SARS-CoV-2 serological findings and exposure risk among employees in school and retail after first and second wave COVID-19 pandemic in Oslo, Norway: a cohort study.

Objectives: The objective was to characterize and compare SARS-CoV-2 serology among Norwegian school employees and retail employees, and describe preventive measures taken at the workplaces.

Material And Methods: A cohort of 238 school and retail employees was enrolled to an ambidirectional cohort study after the first COVID-19 pandemic wave. Self-reported exposure history and serum samples were collected at 10 schools and 15 retail stores in Oslo, Norway, sampled at 2 time-points: baseline (May-July 2020); and follow-up (January-March 2021). SARS-CoV-2 antibodies targeting both spike and nucleocapsid were detected by multiplex microsphere-based serological methods.

Results: At baseline, 6 enrolled workers (5 in retail) presented with positive SARS-CoV-2 serology, higher than the expected 1% prevalence (3%, 95% CI: 1-6, p = 0.019). At followup, school and retail groups presented 11 new seropositive cases altogether, but groups were not significantly different, although exposure and preventive measures against viral transmission at workplaces were different between groups. Self-reported medical history of COVID-19 infection showed that all but one positive SARS-CoV-2 serological findings arising between baseline and follow-up had been diagnosed with virus testing.

Conclusions: Distribution of SARS-CoV-2 positive serology after the first wave was slightly higher than expected. Distribution of infection was not significantly different between the groups at baseline nor at follow-up, despite difference in exposure and protective measures. Nearly all new seropositive cases discovered between baseline and follow-up, had already been diagnosed, highlighting the importance of extensive viral testing among workers. Int J Occup Med Environ Health. 2022;35(5):537-47.