Omicron is the comparatively most transmissible and contagious variant of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2). We conducted a seroprevalence study spring 2022, to investigate the seroprevalence of SARS-CoV-2 antibodies among individuals aged 18 years and older after the Omicron outbreak. The seroprevalence of anti-receptor binding domain (RBD) antibodies was found to be 96.3% (95% CI 95.2-97.2%) compared to 88.2% (95% CI 86.1-90%) in our previous serosurvey. For anti-nucleocapsid (NC) antibodies, the seroprevalence was 39.1% (95% CI 36.6-41.7%) compared to 19.7% (95% CI 17.5-22.2%) earlier. Individuals that experienced breakthrough infections exhibited the highest levels of anti-RBD antibodies. Additionally, saliva samples showed promise as a potential diagnostic biofluid for measuring antibody levels, as they exhibited a strong agreement with the data obtained from serum samples. The near doubling of anti-NC reactivity, a proxy for history of infection, reflects the contagiousness of the omicron variant, but may also have been influenced by a more relaxed approach to precautions in the spring of 2022. Serosurveys repeated at regular intervals monitor the trend of infections in the community, delineate the geographical spread of the infection, and may guide containment measures in communities, and prompt response to future outbreaks.

Recruitment for this study was conducted through invitations, email announcements to the university community, and social media advertising, and potential participants were required to complete an electronic consent and a survey before giving biological specimens. The sample collection was extended for three days, in spring 2022. Participants were initially invited via emails and social media channels. Individuals were eligible for inclusion if they were 18 years of age or older and were able to provide informed consent.

All demographic information and assay results were provided via Excel files. Any participant under the age of 18, those who did not provide a biological sample, or those with non-unique sample IDs were removed from the study. Assay information was first merged together by a sample ID and those in common were kept in the study (inner join). The combined assay data was then merged to the demographic information. Those who provided biological samples but no demographic information were removed from the study (left join). Finally, we performed data cleaning in order to fix input mistakes (e.g. spelling, wrong dates, etc.) and group responses in the appropriate categories.

We corrected input mistakes by participants (e.g. wrong spelling of vaccine source). We also checked whether the vaccination dates were sequential. For example, if the second vaccination date was reported before the first vaccination date, then the second vaccination date was recorded as missing (NA).