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How many people are infected without showing symptoms?

Text updated on 2021-05-03

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.

About 50% of people infected with SARS-CoV-2 will not develop symptoms: these people are called "asymptomatic" or healthy carriers. But even without symptoms, these people can transmit the COVID-19. See the question Can a person without symptoms infect others?

Among those who will develop symptoms, the incubation period, i.e. the time between infection and the appearance of symptoms, may last several days. This is called the "pre-symptomatic" period. The particularity of COVID-19 is that infected people are already contagious during this pre-symptomatic period, during the 2-3 days before the onset of symptoms.

The viral load measured on symptomatic and asymptomatic individuals is equivalent, suggesting that asymptomatic individuals who are unaware that they are carrying the coronavirus play a large role in the spread of the disease.

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On 21 February, the town of Vo (3400 inhabitants), near Padua, was the scene of the first Italian death due to COVID-19. Two days later, the municipality isolated itself and implemented a universal RT-PCR test for SARS-CoV-2 on two occasions (sampling 85.9% and 71.5% of the eligible population) at two-week intervals, allowing analysis of the prevalence and location of cases. A majority of the population was tested, and the results showed that 45% of those infected with the virus were asymptomatic and were equally distributed across all age groups.

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 in Wuhan who never developed symptoms but had COVID-19infected five family members by taking them to see a patient in a hospital 600 km from Wuhan where there were no cases 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.

Based on a meta-analysis of 16 studies, including 2,788 patients infected with SARS-CoV-2, the prevalence of asymptomatic cases was 48.2%.

Syangtan, G., Bista, S., Dawadi, P., Rayamajhee, B., Shrestha, L. B., Tuladhar, R., & Joshi, D. R. (2020). Asymptomatic SARS-CoV-2 carriers: A systematic review and meta-analysis. Frontiers in public health, 8.

Further reading

What are the different types of serological tests?