SARS-CoV-2 prevalence during the initial period of SARS-CoV-2 Omicron variant emergence in Finland

A recent study posted to the medRxiv* preprint server presented the rapid spike in coronavirus disease 2019 (COVID-19) seroprevalence during the emergence of the Omicron variant in Finland.

Background

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant of concern (VOC) emerged at the end of 2021. It altered the epidemiology of the ongoing SARS-CoV-2 pandemic with a swift increase in COVID-19 cases worldwide. On November 29^th, 2021, the first case of the SARS-CoV-2 Omicron variant was detected in Finland. Nevertheless, data on the prevalence and exposure of Omicron in the initial months of Omicron emergence in Finland is not well established.

About the study

In the present research, the scientists assessed alterations in prevalence and exposure of COVID-19 during the initial period of the SARS-CoV-2 Omicron VOC emergence in the Greater Helsinki area, Finland. The team conducted a prospective seroepidemiological evaluation of COVID-19 on 1,600 routine serum specimens delivered to Helsinki University Hospital Laboratory (HUSLAB) for human immunodeficiency virus (HIV) serology. This analysis was performed between November 15^th, 2021, and  March 6^th, 2022. 

The present study was initiated in 46 calendar weeks in 2021 and ended in nine calendar weeks in 2022. A total of 100 serum samples were chosen at random and tested for SARS-CoV-2 immunoglobulin G (IgG) antibodies against the viral receptor-binding domain (RBD) of the spike 1 (S1) and nucleocapsid (N) proteins. Further, the S1 and N proteins were assessed using SARS-CoV-2 IgG II Quant (S protein) and Abbott SARS-CoV-2 IgG (N protein) analyses, respectively, for each calendar week of the study.

The samples for each calendar week were selected by choosing a random initiation point and selecting specimens systematically till 100 specimens, and five extra samples were obtained. Subsequently, the 100 selected specimens were evaluated for COVID-19 antibodies. If the specimen volume was insufficient or the assay failed, the sample was substituted with one of the extra samples corresponding to that calendar week.

The authors estimated the percentage of the subgroups identified based on the serostatus for each calendar week. The statistical analysis of the study was conducted using IBM statistical package for the social sciences (SPSS) software.

Results

The study results demonstrated that the subgroups derived from the serostatus evaluation of the samples were 1) anti-S1 negative/anti-N negative, 2) anti-S1 positive/anti-N negative, 3) anti-S1 positive/anti-N positive, and 4) anti-S1 negative/anti-N positive. These four subgroups respectively indicate 1) no serological proof for previous SARS-CoV-2 infection or vaccine immunization; 2) no proof for recent COVID-19 but seroresponse to vaccination was present, 3) consistent with vaccination status prior infection status unknown, and 4) no proof for vaccination but possible recent infection.

The mean age of the study participants was 33 years, and more than 55% population included in this research were females. During the initial five weeks of the study, the baseline prevalence of SARS-CoV-2 N antibodies was 5.2%, whereas in the last five weeks, it rose to 28.2%. At the same timestamps, the percentages of SARS-CoV-2 seronegative samples dropped from 11.6% to 3.8%, and anti-S1 positive/anti-N negative samples declined from 84.2% to 68.2%.

In people aged less than 30 years, a dramatic increase in N antibody seroprevalence was detected during the research period. The seroprevalence of N antibodies was continuously beneath 10% by the end of 2021, but it demonstrated a quick increase by week 1 of 2022 and exceeded 20% by week 3 of 2022.

Anti-N positive/anti-S1 negative samples started to appear from week 2 in 2022 and accounted for 0.9% of all studied samples, which might indicate a delayed or diminished seroresponse towards S1 in Omicron infections. The declining percentage of seronegative samples from group 1 towards the study's conclusion depicted the elevation in anti-N positive samples in groups 3 and 4. 

Omicron became the dominant SARS-CoV-2 variant in the Greater Helsinki area by the middle of December 2021. The findings demonstrated a rapid incline in the individual-level SARS-CoV-2 exposure, suggesting high COVID-19 transmission. This observation corresponded to prior investigations on the topic. Nearly 4% of the participants were SARS-CoV-2 S and N seronegative at the end of the research. Around 78% of study volunteers exhibited SARS-CoV-2 S antibodies/without N antibodies, reflecting viral immunization from COVID-19 vaccination/without recent infection.

Conclusions

The study findings depicted that the prevalence of SARS-CoV-2 N antibodies rapidly elevated from 5.2% between 46 and 50 weeks in 2021 to 28.2% between five and nine weeks in 2022 during the research period. This data indicates that around 23% of the subjects were SARS-CoV-2-infected during the study period. However, the percentage of seronegative and anti-S1 positive/anti-N negative samples dropped. Anti-N positive samples that were negative for anti-S1 antibodies started to appear by week 2 in 2022.

Overall, the study depicts a significant hike in the prevalence of SARS-CoV-2 N antibody during the initial period of Omicron emergence in Finland, reflecting a high COVID-19 transmission rate. Additionally, the results showed that during the emergence of the Omicron VOC in the Greater Helsinki Area, Finland, a significant number of COVID-19 patients remained undiagnosed. The authors suggested subclinical SARS-CoV-2 infections and reduced reverse transcription-polymerase chain reaction (RT-PCR) tests as reasons for this observation.

*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.

Written by

Shanet Susan Alex

Shanet Susan Alex, a medical writer, based in Kerala, India, is a Doctor of Pharmacy graduate from Kerala University of Health Sciences. Her academic background is in clinical pharmacy and research, and she is passionate about medical writing. Shanet has published papers in the International Journal of Medical Science and Current Research (IJMSCR), the International Journal of Pharmacy (IJP), and the International Journal of Medical Science and Applied Research (IJMSAR). Apart from work, she enjoys listening to music and watching movies.