How many days after contact should I wait for a COVID test?
Text updated on 2020-09-22
If you are sick, the ideal situation is to be tested as soon as you have symptoms of the COVID-19 disease. If you are not ill, you will ideally wait 6-7 days after infectious contact to be tested.
I have symptoms
The most common symptoms of COVID-19 are: fever (over 38°), breathing difficulties, unexplained muscle pain, unusual headaches, loss of taste, loss of smell, unexplained fatigue, diarrhea (more than 3 bowel movements in 24 hours) or vomiting (more than 2 vomits in 24 hours). I isolate myself (without contact with others). I get tested as soon as possible and while waiting for the results, I stay in isolation. In most cases, the symptoms disappear in a few days. The contagious period begins two to three days before the onset of symptoms and lasts 7-8 days after the onset of symptoms. Thereafter, the coronavirus SARS-CoV-2 remains detectable in small quantities in the saliva and nasal cavities of some patients until several weeks after the onset of the disease. But it seems that from 8 days after the onset of symptoms, the viral particles of patients are no longer able to infect cells in culture, suggesting that the virus is no longer contaminating.
I have no symptoms but I am a "contact case".
A person I have been in contact with has just been diagnosed as positive at SARS-CoV-2. I am therefore a "contact case". I isolate myself if I have had prolonged contact with the positive person. See the question What is close contact?
If I have no symptoms, I wait 6 to 7 days (the average incubation time of SARS-CoV-2) after my last contact with the positive person at SARS-CoV-2 to get tested. If the result is positive, I stay in isolation for another 7 days after the test day. If the result is negative, the isolation ends. If I cannot be tested, even if I have no symptoms, I may have an asymptomatic form of COVID-19. I therefore stay 14 days in isolation (7 days after the last contact with the positive person at SARS-CoV-2 + 7 days to stop being contagious).
Why wait to be tested if you have no symptoms?
The coronavirus, once transmitted to a new person, will not be detectable immediately in this new person. It is thus necessary to wait a certain time before making a PCR test to be able to detect the possible presence of the virus. The incubation time (between the time of contact and the appearance of the first symptoms) varies between 1 and 14 days depending on the individual, with an average of 6.2 days. On the other hand, the moment when the coronavirus starts to be contagious is relatively stable between individuals and corresponds to approximately 2-3 days before the appearance of the symptoms. It is therefore important to isolate oneself before testing because the virus can be transmitted before symptoms appear (see below). COVID-19 before the onset of symptoms (see question Can a person without symptoms contaminate other people?).
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 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.
Based on 181 cases in China, it is observed that in 90% of cases the onset of symptoms occurs within 10 days of infection. After the onset of symptoms, patients can theoretically isolate themselves to avoid infecting others.Lauer, S. A., Grantz, K. H., Bi, Q., Jones, F. K., Zheng, Q., Meredith, H. R., ... & Lessler, J. (2020). The incubation period of coronavirus disease 2019 (COVID-19) from publicly reported confirmed cases: estimation and application. Annals of internal medicine, 172(9), 577-582.
This article about 100 patients COVID-19 and 2,761 people who have been in close contact with them indicates that the risk of transmitting the virus is highest in the 5 days after the onset of symptoms and in the days before 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.
A meta-analysis of 5 studies estimates the mean incubation time to be 5.1 days.He, W., Yi, G. Y., & Zhu, Y. (2020). Estimation of the basic reproduction number, average incubation time, asymptomatic infection rate, and case fatality rate for COVID-19: Meta-analysis and sensitivity analysis. Journal of Medical Virology.
Three people were infected on the same day when they went to an aquatic swimming centre. In one person, the first symptoms appeared 6 days after swimming and 9 days afterwards in the other two.Luo, C., Yao, L., Zhang, L., Yao, M., Chen, X., Wang, Q., & Shen, H. (2020). Possible transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a public bath center in Huai'an, Jiangsu Province, China. JAMA network open, 3(3), e204583-e204583.
Official recommendations for contact cases in France.https://solidarites-sante.gouv.fr/IMG/pdf/fiche_personne_contact.pdf
Official recommendations for contact cases in the United Kingdom.https://www.nhs.uk/conditions/coronavirus-covid-19/testing-and-tracing/nhs-test-and-trace-if-youve-been-in-contact-with-a-person-who-has-coronavirus/
A systematic review of 42 studies and a meta-analysis of 11 studies estimated that for COVID-19 the incubation period (time from infection to onset of symptoms) averaged 6.2 days.Dhouib, W., Maatoug, J., Ayouni, I., Zammit, N., Ghammem, R., Fredj, S. B., & Ghannem, H. (2021). The incubation period during the pandemic of COVID-19: a systematic review and meta-analysis. Systematic reviews, 10(1), 1-14.