The effects of Omicron emergence on seroprevalence and hybrid immunity in the Finnish adult population

By Pooja Toshniwal PahariaReviewed by Danielle Ellis, B.Sc.Feb 28 2023

In a recent study posted to the medRxiv* preprint server, researchers evaluated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence and population-level humoral immunity among Finnish individuals between April 2020 and December 2022.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

Coronavirus disease 2019 (COVID-19) has resulted in considerable morbidities and unprecedented mortality across the globe, progressing in varying magnitude and rates in different nations. Evaluating humoral immunity levels against SARS-CoV-2 could provide valuable insights into the population-level SARS-CoV-2 exposure, dynamics, and the effects of COVID-19 containment strategies and measures, including vaccinations.

About the study

In the present report, researchers evaluated humoral immunity levels against SARS-CoV-2 among Finnish adults between 2020 and 2022, emphasizing immune protection against the SARS-CoV-2 Omicron variant of concern (VOC).

Serum samples were obtained at ≤5.0 months, six months to one year, and ≥1.0 years from polymerase chain reaction (PCR)-confirmed COVID-19 patients aged between 18 and 85 years. Anti-SARS-CoV-2 immunity levels were evaluated by assessing serum immunoglobulin G (IgG) titers against SARS-CoV-2 spike (S) and nucleoprotein (N) proteins using fluorescent multiplex immunoassays (FMIA).

In total, 34,619 individuals were mailed study participation invites in 40 sequential population-level surveys conducted between 2020 and 2022. In every survey, individuals residing in the top five biggest Finnish health districts were chosen randomly from the population registry of Finland. The team excluded individuals residing in institutional care settings and individuals who received prior invitations for the study or the follow-up study of SARS-CoV-2 seroprevalence surveys.

COVID-19 vaccination data were retrieved from the national vaccination register and  PCR results from the national infectious diseases register of Finland. In addition, data were obtained on at-home SARS-CoV-2 antigen tests for individuals participating in 2022. The level of humoral protection against SARS-CoV-2 was evaluated by integrating anti-S IgG and anti-N IgG titers with SARS-CoV-2 vaccination data.

The findings were grouped into quartiles of the calendar year, with sera samples obtained between January and March constituting the first quartile (Q1), between April and June comprising Q2, between July and September comprising Q3, and between October and December comprising Q4. Logistic regression modeling was performed for the analysis.

Results

Of 34,619 individuals invited to participate in the study, 28% (n=9,794) of them provided serum samples, among which 61% were women. Participation reduced from 64.0% to 19.0% from the initial specimen in 2020 to the final specimen in 2022. By age, participation of individuals aged between 18 and 29 years was less frequent, whereas those aged between 45 and 64 years and the natives of Finland and speaker groups of Sweden showed disproportionally greater participation.

Most (96%) individuals had received ≥1.0 doses of COVID-19 vaccines prior to 2022, greater than the vaccination rate among Finnish individuals aged 18 to 85 years in Finland (88%) in the period. Anti-N IgG rates were <7.0% till Q4 2021. Following Omicron emergence, anti-N IgG rates elevated swiftly, to 31.0%, with the greatest SARS-CoV-2 seroprevalence of 41% among individuals aged 30 to 44 years in Q1 2022. In the final quartile of 2022, anti-N IgG titers prevalence rates were 54% and 71% among all Finnish adult individuals and those aged between 18 and 29 years, respectively.

For serum samples obtained ≤5.0 months, six months to one year, and >1.0 years of PCR-confirmed SARS-CoV-2 infections, SARS-CoV-2 anti-N IgG titers were observed among 291 (90%), 146 (74%), and 26 (65%) samples, respectively. Integrating anti-N IgG titers with PCR reports and/or home-based SARS-CoV-2 antigen tests yielded similar results. In 2020, anti-N IgG titers were observed among 2.0%, 4.0%, and 2.0% of Finnish adults in Q2, Q3, and Q4m, respectively.

In 2020, anti-S IgG titers were observed among <3.0% of Finnish adults. In 2021, anti-N IgG titers were observed among <7.0% of individuals and were greatest among individuals aged between 30 and 44 years in the final quartile of 2021. In the following years, SARS-CoV-2 seroprevalence increased across ages. From the second quartile of 2022 onward, anti-N IgG titers were most prevalent among individuals aged between 18 years and 29 years, followed by a gradual decline with advancing age.

In the second quartile of 2021, among individuals aged 18 to 29 years, 30 to 44 years, 45 to 64 years, and 65 to 85 years, anti-S IgG titers were observed among 13.0%, 28.0%, 51.0%, and 74.0% of individuals, respectively. Anti-S IgG levels showed rose steeply in 2021, reaching 85% in the final quartile of 2021, and were persistent at >90.0% through 2022. In the final quartile of 2022, 99% of Finish adult indiviudals had anti-S IgG titers.

Among unvaccinated adults, anti-S IgG titers were observed among 2.0% and 9.0% of individuals in Q1 and  Q2 of 2021, respectively. In the preceding year, most individuals did not have humoral immunity against SARS-CoV-2, and most had vaccine-elicited immunity only in 2021. From July 2021 onward, 2,937 of 3,038 individuals received ≥1.0 COVID-19 vaccine. Infection-induced and vaccine-induced (hybrid) immunity was low till the initial days of 2022, with hybrid immune protection levels elevating from 4.0% in Q4 2021 to 31.0% in Q1 2022.

Hybrid immunity levels were 53% and 71% among adults below 29 years in Q2 and Q4 2022, respectively, with least rates of 39% among older individuals aged between 65 years and 85 years through Q4 2022. Among Finnish adults, hybrid immunity levels increased from 37% in the second quartile of 2022 to 51% in Q4 2022.

Conclusion

The study findings showed the changing trends in SARS-CoV-2 seroprevalence among Finnish adults between 2020 and 2022 due to COVID-19 vaccinations and previous SARS-CoV-2 infections. The results indicated that SARS-CoV-2 containment strategies and measures, combined with vaccination coverage, could curtail SARS-CoV-2 transmission.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.