occupations most at risk of catching the coronavirus SARS-CoV-2 are, after care givers, bus and taxi drivers and workers in the transport sector.

Lan, F. Y., Wei, C. F., Hsu, Y. T., Christiani, D. C., & Kales, S. N. (2020). Work-related COVID-19 transmission in six Asian countries/areas: A follow-up study. PloS one, 15(5), e0233588.

Data from a June 2020 survey of 1,030 individuals who had the COVID-19 show that the risk of having the COVID-19 is 4.3 times higher among those who take public transport, 16 times higher among those who have visited a place of worship.

Clipman, S. J., Wesolowski, A. P., Gibson, D. G., Agarwal, S., Lambrou, A. S., Kirk, G. D., ... & Solomon, S. S. (2020). Rapid real-time tracking of non-pharmaceutical interventions and their association with SARS-CoV-2 positivity: The COVID-19 Pandemic Pulse Study. medRxiv.

A survey conducted in March 2020 with individuals who have had the COVID-19 to determine the situations most at risk. Of the 375 individuals, 265 (73%) did not know the index case (the person with the COVID-19 who infected them). The most frequently reported activities in the two weeks prior to illness included attending gatherings of more than 10 people (116; 44%), traveling within the country (76; 29%), working in a health care facility (75; 28%), visiting a health care facility without being a health care worker (61; 23%), and using public transportation (57; 22%).

Marshall, K., Vahey, G. M., McDonald, E., Tate, J. E., Herlihy, R., Midgley, C. M., ... & Spillane, M. (2020). Exposures Before Issuance of Stay-at-Home Orders Among Persons with Laboratory-Confirmed COVID-19 Colorado, March 2020. Morbidity and Mortality Weekly Report, 69(26), 847.

How to reduce the transmission of SARS-CoV-2.

Prather, K. A., Wang, C. C., & Schooley, R. T. (2020). Reducing transmission of SARS-CoV-2. Science, 368(6498), 1422-1424.

Modelling study showing that ventilation of a room combined with the wearing of a mask by all individuals in the room can reduce the risk of infection by 5 to 10.

Lelieveld, J., Helleis, F., Borrmann, S., Cheng, Y., Drewnick, F., Haug, G., ... & Pöschl, U. (2020). Model Calculations of Aerosol Transmission and Infection Risk of COVID-19 in Indoor Environments. International Journal of Environmental Research and Public Health, 17(21), 8114.

Particles from SARS-CoV-2 can remain stable in aerosols and potentially capable of infecting human cells for up to 16 hours.

Fears, A. C., Klimstra, W. B., Duprex, P., Hartman, A., Weaver, S. C., Plante, K. S., ... & Roy, C. J. (2020). Persistence of severe acute respiratory syndrome coronavirus 2 in aerosol suspensions. Emerging infectious diseases, 26(9), 2168.

In the immediate environment of patients with COVID-19patients, 17% of the air sampled contained SARS-CoV-2 RNA, and the sampled virus was viable in 9% of cultures.

Birgand, G., Peiffer-Smadja, N., Fournier, S., Kerneis, S., Lescure, F. X., & Lucet, J. C. (2020). Assessment of Air Contamination by SARS-CoV-2 in Hospital Settings. JAMA network open, 3(12), e2033232-e2033232éar

Review of the literature on outdoor contamination at SARS-CoV-2 . Less than 10% of infections occurred outdoors. The risk of transmission of SARS-CoV-2 indoors is almost 20 times higher than outdoors.

Bulfone, T. C., Malekinejad, M., Rutherford, G. W., & Razani, N. (2020). Outdoor Transmission of SARS-CoV-2 and Other Respiratory Viruses, a Systematic Review. The Journal of infectious diseases.

Modelling study that evaluates the effects of a mass testing campaign in terms of test sensitivity (the rate of detection of infection among infected individuals) and adherence to the testing protocol (the rate of people who will accept to be tested). The results on the French data show that in order to reduce the daily infection rate, it is more effective to increase adherence to the test with less sensitive tests, than to have very sensitive tests that are not accepted by the population, and therefore only a small part of the population will be tested. A 90% sensitive test performed in 25% of the population (i.e. 15 million people tested, knowing that in March 2021, there are about 60,000 tests performed per week) reduces the daily infection rate by 7%, while a 75% sensitive test performed in 50% of the population reduces the daily infection rate by 12%, and if it is performed in 75% of the population, the daily infection rate is reduced by 17%.

Bosetti, P., Kiem, C. T., Yazdanpanah, Y., Fontanet, A., Lina, B., Colizza, V., & Cauchemez, S. (2021). Impact of mass testing during an epidemic rebound of SARS-CoV-2: a modelling study using the example of France. Eurosurveillance, 26(1), 2001978.

Study of a pooling strategy for the detection of SARS-Cov-2 in saliva samples applied to a population of 400,000 school children in New York State. The study reveals an increased contamination during the Halloween and New Year's holidays.

Chongfeng Bi et al. bioRxiv Pooled Surveillance Testing Program for Asymptomatic SARS-CoV-2 Infections in K-12 Schools and Universities, bioRxiv (February 2021)

Further reading

Do I have to wear a mask if I don't have any symptoms?

Why put on a mask?

What precautions should be taken in the workplace to limit airborne transmission of the virus?

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

What approaches could accelerate large-scale screening?

Why can measuring CO2 levels help us fight COVID-19 ?

What sample to test for COVID-19: nasopharyngeal or buccal?