Background: Qatar has experienced a large SARS-CoV-2 epidemic . Our first objective was to assess the proportion of the urban population that has been infected with SARS-CoV-2, by measuring the prevalence of detectable antibodies . Our second objective was to identify predictors for infection and for having higher antibody titers .
Methods: Residual blood specimens from individuals receiving routine and other clinical care between May 12-September 9, 2020 were tested for anti-SARS-CoV-2 antibodies . Associations with seropositivity and higher antibody titers were identified through regression analyses . Probability weights were applied in deriving the epidemiological measures .
Results: We tested 112,941 individuals (~10% of Qatar urban population), of whom 51.6% were men and 66.0% were 20-49 years of age . Seropositivity was 13.3% (95% CI : 13.1-13.6 %) and was significantly associated with sex, age, nationality, clinical-care type, and testing date . The proportion with higher antibody titers varied by age, nationality, clinical-care type, and testing date . There was a strong correlation between higher antibody titers and seroprevalence in each nationality, with a Pearson correlation coefficient of 0.85 (95% CI : 0.47-0.96), suggesting that higher antibody titers may indicate repeated exposure to the virus . The percentage of antibody-positive persons with prior PCR-confirmed diagnosis was 47.1% (95% CI : 46.1-48.2 %), severity rate was 3.9% (95% CI : 3.7-4.2 %), criticality rate was 1.3% (95% CI : 1.1-1.4 %), and fatality rate was 0.3% (95% CI : 0.2-0.3 %).
Conclusions: Fewer than two in every 10 individuals in Qatar urban population had detectable antibodies against SARS-CoV-2 between May 12-September 9 , 2020, suggesting that this population is still far from the herd immunity threshold and at risk from a subsequent epidemic wave.