In France, the population’s immunity to SARS-CoV-2 at around 4.4% in May, modeling suggests

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As of May 11, when the lockout restrictions were relaxed in France, approximately 4.4% of the French population had been infected with SARS-CoV-2, according to a new modeling study. These estimates fall far short of what would be required for collective immunity, say the authors. Understanding the level of immunity to SARS-CoV-2 will be essential to avoid a rebound in the epidemic as populations around the world ease the blockages. Serological studies – which detect anti-SARS-CoV-2 antibodies in patients’ blood – are important to shedding light on immunity issues, but mounting evidence suggests that many people, especially asymptomatic, take time to seroconvert. Meanwhile, researchers seeking to understand viral immunity must rely on indirect measures, including data on hospitalizations and deaths. France was severely affected by the SARS-CoV-2 epidemic and went into detention on March 17, 2020. Following the expected reduction in cases, the French government announced that it would relax the restrictions on May 11. To better understand the underlying level of immunity and infection of the population at that time, Henrik Salje and his colleagues studied the depersonalized hospital records of COVID-19 patients from all hospitals in France, as well as the data from surveillance of these hospitals and of the Diamond Princess cruise ship. About 3.6% of those infected in France were hospitalized and 0.7% of those infected died, according to them, 10.1% of those infected over 80 dying. At all ages, according to the authors, men were more likely to be hospitalized than women. By applying their data to national models of disease transmission, Salje et al. estimate that locking in France resulted in a 78% reduction in transmission. As of May 11, when restrictions were relaxed in France, 4.4% of the population was likely infected, although infection rates were closer to 10% in the two French regions most affected. However, as around 65% of the population would need to be immunized for the epidemic to be controlled solely by immunity, the population’s immunity seems insufficient to avoid a second wave, the authors say. Effective control measures must be maintained. The authors note that their model projections can support French government health care planning by forecasting hospital bed capacity requirements.

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