Transmission of Covid-19: what is R0, a determining indicator on the resumption of the epidemic?

At a time when “weak signals” of epidemic resumption have been observed, barrier gestures are more than ever in order. Wearing a mask, washing hands and physical distancing are therefore required to limit the spread of the coronavirus. But these are not the only effective measures to prevent a possible second wave of the Covid-19 epidemic. The health authorities also observe a key marker to measure the evolution of the epidemic on the territory: the R0. But what is it all about?

Covid-19: what is R0 or R “zero”?

The R0 or R “zero” is the average reproduction rate or the average rate of contagion of the virus. This means that this indicator makes it possible to determine how many people on average will be infected by contact with a single individual carrying Covid-19. This measurement is the result of three factors: the transmission of the virus either by contact, through droplets or infected surfaces, the number and degree of contacts per day and the duration of contagiousness.

This marker makes it possible to evaluate the circulation of the virus and to avoid a second wave of the epidemic. “If the R0 (or R) is greater than 1, a patient will infect more than one person, so the epidemic will increase. If it is less than 1, gradually the patients contaminate less than people and therefore the epidemic can diminish or even disappear “, explained the Minister of Health Olivier Véran on April 6.

The reproduction rate is increasing in France

This indicator must therefore be kept below 1 to break the chains of transmission, reduce the number of cases of coronavirus and put an end to the epidemic. When this marker is between 1 and 1.5, we speak of the vigilance threshold. And when it exceeds 1.5, we speak of an alert threshold.

The confinement had made it possible to reduce the rate of reproduction of the virus, as indicated by the Scientific Council in an opinion delivered last April. “The available data (…) indicate that the confinement practiced (…) made it possible to reduce the transmission of the virus by 84%, with an estimated reproduction number of 0.5 during the confinement, while it was of 3.3 before the initiation of confinement. This resulted in a significant decrease in the number of intensive care admissions “.

But after deconfinement, the rate of reproduction of the virus had increased again. Throughout France, this increase is still ongoing: on July 23 the R0 was at 1.29, according to data from Public Health France. And in Brittany, where the situation was particularly worrying – the virus reproduction rate was over 2 – the R0 is now at 1.87.



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