Statistical analysis
Qualitative variables were described with frequency and proportions for each category. The description of quantitative variables was performed using mean and standard deviation and/or median, minimum and maximum values. N-protein and S-protein IgG levels were compared in residentsi ) with and without prior COVID-19, ii ) with prior COVID-19 during the first or second wave of the epidemic, andiii ) between those who remained or did not remain seropositive for N-protein IgG. Wilcoxon–Mann–Whitney 2-sided tests were used, qualitative variables were compared using a chi-2 test, and the statistical significance threshold was set at 5%. Analyses were performed using the SAS Enterprise Guide, v7.3 (SAS Institute Inc.).