The findings were posted ahead of peer review on Wednesday in medRxiv. Researchers from Public Health France, Department of Infectious Diseases, among others, said the study captures mild and asymptomatic cases that typically go unreported.
Study authors estimated that, throughout the first wave of the epidemic, nationwide prevalence of antibodies climbed from 0.41% in March to nearly 5% by May, which translates to 3,292,000 infections, according to the study.
Also, about 70% of those with antibodies had detectable neutralizing antibodies, which varied across ages and regions, authors wrote.
Blood samples were analyzed from 11,021 people and then extrapolated to the nationwide population.
“Seroprevalence estimates confirm that the nationwide lockdown substantially curbed transmission and that the vast majority of the French population remains susceptible to SARS-CoV-2 [the virus causing COVID-19 disease],” study authors wrote.
Antibody prevalence was highest in the area including Paris, where researchers noted the spread of infections happened earlier and more intensely.
“Our results are within the same order of magnitude as studies carried out at comparable epidemic stages in Europe,” the authors also wrote.
The study also found low infections among schoolchildren and suggested that the age group has restricted susceptibility and/or transmissibility.
“Our results provide a critical understanding of the progression of the first epidemic wave and provide a framework to inform the ongoing public health response as viral transmission is picking up again in France and globally,” they concluded. “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.”