14 May 2020

Estimating the burden of SARS-CoV-2 in France

A new study by the Institut Pasteur reveals the burden of SARS-CoV-2 from data collected in France on a national scale. Using models applied to hospital and death data, this new study estimates the impact of the lockdown and current population immunity.
France has been heavily affected by the SARS-CoV-2 epidemic and went into lockdown on the 17th of March 2020, with relieving measures starting on 11th of May. In order to appropriately exit from the lockdown, researchers need to understand the underlying level of population immunity and infection, identify those most at risk for severe disease and the impact of current control efforts.
Daily reported numbers of hospitalizations and deaths only provide limited insight into the state of the epidemic. Many people will either develop no symptoms or symptoms so mild they will not be detected through healthcare-based surveillance. 
By 11th of May, Institut Pasteur projected that 5.7% of the population, roughly 4 million people, will have been infected, the lockdown reducing the reproductive number from 3.3 to 0.5 (84% reduction). They also found that while 2.6% of infected individuals required hospitalization.
On the basis of these figures, one can estimates that there was an overall 0.5-0.6% of mortality from all COVID-19 patients, much less than the 19% case fatality rate based on the number of laboratory confirmed cases and total number of reported deaths on the same day.
Detailed data shows large variation depending on age: 0.001% of mortality rate in those under 20 years old, to 8.3% in those over 80 years old. The concentration of hospitalized cases in older individuals has led to hypotheses that there may be widespread ‘silent’ transmission in younger individuals. Moreover, across all ages, men are more likely to be hospitalized, enter intensive care, and die than women.