Can a person without symptoms infect others?
Text updated on 2021-05-03
Yes, the absence of symptoms does not mean that I am not contagious. Even without symptoms, I can infect other people.
Infection with the virus SARS-CoV-2 generates a highly variable response over time and from one individual to another. People infected without symptoms can be either people who will develop symptoms later ("pre-symptomatic") or infected people who will not develop symptoms ("asymptomatic"). See the question How many people are infected without showing symptoms?
High viral loads are found in asymptomatic and symptomatic people. Studies show that all these people can be contagious. Protecting yourself and others by wearing a mask in a social setting is therefore crucial. See the question What to do if my test is positive COVID-19 .
Epidemiological studies have shown that about half of all infections are caused by people who do not show symptoms. The presymptomatic and asymptomatic transmission observed for SARS-CoV-2 greatly accelerates the spread of the virus.
Viral loads do not differ significantly between asymptomatic and symptomatic individuals:Zou, L., Ruan, F., Huang, M., Liang, L., Huang, H., Hong, Z., ... & Guo, Q. (2020). SARS-CoV-2 viral load in upper respiratory specimens of infected patients. New England Journal of Medicine, 382(12), 1177-1179.
Elevated viral loads were found in asymptomatic patients:Kimball, A. (2020). Asymptomatic and presymptomatic SARS-CoV-2 infections in residents of a long-term care skilled nursing facility-King County, Washington, March 2020. MMWR. Morbidity and mortality weekly report, 69.
At the very beginning of the epidemic, a person in Wuhan who never developed symptoms but who was affected by COVID-19 infected five members of his family by taking them to see a patient in a hospital 600 km from Wuhan where there was still no case of COVID-19.Bai, Y., Yao, L., Wei, T., Tian, F., Jin, D. Y., Chen, L., & Wang, M. (2020). Presumed asymptomatic carrier transmission of COVID-19. Jama.
In one restaurant, a pre-symptomatic COVID-19 person contaminated at least two other people who were eating at two adjacent tables more than 2 meters away.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).
An asymptomatic person infected two people following a business meeting that lasted more than a day. One of the contaminated individuals then contaminated two other colleagues following 2 (and 4) brief contacts at the workplace when she was not yet symptomatic.Rothe, C., Schunk, M., Sothmann, P., Pretzel, G., Froeschl, G., Wallrauch, C., ... & Seilmaier, M. (2020). Transmission of 2019-nCoV infection from an asymptomatic contact in Germany. New England Journal of Medicine, 382(10), 970-971.
In January 2020, two pre-symptomatic people from Wuhan infected two family members in Shanghai after staying in their home.Yu, P., Zhu, J., Zhang, Z., & Han, Y. (2020). A familial cluster of infection associated with the 2019 novel coronavirus indicating possible person-to-person transmission during the incubation period. The Journal of infectious diseases.
Transmission demonstrated by infected individuals before the onset of symptoms in SingaporeWei, W. E., Li, Z., Chiew, C. J., Yong, S. E., Toh, M. P., & Lee, V. J. (2020). Presymptomatic Transmission of SARS-CoV-2-Singapore, January 23-March 16, 2020. Morbidity and Mortality Weekly Report, 69(14), 411.
Modelling of infection dynamics in China is consistent with the fact that asymptomatic individuals are involved in the transmission of the virus.Li, R., Pei, S., Chen, B., Song, Y., Zhang, T., Yang, W., & Shaman, J. (2020). Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2). Science 16: eabb3221.
Estimated time to onset of symptoms (5.20 and 3.95 days) and percentage of transmission by pre-symptomatic individuals in, respectively, Singapore and Tianjin (China): 48% (95%CI 32-67%) and 62% (95%CI 50-76%)Ganyani, T., Kremer, C., Chen, D., Torneri, A., Faes, C., Wallinga, J., & Hens, N. (2020). Estimating the generation interval for COVID-19 based on symptom onset data. medRxiv.
By measuring viral shedding in 94 patients with an infectivity profile, COVID-19 was confirmed in the laboratory, and by analyzing 77 contaminant-contaminated transmission pairs, it was estimated that 44% (95% confidence interval, 30-57%) of the infections occurred during the presymptomatic stage.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, 26(5), 672-675.
In this modeling study, it is considered that cases of SARS-CoV-2 contamination originate in 46% of cases from presymptomatic (before presenting symptoms) persons, in 38% of cases from symptomatic persons, in 10% of cases from asymptomatic persons (who never present symptoms) and in 6% of cases from indirect transmission via the environment. Estimates for the last two routes are speculative.Ferretti, L., Wymant, C., Kendall, M., Zhao, L., Nurtay, A., Abeler-Dörner, L., ... & Fraser, C. (2020). Quantifying SARS-CoV-2 transmission suggests epidemic control with digital contact tracing. Science, 368(6491).
Study which shows that in individuals who are going to have symptoms, 3/4 of the contaminations take place in a period which starts 2-3 days before the beginning of the symptoms and 2-3 days after. 41% of contaminations occur during the pre-symptomatic period, 35% of contaminations between the day of symptoms and the following day.Ferretti, L., Ledda, A., Wymant, C., Zhao, L., Ledda, V., Abeler-Dörner, L., ... & Fraser, C. (2020). The timing of COVID-19 transmission.
Study showing that the risk of coronavirus transmission through close contacts is similar between symptomatic and asymptomatic people.Yin, G., & Jin, H. (2020). Comparison of transmissibility of coronavirus between symptomatic and asymptomatic patients: reanalysis of the Ningbo COVID-19 data. JMIR public health and surveillance, 6(2), e19464.
The viral load of asymptomatic people with COVID-19 is not different from the viral load of symptomatic people.Atripaldi, L., Sale, S., Capone, M., Montesarchio, V., Parrella, R., Botti, G., ... & Madonna, G. (2021). Could asymptomatic carriers spread the SARS-CoV-2 infection? Experience from the Italian second wave. Journal of Translational Medicine, 19(1), 1-4.