Corona Immunitas Fribourg : Immunité de la population Episode 2/2021

Corona Immunitas Fribourg aims at estimating the number and proportion of people who have been infected with the SARS-CoV-2 virus and have developed antibodies in the Canton of Fribourg. It is carried out within the framework of the national Corona Immunitas program coordinated by the Swiss School of Public Health (SSPH+) and is planned in 5 phases. Several Cantons have joined the program at different phases. In Fribourg, a first study took place from July 2020 to October 2020 (phase II of Corona Immunitas). The results, published in the report "Corona Immunitas Fribourg: Population Immunity, Episode 1/2020", revealed that after the first wave about 8% of the population in the Canton of Fribourg has developed antibodies against the SARS-CoV-2 virus. For the second study in Fribourg (phase III of Corona Immunitas), the aim was to estimate the proportion of people who developed antibodies during the second epidemic wave. 3’841 adults aged 20 years and older were invited to participate and 449 (12%) agreed to undergo serological testing between November 30, 2020 and February 5, 2021 (55% women; mean age: 54 years). The proportion of people with antibodies was 19% (95% confidence interval: 15% to 23%). The small number of people vaccinated at the end of January represents a negligible share of this proportion. Therefore, since the beginning of the pandemic and until the end of January 2021, that is during the second wave and before the deployment of the vaccination campaign, we estimate that about 50'000 (between 40'000 and 60'000) adults aged 20 years and more in the Canton of Fribourg have been infected by the virus. During this same period, more than 23’321 adults aged 20 years and older were diagnosed as positive for SARS-CoV-2 (by PCR or rapid antigen test) in the Canton of Fribourg. We can therefore estimate that approximately 1 out of 2 infections was diagnosed. If the period from October 2020 to January 2021 is considered only, approximately 3 out of 4 infections were diagnosed. By comparison, after the first wave and before the second wave, we estimated that only 1 out of 8 infections was diagnosed.