Corona Virus Herd Immunity in Estonia: KoroSe-ro-EST Study
The first COVID-19 case in Estonia was registered on 26 February 2020. One year later, in spring 2021, over 100,000 people have been diagnosed and over 1,000 deaths have been registered in connection with COVID-19.
Stopping the spreading of the disease and restoring the normal way of life is possible when the majority in the society has developed a permanent immunological defence, i.e. the herd immunity. To achieve that, the percentage of people with SARS-CoV-2 antibodies in the total population should probably be 50–75%.
The purpose of KoroSero-EST or corona virus seroepidemiological study in Estonia has been to determine the percentage of the population who have SARS-CoV-2 anti-bodies, i.e. the seroprevalence in Estonia at different times, and to assess the persis-tence of immunity after recovery.
In order to assess seroprevalence, we have repeatedly organised cross-sectional seroprevalence studies, where we have tested the prevalence of SARS-CoV-2 antibodies in all age groups and in different counties. We have estimated the persistence of both the antibody-mediated humoral as well as cell-mediated immunity in a case-control study within the KoroSero-EST study among individuals with SARS-CoV-2 antibodies, i.e. seropositive individuals.
In July 2020, a study conducted in Järveotsa General Practice in Tallinn showed a 1.5% SARS-CoV-2 seroprevalence (95% CI 0.9–2.5%), while Kuressaare General Practice on the Island of Saaremaa, which was the epicentre of the infection at the time, showed a 6.3 % rate (95% CI 5.0–7.9%). In both General Practices, the seroprevalence exhibited similar rates among men and women, and among different age groups. 80% of the seropositive had not experienced any symptoms, i.e. had been asympto-matic. The estimated prevalence of the infection in Harju County was about 13 times higher than the national statistics based on PCR analyses, and in Saaremaa about four times higher.
In September 2020, the national seroprevalence was 0.9% (95% CI 0.6–1.3%), reaching the highest level in Saaremaa. No seropositive individuals were found in the sample groups in eight counties.
In March 2021, the national seroprevalence rate was 20.1% (95% CI 18.5–21.7), being the highest in Harju County with 28.0%, and the lowest in Lääne-Viru County with 4.6%. Seroprevalence was similar across all the age groups. The data in the Patient Portal shows that around one in three seropositive individuals had not been aware of their COVID-19 infection, and were not vaccinated against SARS-CoV-2.
After recovering from a light or asymptomatic SARS-CoV-2 infection, the anti-bodies and T cell immunity persisted for at least six months in 80% of the tested individuals; however, the high level of infection markers shows that the disease can have long-term health effects.
Therefore, one year after the start of the pandemic, 20–30% of the Estonian po-pulation have SARS-CoV-2 antibodies; this means that we have not achieved herd immunity and a large part of the population is receptive to SARS-CoV-2 infection. A large majority of the seropositive have experienced light symptoms or been comple-tely asymptomatic, and have remained immune for at least six months.