Prevalence of IgG antibodies to SARS-CoV-2 was 89.8% (95% CI 88.2-91.3%) in COVID-19 patients (Table 2) compared to 4.0% (95% CI 3.4-4.7%) in healthcare providers, 4.6% (95% CI 4.3-4.9%) in general workers, and 1.0% in other patients (p all <0.001 for comparing to COVID-19 patients). Only the comparison of IgG prevalence between healthcare workers and general workers was not significant (p=0.39). IgG prevalence increased significantly by age among healthcare providers, and was 2.8% in those <30 years old, 9.6% in those 60-69 years old and 10.0% in those ≥ 70 years old (p<0.001 for trend). IgG prevalence also increased significantly by age among general workers. Prevalence of IgM antibodies to SARS-CoV-2 was 31.4% (95% CI 29.0-33.7%) in COVID-19 patients, 1.5% (95% CI 1.1-1.8%) in healthcare providers, 1.3 (95% CI 1.1-1.5%) in general workers, and 0.2% (95% CI 0-0.4%) in other patients (Table 3).

Among COVID-19 patients, mean age was similar between those with IgG antibodies to SARS-CoV-2 and those without (Table 4). Presence of IgG antibodies to SARS-CoV-2 was not associated with most demographic characteristics, disease severity, presence of comorbidities, treatment received, and clinical characteristics, except for antibiotics treatment, chloroquine/hydroxychloroquine treatment, and needing intubation. IgG prevalence and IgM prevalence among hospitalized patients with COVID-19 by demographic and clinical characteristics are presented in Table 5. Mortality rate was 1.3% (95% CI 0.7-1.9%) in those with IgG antibodies to SARS-CoV-2 and was 3.3% (95% CI 0.4-6.2%) in those without (Figure 1). Mortality risk was similar between those with IgG antibodies to SARS-CoV-2 and those without (adjusted hazard ratio 0.45 95% CI 0.16-1.24, P=0.12).