Rapid increase in SARS-CoV-2 seroprevalence during the emergence of Omicron variant, Finland

Abstract

Objectives The aim of this study was to assess changes in exposure and prevalence of SARS-CoV-2 infection during the first months of emergence of Omicron variant in the Greater Helsinki area, Finland. Methods A prospective seroepidemiological survey of SARS-CoV-2 was conducted on 1,600 serum specimens sent to Helsinki University Hospital Laboratory (HUSLAB) for HIV serology between 15 November 2021 and 6 March 2022 (calendar weeks 46/2021 - 9/2022). For each calendar week, 100 serum specimens were randomly selected and analysed for SARS-CoV-2 IgG antibodies against nucleocapsid (N) and spike 1 (S1) protein with Abbott SARS-CoV-2 IgG (N protein) and SARS-CoV-2 IgG II Quant (S protein) tests, respectively. Results The prevalence of N antibodies increased from 5.2% (weeks 46-50/2021) to 28.2% (weeks 5-9/2022) during the study period. The proportion of seronegative samples as well as anti-N negative, anti-S1 positive samples decreased correspondingly from 11.6% to 3.8%, and 84.2% to 68.2%, respectively. Anti-N positive samples that were anti-S1 negative only began to appear as of week 2/2022. Conclusions A rapid increase in the N antibody prevalence was observed over the study period, suggesting a high transmission rate. A substantial proportion of COVID-19 cases remained undiagnosed during the emergence of Omicron variant in the Greater Helsinki Area, Finland.

SARS-CoV-2 seroepidemiology in paediatric population during Delta and Omicron predominance

“…By April 2022, more than half of the children included in the study had serological evidence of previous exposure to SARS-CoV-2. These findings indicate the high infection rate of the Omicron variant in children and are in line with other contemporary seroprevalence surveys from the USA and Europe [10], [11].…”

“…By April 2022, more than half of the children included in the study had serological evidence of previous exposure to SARS-CoV-2. These findings indicate the high infection rate of the Omicron variant in children and are in line with other contemporary seroprevalence surveys from the USA and Europe [10,11]. In the USA, approximately 75% of children and adolescents had detectable antibodies for natural SARS-CoV-2 infection by February 2022, with a marked increase in seropositivity being detected after the appearance of the Omicron in December 2021 [10].…”

“…In the USA, approximately 75% of children and adolescents had detectable antibodies for natural SARS-CoV-2 infection by February 2022, with a marked increase in seropositivity being detected after the appearance of the Omicron in December 2021 [10]. At the beginning of 2022 in Finland, seropositivity in children and young adults <30 years increased considerably to 40% and was even higher compared to adults 30-45 years (30%) and >45 years (20%) [11].…”

Unusual late epidemic peak during influenza season 2021–2022: A nationwide register‐based analysis in Finland

“…Many countries have reported similar findings of atypical epidemics after the ending of social restrictions 8–10 . Although the restrictions were ended in September 2021, the arrival of Omicron in December 2021 lead to restrictions set back to use in January 2022 11 . As the omicron wave started to decrease, the restrictions were again loosened in March 2022.…”

“…[8][9][10] Although the restrictions were ended in September 2021, the arrival of Omicron in December 2021 lead to restrictions set back to use in January 2022. 11 As the omicron wave started to decrease, the restrictions were again loosened in March 2022. The aim of our study was to analyze the influenza season 2021-2022 and how the ending and reimplementation of social restrictions affected the epidemic.…”