< Transmission

Can a person without symptoms infect others?

Text updated on 2020-09-20

Yes! The absence of symptoms does not mean the absence of contagiousness. Even without symptoms, even cured, one can still infect other people.

Infection with the SARS-CoV-2 virus generates a highly variable response over time and among individuals.

Symptomless infected individuals can be either individuals who will develop symptoms later ("pre-symptomatic") or infected individuals who will not develop symptoms ("asymptomatic") or after they develop symptoms ("post-symptomatic").

High viral loads are found in pre-symptomatic, asymptomatic, symptomatic, and post-symptomatic individuals, suggesting that infected people can be contagious regardless of their symptoms, so protecting oneself and others by wearing a mask in a social setting becomes critical.

Epidemiological studies have shown that about half of the contaminations are caused by people who do not show symptoms. The presymptomatic transmission observed for SARS-CoV-2, which differs from SARS-CoV-1 in which patients were only contagious after the development of symptoms, greatly accelerates the spread of the virus.

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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 Singapore

Wei, 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).

Further reading

How many people are infected without showing symptoms?

How many people are contagious with COVID around me?

What strategies to detect contagious people at the entrance of a bar or an airplane?

Can we catch COVID two times?

What is close contact?

Why put on a mask?

Does the severity of the COVID disease depend on the dose of virus received?