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.
If I have symptoms: fever (over 38°C), breathing difficulties, unexplained muscle pain, unusual headaches, loss of taste, loss of smell, unexplained fatigue, diarrhea (more than 3 episodes in 24 hours) or vomiting (more than 2 episodes in 24 hours) I must quarantine myself (i.e., no contact with others). I must get tested as soon as possible and while waiting for the results, I will remain in quarantine. In the majority of cases, the symptoms disappear within a few days. The contagious period begins 2-3 days before the onset of symptoms and lasts 7-8 days after the onset of symptoms. Thereafter, the SARS-CoV-2 coronavirus remains detectable in small amounts 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 the patients are no longer able to infect cells in culture, suggesting that the virus is no longer contaminating.
If I don't have symptoms but I'm a "contact case"... One person I've been in contact with just found out that she's positive with SARS-CoV-2. So I'm a "contact case". I must quarantine myself if I have had prolonged contact with the positive person. See the question What is close contact?
If I don't have any symptoms, I must wait 5 to 7 days (the average incubation time of SARS-CoV-2) after my last contact with the infected person (who was positive with SARS-CoV-2) to get tested. If the result is positive, I will stay in quarantine for another 7 days after the day of the test. If the result is negative, the quarantine ends. If I can't get tested, even if I don't have symptoms, I may have an asymptomatic form of COVID-19. So, I will stay 14 days in quarantine (7 days after the last contact with the positive person SARS-CoV-2 plus 7 days to no longer be contagious).
Why wait to get tested if you don't have any symptoms? The coronavirus, once transmitted to a new person, is not going to be detectable right away in that new person. Therefore, it is necessary to wait a certain amount of time before doing 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 5.1 days. However, the time at which the virus begins to be detected is relatively stable between individuals and corresponds to approximately 1-2 days before the onset of symptoms.
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, 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.
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/