The coronavirus disease (COVID-19) pandemic first started in late December 2019 in Wuhan City, China. From there, it has spread across the globe. During the first peak of cases in March, France is one of the hardest-hit countries, with the cases now reaching more than 1 million, with at least 34,000 deaths.
The government has imposed an initial lockdown in March, banning large gatherings and closing schools. In August, when restrictions were eased, there was a resurgence of new COVID-19 cases.
Now, a new study by researchers at the Santé Publique France conducted seroprevalence estimates in France, one of the countries with high COVID-19 cases in Europe.
The team estimates that the nationwide lockdown substantially contained the virus transmission. However, the researchers also found that a vast majority of the French population remains vulnerable to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19.
In the study, published on the pre-print server medRxiv*, the researchers aimed to assess the cumulative incidence of SARS-CoV-2 infections, which is critical for monitoring the pandemic's course and extent.
To arrive at the study findings, the researchers conducted nationwide serological surveillance. They performed a serial cross-sectional sampling of residual sera over three periods – before the lockdown on March 9 to 15, during the lockdown on April 6 to 12, and following the national lockdown on May 11 to 17.
The team tested each sample for anti-SARS-CoV-2 immunoglobulin G (IgG) antibodies, which target the nucleoprotein and spike protein using two Luciferase-Linked ImmunoSorbent Assays. They also examined for the presence of neutralizing antibodies using a pseudo-neutralization assay.
Estimated prevalence of SARS-CoV-2 antibodies in the French population by region.
What the study found
The team has collected more than 9,100 residual sera in France, which were randomly selected from available sera at three collection periods.
They also found that the nationwide seroprevalence of SARS-CoV-2 infections increased from 0.41 percent to 4.14 percent and 4.93 percent between the collections made in March, April, and May. The data correspond to more than 3.2 million people having been infected as of May.
"Note that when taking into account the inherent delay between infection and IgG-mediated antibody responses, this estimate provides the number of infections which occurred approximately two weeks before the collection periods," the team explained in the study.
The researchers also revealed that the prevalence of pseudo-neutralizing antibodies for the SARS-CoV-2 spike protein increased from 0.06 percent to 3.33 percent over the same period. Further, from mid-March to mid-May, the prevalence of infections increased in all age groups. However, the highest was among the 50 to 59-year-old and 60 to 60-year-old residents, and the lowest was among children who are 10 years old and below.
Based on the study findings, the team noted that following the first wave of the COVID-19 epidemic, seroprevalence remained low, with about 5 percent of the population developing a detectable humoral response to the virus.
"Estimates at multiple points of the French epidemic show a sharp increase between the first two collection periods, immediately preceding and during the generalized lockdown, followed by little progression observed at the final collection period just after the lockdown ended. This confirms its substantial impact in almost halting community transmission," the authors wrote.
The seroprevalence estimates, including the number of people having produced pseudo-neutralizing antibodies, confirm that post lockdown, most of the residents in France remain susceptible to SARS-CoV-2, including those living in regional hotspots.
The team concludes that the study provides a better understanding of the first outbreak's progression in the country. It can help provide a framework to help in the ongoing battle against the second wave of the country's COVID-19 pandemic.
"Serological surveillance based on residual sera will continue to be used to provide timely seroprevalence estimates as the epidemic evolves and through the 2020-2021 winter season to monitor the progression of population-level immunity and guide public health response," the team added.
France is not the only country that has reported a second wave of infections caused by SARS-CoV-2. The United Kingdom is experiencing a new wave of infections after easing its restrictions and lockdown orders. The U.K. has more than 792,000 cases.
Globally, the number of cases has reached more than 41 million people, with at least 1.13 million deaths.
The United States has the highest number of cases, topping 8.4 million infections, followed by India, with more than 7.7 million cases.
The other countries with high infection tolls include Brazil, with more than 5.29 million cases, Russia, with more than 1.45 million cases, and Argentina, with more than 1.05 million cases.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.