Over half of Finns have developed hybrid immunity to the coronavirus by the end of 2022 through a combination of vaccination and natural infection, according to a study conducted by the Finnish Institute for Health and Welfare (THL). This hybrid immunity provides long-lasting, high protection against severe disease.
THL has been monitoring the occurrence of antibodies produced in response to coronavirus infection in the population since April 2020.
The study invited randomly selected adults aged 18-85 from across Finland, and blood samples were collected from nearly 9,800 individuals.
In addition, antibodies produced in response to coronavirus infection in children and young people were studied using anonymized surplus serum samples from the HUS Diagnostic Center.
These studies have made it possible to estimate the occurrence of coronavirus infections in the population regardless of the proportion of infections detected by coronavirus tests reported to the infectious disease register. Since the start of coronavirus vaccinations, the study has also been monitoring the occurrence of antibodies produced by the vaccines.
The prevalence of antibodies indicating a coronavirus infection rose sharply in 2022
According to the study, the prevalence of antibodies indicating a coronavirus infection was less than 7% on average until the end of 2021, but rose to 31% in January-March 2022 and 54% in October-December 2022.
Based on data from the vaccination register and antibody studies, it is estimated that 51% of 18-85 year olds had hybrid immunity produced by the combination of vaccination and infection by the end of 2022.
"In Finland, coronavirus infections began to increase only when vaccination coverage among young adults was already high. Although vaccines have not been able to prevent infections caused by new variants to a large extent, they have provided protection against severe disease for a large proportion of the population. Infections further enhance protection against severe disease," says Merit Melin, research director at THL.
Antibodies indicating a coronavirus infection were most prevalent in the youngest adult age groups from April-June 2022 onwards. For example, in October-December 2022, 71% of 18-29 year olds who participated in the study had antibodies indicating a coronavirus infection, compared to 40% of those aged 65 and over. The latest estimate for children is from August 2022, when antibodies indicating a coronavirus infection were found in 77% of children under 12 and 81% of those aged 12-17.
Changes in the proportions of antibodies obtained through infection and vaccination detected through these studies have been used to assess the effects of epidemic control measures, vaccinations, and new variants on the number of infections.
Over half of Finns have both vaccine and disease-induced COVID-19 antibodies: Study
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