< Propagation

How many people are actually infected relative to the number of confirmed COVID-19 cases?

Text updated on 2020-04-27

The amount of work required to perform large-scale testing, as well as the sensitivity of existing tests to detect the virus, gives us a partial picture of the number of people infected with the SARS-CoV-2 virus.

At this stage of the epidemic, the number of confirmed cases is much lower than the number of actual cases in the population. Most European countries are limited in terms of testing and have therefore often had to restrict the detection of COVID-19 to patients in respiratory distress or to people who could infect vulnerable people in their work environment. A statistical analysis was conducted on a large sample in Iceland of approximately 5,000 volunteers with no symptoms or only mild symptoms. It is estimated that when testing is limited to patients in respiratory distress or exposed persons, approximately 88% of the cases of COVID-19 are not detected. This means that the actual cases should be 10 times higher than the identified cases. In France, where the tests have not been implemented as much as in Iceland (7 tests / 1,000 people in France versus 132 tests / 1,000 people in Iceland as of April 24, 2020), it is reasonable to think that the number of infected people is even higher than ten times the number of confirmed cases. Similarly, in Italy, it is estimated that only one infected person out of 35 has been confirmed as having COVID-19, and many COVID-19 patients are not always detected by the test taken from nasopharyngeal secretions.

facebook twitter


A large number of tests have been carried out on the Icelandic population and these tests show a high rate of asymptomatic patients.

Stock, J. H., Aspelund, K. M., Droste, M., & Walker, C. D. (2020). Estimates of the undetected rate among the sars-cov-2 infected using testing data from iceland. medRxiv.

Testing of 5,000 volunteers in Iceland, including many symptom-free people, has identified asymptomatic cases in the population:

Gudbjartsson, D. F., Helgason, A., Jonsson, H., Magnusson, O. T., Melsted, P., Norddahl, G. L., ... & Eiriksdottir, B. (2020). Early Spread of SARS-Cov-2 in the Icelandic Population. medRxiv.

In France, according to a study based on data from April 14, 2020 and posted online on April 21 by the Pasteur Institute, approximately 6% of the French population will have been contaminated by COVID-19 by May 11, 2020 (which coincides with the announced day of the start of deconfinement) with approximately ~20,000-30,000 deaths from COVID-19 disease, a fatality rate of ~0.5%.

Salje, H., Kiem, C. T., Lefrancq, N., Courtejoie, N., Bosetti, P., Paireau, J., ... & Le Strat, Y. (2020). Estimating the burden of SARS-CoV-2 in France.

An American study estimates that R0 was 5.7 in Wuhan City at the beginning of the epidemic while its estimate was much lower in smaller European cities.

Sanche, S., Lin, Y. T., Xu, C., Romero-Severson, E., Hengartner, N., & Ke, R. (2020). High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2. Emerging Infectious Diseases, 26(7).

A study from the University of California Berkeley in April 2020 estimated that from the monthly mortality observed between 2015 to 2019 in Italy, the actual number of victims of COVID-19 is likely twice the number reported in hospitals and that only one person out of 35 infected people has been confirmed. The researchers propose that in cities such as Bergamo, where there were 6,000 victims per 1 million inhabitants, herd immunity, if it exists, should have be reached.

Modi, C., Boehm, V., Ferraro, S., Stein, G., & Seljak, U. (2020). Total COVID-19 Mortality in Italy: Excess Mortality and Age Dependence through Time-Series Analysis. medRxiv.

Further reading

Do all infected people show symptoms?

Can a person without symptoms infect others?

What is the test to find out if I am infected with SARS-CoV-2?