Study: Over 70 per cent of Finns have antibodies to COVID-19 produced by the virus
According to an antibody study conducted by the Finnish Institute for Health and Welfare (THL), the vast majority of the population have antibodies that indicate COVID-19 infection.
Based on the study, 73 per cent of the adult population have been infected before the 2022 or the spring 2023 sampling points. 86 per cent of adults under the age of 30 have been infected, and 60 per cent of adults over 65.
According to Merit Melin, Research Manager at THL, the antibody study indicates that COVID-19 infections have been very common among the population and also among those previously infected.
“As a result of vaccinations and infections, the population has built up immunity against various Omicron variants, which maintains good protection against a severe disease,” Melin says.
“However, those who have been infected are not evenly distributed, as for a significant proportion of people, especially the elderly, the protection is based on immunity from vaccination alone,” explains Melin.
The proportion of infected people has continued to grow in all age groups included in the study during the past six months. Re-infections have also been common.
THL has monitored the presence of antibodies from COVID-19 infection in the adult population since April 2020. A random sample of 18–85-year-olds from the five hospital districts of the time have been invited to take part in the study.
Until the end of 2021, the prevalence of antibodies indicating infection was still below 7 per cent. In January–March 2022, the prevalence was 31 per cent and increased to 54 per cent in October–December. (Reference 1)
One quarter had a new infection in spring 2023
Participants in the survey conducted at the end of 2022 were invited to a follow-up visit in spring 2023. The samples were evaluated for both the preservation of antibodies indicating infection and the prevalence of antibodies indicating new infections.
In spring 2023, the prevalence of antibodies indicating infection was 62 per cent in the adult population.
A total of 80 per cent of those who were tested for antibodies indicating infection at the end of 2022 were also found to have antibodies six months later in spring 2023. One quarter of them had had a new infection between October–December 2022 and April–June 2023.
The study shows that 38 per cent of those who did not have antibodies indicating infection in 2022 had been infected before the spring 2023 sampling.
XBB sub-lineages are the dominant variant almost all over the world
The evolution and sub-lineages of COVID-19 have been monitored in Finland since the end of 2021 by sequencing a geographically comprehensive sample of PCR positive samples.
In 2023, the XBB sub-lineages of Omicron have become the dominant variant almost all over the world, displacing the BA.5 and BA.2 Omicron sub-lineages. Currently, XBB.1.5, XBB.1.9.1, XBB.1.9.2 and XBB.1.16 are the most common sub-lineages, accounting for up to 86 per cent of all sequenced samples in Finland at the end of May.
The population's immunity increases as a result of vaccinations and the continuous large-scale circulation of the SARS-CoV-2 virus. This creates evolutionary pressure for viruses to mutate so that they can dodge previous immunity.
“There are more mutations in the receptor-binding domain of the spike protein of the XBB sub-lineage viruses compared to previous variants. These changes affect the ability of antibodies to detect mutated viruses. Because of the mutations, the XBB sub-lineages have caused infections regardless of the immune produced by previous vaccines and infections,” explains Erika Lindh, Senior Researcher at THL.
Autumn vaccination recommendations will be specified after the summer
According to THL's estimate, a booster dose of the COVID-19 vaccine is likely to be recommended for elderly people and certain risk groups in the autumn. The vaccine is to be administered simultaneously with the influenza vaccine in November–December. The estimate of future recommendations has been given so that the wellbeing services counties can prepare in time for organising the autumn vaccinations.
The autumn vaccination recommendation is likely to be specified in August, when international research data on new XBB vaccine products is expected to become available. The epidemic situation in Finland is also continuously monitored.
THL will later provide information on which vaccines will be used in the autumn booster doses and when new vaccine batches will be received in Finland.
All COVID-19 vaccines currently in use in Finland protect against a severe disease.
Further information
Reference 1: Solastie A, Nieminen T, Ekström N, Nohynek H, Lehtonen L, Palmu AA, Melin M. Changes in SARS-CoV-2 seroprevalence and population immunity in Finland, 2020-2022. Emerg Microbes Infect. 2023 Jun 8:2222849. Epub ahead of print.
Merit Melin
Research Manager, THL
[email protected]
Erika Lindh
Senior Researcher, THL
firstname.lastname@thl
