Incidence of SARS-CoV-2 infection in a cohort of workers from the University of Porto

BackgroundRepeated serosurveys in the same population provide more accurate estimates of the frequency of SARS-CoV-2 infection and more comparable data than notified cases. We aimed to estimate the incidence of SARS-CoV-2 infection, identify associated risk factors, and assess time trends in the ratio of serological/molecular diagnosis in a cohort of university workers. MethodsParticipants had a serological rapid test for SARS-CoV-2 Immunoglobulins M and G, and completed a questionnaire, in May-July 2020 (n=3628) and November 2020-January 2021 (n=2661); 1960 participated in both evaluations and provided data to compute the incidence proportion and the incident rate. Crude and adjusted incidence rate ratios (aIRR) and 95% confidence intervals (CI) were computed using generalised linear models with Poisson regression. ResultsThe incidence rate was 1.8/100 person-month (95%CI 1.6-2.1), and the 6 months cumulative incidence was 10.7%. The serological/molecular diagnosis ratio was 10:1 in the first evaluation and 3:1 in the second. Considering newly identified seropositive cases at the first (n=69) and second evaluation (n=202), 29.0% and 9.4% never reported symptoms, respectively, 14.5% and 33.3% reported contact with a confirmed case and 82.6%, and 46.0% never had a molecular test. Males (aIRR: 0.59; 95%CI: 0.42-0.83) and "high-skilled white-collar" workers (aIRR: 0.73, 95%CI: 0.52-1.02) had lower incidence of infection. ConclusionUniversity workers presented a high SARS-CoV-2 incidence while restrictive measures were in place. The time decrease in the proportion of undiagnosed cases reflected the increased access to testing, but opportunities continued to be missed, even in the presence of COVID-19 like symptoms. What is already known on this subjectO_LIThe median ratio of seroprevalence to the corresponding cumulative incidence is 18, however, there is great variability between studies. C_LIO_LISeroprevalence studies are essential to estimate the true burden of the infection. C_LIO_LIFew cohort studies focused on essential non-healthcare workers, such as university workers. C_LI What this study addsO_LIThis longitudinal seroprevalence study among university workers found a SARS-CoV-2-specific IgM or IgG incidence rate of 1.8/100 person-month, and a 6 months cumulative incidence of 10.7%. C_LIO_LIThe undiagnosed fraction was 3:1 in the second evaluation, representing a decrease from a 10:1 in the first evaluation in the same population showing that a gap to test-trace-isolate remained in this highly educated working population. C_LIO_LISeropositive participants were mostly pauci- or symptomatic with no known contact with a COVID-19 confirmed case; "high-skilled white-collar" workers were at lower risk of being an incident seropositive case. C_LI