What is “Rt”?
Considered as a concrete indicator of the spread of the coronavirus through society, the reproduction rate globally represents the contagiousness of SARS-CoV-2.
First calculated from three factors (the duration of contagiousness after infection, the probability of an infection after contact between an infected person and a person likely to be, and the frequency of human contact), this rate of reproduction, formerly “R0”, has transformed over the months into “Rt”, a more regular index which also changes more often according to the sanitary measures adopted.
“If this reproduction rate remains below 1, this means that each infected person will infect less than one person, and that the epidemic will gradually disappear, said Frédérique Jacobs, spokesperson for the Covid-19 crisis center. during the summer. But if the R is greater than 1, each person will infect more than one person and the epidemic will continue […] The authorities’ goal is naturally to lower this “Rt” as much as possible and to keep it below 1 in order to control the epidemic, and then stop it. “
Importantly, the reproduction rate “is calculated according to parameters that attempt to reproduce human behavior”, underlines Sciensano. “This rate (which is obtained on the basis of complex mathematical models, Editor’s note) must be appreciated alongside other indicators”, such as the daily number of hospitalizations and deaths. Concretely, it is not because the “Rt” decreases that the virus is no longer contagious or that it is no longer circulating.
The K factor, the other factor
Because SARS-CoV-2 first appeared almost two years ago, experts continue to learn more and more about the virus and how it is spread. Thus, at the end of last year, several studies confirmed that 10% of people who tested positive were responsible for 80% of contamination.
Therefore, if the “Rt” (the reproduction rate) makes it possible to calculate the total number of individuals that an infected person can infect, it must be correlated with the factor K, that is to say the coefficient of dispersion of the virus.
According to a prepublication from the London School of Hygiene & Tropical Medicine (LSHTM), the K factor of SARS-Cov-2 is 0.1 against 0.16 for the previous SARS-CoV (2003), 0.25 for MERS (0.25) and 1 for influenza.
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