The estimated impact of the COVID-19 epidemic in the general population of France

Background Model-based predictions for the COVID-19 outbreak revealed the potential for extraordinary mortality and saturation of health care systems if no action was taken. The pandemic hit France in late January 2020. Lockdown was implemented on March 17, 2020. The goal was to drastically reduce the number of infections, but at the cost of fear of a second epidemic wave when easing the lockdown. Our aim was to characterize the dynamics of the COVID‑19 spread in France and estimate the proportion of coronavirus-infected individuals using ground truth from syndromic surveillance data. Methods National health authorities provide data from syndromic surveillance of the diagnosis of suspected COVID-19 reported by a sample of primary care physicians and from epidemic surveillance of confirmed cases, originating from hospitals. By extrapolation, COVID-19 incidence in the general population can be estimated. In turn, a back-calculation model can infer the number of contagious individuals, providing an idea of the spread of the epidemic before the implementation of lockdown measures. Results This study estimated that 12.3 million individuals were diagnosed 9suspected COVID-199 on May 6, 2020. At lockdown start, 2.5 million people were already contagious. The infection attack rate peaked on March 27, ten days after lockdown. The predicted sharp drop was not observed. The dynamics of the epidemic followed a continuous curve with a decline phase 2.35 times slower than the growth phase. 80% of infections occurred after lockdown. Conclusions These results call into question the effectiveness of lockdown. The epidemic would have followed its 9natural trajectory9, beginning long before the health system detected the first cases. This hypothesis does not dispute the caution required with regard to the extraordinary spread of the epidemic, with less affected geographic areas becoming a source of susceptible individuals.