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Popularization of research advances on COVID-19

A model to study the epidemic’s dynamics

At the end of 2019, China reported unusual cases of pneumonia in the province of Wuhan. A few weeks later, the first cases of COVID-19 were detected, the 21st of January in the United States and the 24th of January in France. But there was a great deal of uncertainty concerning the introduction of the virus into these countries and the transmission patterns of COVID-19 before the beginning of the 1st wave in March 2020. The low number of diagnostic tests and the absence of monitoring of travellers entering the countries did not allow the timing of contaminations to be easily reconstructed. In this study, American researchers (at Northeastern University, Boston) have nevertheless tried to understand how the virus spread in the beginning.

The researchers developed an epidemiological model to understand its dynamics. The model used a great deal of information available on the beginnings of the epidemic in the United States and in 30 European countries, notably data on people’s mobility.

This model firstly allowed an estimation of the number of real cases, which is much higher than the number of cases officially reported. This underlines the significance of the virus’ transmission before countries put their diagnostic testing programmes into operation. Only 1 to 3 cases of infections out of 100 had been detected at the beginning of March 2020.

The start of the epidemic in European countries and in the United States is clearly linked to cases arriving from China. For example, according to this model, 72% of early COVID-19 cases, before the 1st wave of the epidemic in Italy, came from China. But the role of China in the virus’ circulation is in the main limited to these early cases. Later diffusion of SARS-CoV-2 occurred at a local and intra-community level.

With regard to mortality, there is a clear correlation between the number of deaths in April and May 2020 and the easing of sanitary precautions. Different arguments show that these measures, including lockdowns, enabled the number of deaths linked to COVID-19 to be limited.

In conclusion, in this study the researchers have provided a model analysing the dynamics of the early phases of the epidemic. This model, used in real time, could allow us to predict those areas where the virus would be most likely to spread and set up diagnostic testing and other appropriate preventive measures.

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