Last update and review: December 26, 2020.
Asympotomatic SARS-CoV-2 infections accounted for 40% of all SARS-CoV-2 infections in the control arm of Astra Zeneca-Oxford vaccine trial that did weekly swabs.
Recent data from the ongoing vaccine trials provide us with a good estimation of the proportion of asymptomatic SARS-CoV-2 infections.
The New Neander’s Medical on December 14, 2020:
COVID-19 epidemiology.
Astra Zeneca’s large trial gives us an idea of the percentage of SARS-CoV-2 infections that are asymptomatic. In the control arm of COV002 group, the one that did weekly swabs, the “rate of attack” for asymptomatic cases was 1.2%. The rate of attack for symptomatic SARS-CoV-2 infections, defined as COVID-19, was 1.8%. The proportion of asymptomatic SARS-CoV-2 infection was therefore 40%. Calculations: 1.2%/(1.2%+1.8%)=40%.
An update and a correction from December 20, 2020.
This article was originally published on May 2, 2020. The calculation in the text of the original article turned out to be incorrect later. In fact, the information in the bulletin published by the IHU hospital in Marseille, France, at the time, was confusing. Indeed, “25858 people tested” refers to the total number of PCR tests done at the IHU hospital in Marseille. How many of them were positive is NOT mentioned at all. Of the 16080 “people with symptoms”, not all were positive for SARS-CoV-2. Their symptoms were caused by something else or the SARS-CoV-2 was not detectable.
We kept the initial text of the article as a snapshot of an error caused by a piece of misleading information.
The text below contains an incorrect calculation.
On April 27, 2020, infectious disease specialist Didier Raoult, head of a large infectious disease hospital network in the area of the city of Marseille, France, published a slide with the current data on the COVID-19 epidemic.
The slide (see below) is in French. There is a lot of interesting information. But what interests us most for this short post is the percentage of asymptomatic people in whom the SARS-CoV-2 virus infection was confirmed by a PCR-test.
This information is in the left upper square:
25858 individuals tested positive for SARS-CoV-2 in the hospitals of the network. Of those who tested positive, 16080 were symptomatic and 1014 were hospitalized.
We can calculate that the percentage of asymptomatic patients is 37%.
(25858-16080)/25858)=37%.
There is a large number of tests and given the strong citizen position of Didier Raoult, any manipulation of data is unlikely. With these data, we, therefore, get a good representation of reality in terms of the proportion of asymptomatic among those who tested positive in a PCR test for SARS-CoV-2.
It is important to note that being “positive for SARS-CoV-2” and being “infected with SARS-CoV-2” is not the same. Many infected with SARS-CoV-2 will not test positive. The viral load may be too low in them. Or, they may be “false negative”.
This being said, 37% of asymptomatic is a lot. If people test positive on a PCR-test for SARS-CoV-2, they are likely to shed virus for between one week and three weeks and, thus, to infect others. This high proportion of infectious asymptomatic is one of the possible explanations of why COVID-19 could not effectively be contained.
The number of new cases dropped in many countries. But there are new cases and they seem to have stabilized at a low steady rate. In the tables below, there is a column with the daily increase in new cases. It uses the number of cases that are still active. As an approximation, all the new cases since the last 20 days are considered as active. The most informative data are from countries with a high number of total cases, such as, for example, Spain, Germany, Italy.
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