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.