< Transmission

How long is a person contagious?

Text updated on 2020-05-31


The length of time a person is contagious is not known for certain. An optimistic estimate is that infected people remain contagious for about 10 days, from 2-3 days before to 8 days after the onset of symptoms of the disease. However, the virus can still be found in saliva 4 to 5 weeks after the onset of symptoms, suggesting that wearing a mask is important between 2 and 4 weeks after the onset of symptoms.

Infected people become contagious fairly quickly: two to three days before developing the first symptoms of the disease, with a peak of "contagiousness" between 2 days before and 1 day after the onset of symptoms. This would explain the high proportion of contagion from presymptomatic people (who have not yet developed symptoms) - up to 60% in some parts of China!

Thereafter, the SARS-CoV-2 coronavirus remains detectable in small amounts in the saliva of some patients until several weeks after the onset of disease. This is also the case for patients who are considered to be cured and who no longer have any clinical manifestations and for whom an X-ray of the lungs is normal. However, the presence of viruses does not necessarily imply that patients are contagious. Indeed, German virologists have observed that from eight days after the onset of symptoms, viral particles from patients are no longer able to infect cells in culture, suggesting that the virus is no longer necessarily contaminating.


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Sources

This article compares the interval between the onset of symptoms in 77 pairs of individuals, one of whom has been infected by the other (this is called the serial interval, estimated here at 5 to 6 days) with the mean incubation time of the disease. The authors deduce that individuals carrying the SARS-CoV-2 virus are contagious as early as 2 to 3 days before the onset of the first symptoms and that the peak of contagiousness is between 2 days before and 1 day after the onset of symptoms.

He, X., Lau, E. H., Wu, P., Deng, X., Wang, J., Hao, X., ... & Mo, X. (2020). Temporal dynamics in viral shedding and transmissibility of COVID-19. Nature medicine, 1-4.

This article, based on 100 COVID-19 patients and 2,761 people who had close contact with them, indicates that the risk of transmitting the virus is highest in the 5 days following the onset of symptoms and in the days preceding the onset of symptoms.

Cheng, H. Y., Jian, S. W., Liu, D. P., Ng, T. C., Huang, W. T., & Lin, H. H. (2020). Contact tracing assessment of COVID-19 transmission dynamics in Taiwan and risk at different exposure periods before and after symptom onset. JAMA Internal Medicine.

This short report is based on observations on 41 Chinese patients who developed severe forms of COVID-19. PCR tests revealed the presence of coronavirus in throat specimens for a minimum of 18 days and up to 48 days after the onset of symptoms in these patients. In half of the cases, the virus was present for 31 days or more.

Zhou, B., She, J., Wang, Y., & Ma, X. (2020). The duration of viral shedding of discharged patients with severe COVID-19. Clinical Infectious Diseases.

In this study of 56 Wuhan patients who developed mild to moderate forms of the COVID-19 disease, PCR tests revealed the presence of the virus up to six weeks after the onset of disease symptoms.

Xiao, A. T., Tong, Y. X., & Zhang, S. (2020). Profile of RT-PCR for SARS-CoV-2: a preliminary study from 56 COVID-19 patients. Clinical Infectious Diseases.

In this study conducted in New York, PCR assays were performed on nasopharyngeal samples. These revealed that the genome of the SARS-CoV-2 virus can be detected in patients up to 28 days after the symptoms of the disease have disappeared (41 days since their appearance).

Wajnberg, A., Mansour, M., Leven, E., Bouvier, N. M., Patel, G., Firpo, A., ... & Houldsworth, J. (2020). Humoral immune response and prolonged PCR positivity in a cohort of 1343 SARS-CoV 2 patients in the New York City region. medRxiv.

This very interesting paper studies in detail the dynamics of replication of the SARS-CoV-2 virus in different tissues of infected patients. Among other things, it questions the relationship between the presence of the virus in these tissues and its infectivity, measured as its ability to infect cells in culture. According to the authors, while sputum and nasopharyngeal specimens isolated during the first week of symptoms are highly infectious, those collected 8 or more days after the onset of symptoms are devoid of any infectious properties. Similarly, fecal or urine specimens, although containing viral particles, are not capable of infecting cells in culture, regardless of when they are collected.

Wölfel, R., Corman, V. M., Guggemos, W., Seilmaier, M., Zange, S., Müller, M. A., ... & Hoelscher, M. (2020). Virological assessment of hospitalized patients with COVID-2019. Nature, 1-5.

Further reading

How long is the coronavirus infectious?

What is the test to find out if I am infected with SARS-CoV-2?

Do all infected people show symptoms?