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How many people are infected without showing symptoms?

Text updated on 2020-04-27

It is estimated that nearly 50% of people infected by the coronavirus would be asymptomatic but contagious!

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

A special feature of CoV-2 SARS, like CoV-1 SARS, is that infected people are contagious before they show symptoms (for at least one day): these people are called "pre-symptomatic". On the other hand, some people, once infected, will not develop symptoms: these people are said to be "asymptomatic" or healthy carriers. Finally, studies have shown that "post-symptomatic" people retain the SARS-CoV-2 virus in their oral and nasal secretions for several weeks after symptoms have disappeared.

Cases of infection by pre-symptomatic individuals in the days before the onset of symptoms have been observed. In addition, the viral load measured in symptomatic and asymptomatic individuals is equivalent, suggesting that asymptomatic individuals could play a large role in the spread of the disease in the population by unknowingly infecting healthy individuals.

Several large-scale studies including people without symptoms of COVID-19 have revealed a high proportion of asymptomatic patients. In particular, an almost exhaustive study in the Italian municipality of Vo' (3,400 inhabitants), where the majority of the population was tested, revealed that 45% of the people infected with the virus were asymptomatic and were equally distributed across all age groups.

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Most of the inhabitants of the village of Vo' in Italy were tested at two-week intervals which made it possible to analyse the prevalence and location of cases.

Lavezzo, E., Franchin, E., Ciavarella, C., Cuomo-Dannenburg, G., Barzon, L., Del Vecchio, C., ... & Abate, D. (2020). Suppression of COVID-19 outbreak in the municipality of Vo, Italy. medRxiv.

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.

At the very beginning of the epidemic, a person from Wuhan who never developed symptoms but who was COVID+ 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.

Documentation of infections by asymptomatic and pre-symptomatic patients in a Washington state nursing home:

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.

Individuals infected with SARS-CoV-2 are contagious before the onset of symptoms -- a major difference from SARS-CoV-1:

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.

Further reading

What are the different types of serological tests?