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

Abstract

Background: During initial phases of the coronavirus disease 2019 (COVID-19) pandemic, many workplaces were affected by closures and various preventive measures intended to limit infections. Here, we characterize and compare in an ambidirectional cohort study SARS-CoV-2 serology among Norwegian school employees and retail employees at baseline following the first epidemiological wave, and at follow-up after a second wave. Methods: We enrolled a cohort of 238 school and retail employees 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 two time-points, baseline (May 18. to July 2. 2020) and follow-up (Jan 7. to Mar 17. 2021). SARS-CoV-2 antibodies targeting both spike and nucleocapsid were characterized by multiplex microsphere-based serological methods. Results: At baseline, 6 enrolled workers presented with positive SARS-CoV-2 serology (3%; CI [1, 6]; P=0.019), which was significantly higher than the expected 1% prevalence in the general Oslo-population at this time-point. Five of the positive cases were retail employees. However, school and retail groups distributions at baseline were not significantly different as the number of seropositive observations were limited. Due to a school closure effectuated during the first wave, half of the school employees reported [≤]2 days of physical workplace presence per week, while 65% of the retail employees reported [≥]5 days per week. Eight months later, after passing a second epidemiological wave, school and retail groups presented 11 new seropositive cases altogether, but there was still no significant differences between the groups. Physical attendance at the workplace was similar between the groups during the second wave, but some preventive measures against viral transmission at workplaces were different. Self-reported virus diagnostics (RNA) for the same period were compared to the serological data obtained in this study, showing that all but one positive SARS-CoV-2 serological findings arising between baseline and follow-up had been diagnosed with virus testing. Conclusions: After the first wave, distribution of SARS-CoV-2 positive serology was slightly higher than expected in a cohort of school and retail employees. Distribution of infection was not significantly different between the groups at baseline nor at follow-up, even though physical workplace attendance had been different. Nearly all new seropositive cases discovered in this study between baseline and follow-up, had already been diagnosed due to widespread virus testing during the second wave. This highlights the importance of extensive viral testing among workers.