40% of Covid-19 cases are asymptomatic

The Covid-19 can take an asymptomatic form as well asa serious form potentially fatal according to those affected. If the confirmed asymptomatic cases, that is to say those which do not develop any symptoms at all, are not a burden for the hospitals, they can transmit the coronavirus without realising it. Estimating the proportion of asymptomatic cases among the infected is a precious epidemiological data.

Several estimates of the proportion of asymptomatic cases have been made through meta-analyzes, studies that draw on previously published scientific literature for statistical analyzes. The most recent was published in Jama Network on December 14, 2021. Scientists from the Beijing School of Public Health included 94 different studies, written in Chinese and English, for a total of over 29 million individuals considered. They estimate that 40.5% of confirmed cases of Covid-19 are asymptomatic.

40.5% asymptomatic among confirmed cases of Covid-19

In this meta-analysis, scientists included true asymptomatic patients and patients in the pre-symptomatic phase, that is, patients who did not present symptoms at the time of screening but who develop it afterwards. The percentage of asymptomatic cases was calculated for two large cohorts: people who will be screened and people with confirmed infection.

For the first cohort, 94 studies for 29,776,306 million individuals, including 11,516 asymptomatic, were examined. Taken together, the percentage of asymptomatic people among the population being screened is 0.25%.

The analysis on confirmed cases of Covid-19 was carried out on a smaller sample: 77 studies for 19,884 cases of Covid-19 including 11,069 asymptomatic forms. In total, the percentage of cumulative asymptomatic form among cases of Covid-19 confirmed is 40.5%.

Limited statistical data

Depending on the sub-populations considered, the percentage changes. For example, there are 3.69% of asymptomatics among the over 60s who are screened; 60.21% of asymptomatic in children under 20 with confirmed Covid-19.

Like all meta-analyzes, these analyzes are purely statistical and cannot necessarily be generalized to the population as a whole. In this case, the researchers note that each study looked at a specific population, such as caregivers or seniors for example, and calculated the share of asymptomatics according to different criteria.

Covid-19: there are 17 to 25% of asymptomatic people

Article published on October 5, 2020 by Julien Hernandez

A recent meta-analysis responds a little more precisely to the question of the number of asymptomatic people and their role in the transmission of the disease. virus.

Since the start of the pandemic, this is a question that has tormented scientists. How many asymptomatic people are there and what is their role in the transmission of the virus. A recent meta-analysis published in PLoS Medicine and carried out by researchers from the Swiss Institute of Social and Preventive Medicine, answers this question a little more precisely.

17 to 25% of asymptomatic people

The investigators looked for in various data base scientists with information to answer their question, published between March 25 and June 10. Studies of people with SARS-CoV-2 diagnosed by RT-PCR that documented follow-up and symptom status at the start and end of follow-up studies or modelization have been included. A total of 94 studies were finally selected because they met the inclusion criteria.

The overall estimate of the proportion of people infected with SARS-CoV-2 and remaining asymptomatic throughout infection is 17 to 25%. The ability to predict, on the basis of these results, the course of the disease of a future case is between 3 and 67%. To compensate for certain biases of various studies, the scientists carried out other analyzes by taking into account only the seven studies of population which they had. Here, we see a proportion of asymptomatic people ranging from 26 to 37% and a predictive capacity between 24 and 38%.

Limited transmission capacity

However, according to these data, it would seem that the contamination by asymptomatic people is weak. The authors note a relative risk of infection reduced by 75% (with a fairly wide confidence interval, between -90 and + 27%) following contact with an asymptomatic person, compared to a symptomatic person. In modeling studies, presymptomatic individuals are the most contagious. Several limitations are nevertheless present in this study, such as the fact that the design of the included studies was not designed to estimate the proportion of asymptomatic infections with SARS-CoV-2 and was at risk of selection bias. Likewise, the researchers did not take into account the possibility of false negatives which would therefore underestimate the proportion of asymptomatic individuals.