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Do all infected people show symptoms?

Text updated on 2020-04-23


Today, most European countries are resource-limited in terms of testing and have had to restrict the detection of COVID-19 on patients in respiratory distress or on workers in close proximity to vulnerable people. As a result, the number of confirmed cases is lower than the number of actual cases in the population. A recent statistical analysis in Iceland, where tests have been carried out on all volunteers, whether symptomless or with mild symptoms, estimates that about 88% of COVID-19 cases go undetected under these conditions.

Several large-scale studies involving symptom-free people with COVID-19 have shown a high proportion of asymptomatic patients. In particular, a recent study in the Italian municipality of Vo' (1,800 inhabitants), where a majority of the population was tested and where COVID-19 was eradicated in April 2020, revealed 45% patients are asymptomatic in all age groups.

Asymptomatic individuals nevertheless have viral loads similar to symptomatic patients, suggesting that they could very well infect healthy individuals in the same way that pre-symptomatic patients, i.e., those who are infected but do not yet show symptoms, are contagious in the days before the onset of symptoms.


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A valuable study recently conducted in a village in Italy testing most people two weeks apart after the beginning of the COVID-19 outbreak and analysing prevalence and tracing cases. Suppression of COVID-19 outbreak in the municipality of Vo, Italy.


A great effort in testing the Icelandic population and finding a high rate of asymptomatic patients, and the minority of people being tested upon criteria currently used (severe symptoms or essential work)). Estimates of the Undetected Rate among the SARS-CoV-2 Infected using Testing Data from Iceland


Guobjartsson et al. 2020. Early Spread of SARS-Cov-2 in the Icelandic Population.


Zou et al, SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients. N Engl J Med 2020; 382:1177-1179 DOI: 10.1056/NEJMc2001737.


Pan X, Chen D, Xia Y, et al. Asymptomatic cases in a family cluster with SARS-CoV-2 infection. The Lancet Infectious diseases. 2020.


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Kimball A HK, Arons M, et al. 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. 2020; ePub: 27 March 2020.


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Li R, Pei S, Chen B, et al. Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2). Science (New York, NY). 2020.