The SARS-CoV-2 infection among students in the University of Porto: a cross-sectional study

Introduction: Incidence based on notified cases of SARS-CoV-2 infection underestimates the real extension of the infection. We aimed to quantify SARS-CoV-2 specific antibodies seroprevalence among University students in Porto. Methods: A rapid point of care testing for SARS-CoV-2 specific immunoglobulin (Ig) M and IgG antibodies was performed, and a questionnaire was applied to the 6512 voluntary students from September to December 2020. We computed the apparent IgM, IgG and IgM or IgG prevalence, and the true prevalence and 95% credible intervals (95% CI) using Bayesian inference. Results: We found an apparent prevalence (IgM or IgG) of 9.7%, the true prevalence being 7.9% (95% CI 4.9-11.1). Prevalence was significantly higher among males (10.9% vs 9.2%), international students (18.1% vs 10.4% local vs 8.8% nationally displaced) and increased with age. Those with a known risk contact, that experienced quarantine, had symptoms, or a previous negative molecular test had a higher seroprevalence. Of the 91 (1.4%) students who reported a molecular diagnosis, 86.7% were reactive for IgM or IgG. Conclusion: Based on immunological evidence infection was 5.6 times more frequent than if based on a molecular diagnosis. The higher seroprevalence among male, older, and international students emphasizes the importance of identifying particular groups.