Serial population based serosurvey of antibodies to SARS-CoV-2 in a low and high transmission area of Karachi, Pakistan

Background Pakistan is among the first low- and middle-income countries affected by COVID-19 pandemic. Monitoring progress through serial sero-surveys, particularly at household level, in densely populated urban communities can provide insights in areas where testing is non-uniform. Methods Two serial cross-sectional household surveys were performed in April (phase 1) and June (phase 2) 2020 each in a low- (District Malir) and high-transmission (District East) area of Karachi, Pakistan. Household were selected using simple random sampling (Malir) and systematic random sampling (East). Individual participation rate from consented households was 82.3% (1000/1215 eligible) in phase 1 and 76.5% (1004/1312 eligible) in phase 2. All household members or their legal guardians answered questions related to symptoms of Covid-19 and provided blood for testing with commercial Elecsys Anti-SARS-CoV-2 immunoassay targeting combined IgG and IgM. Seroprevalence estimates were computed for each area and time point independently. Given correlation among household seropositivity values, a Bayesian regression model accounting for household membership, age and gender was used to estimate seroprevalence. These estimates by age and gender were then post-stratified to adjust for the demographic makeup of the respective district. The household conditional risk of infection was estimated for each district and its confidence interval were obtained using a non-parametric bootstrap of households. Findings Post-stratified seroprevalence was estimated to be 0.2% (95% CI 0-0.7) in low-and 0.4% (95% CI 0 - 1.3) in high-transmission areas in phase 1 and 8.7% (95% CI 5.1-13.1) in low- and 15.1% (95% CI 9.4 -21.7) in high-transmission areas in phase 2, with no consistent patterns between prevalence rates for males and females. Conditional risk of infection estimates (possible only for phase 2) were 0.31 (95% CI 0.16-0.47) in low- and 0.41(95% CI 0.28-0.52) in high-transmission areas. Of the 166 participants who tested positive, only 9(5.4%) gave a history of any symptoms. Interpretation A large increase in seroprevalence to SARS-CoV-2 infection is seen, even in areas where transmission is reported to be low. Mostly the population is still seronegative. A large majority of seropositives do not report any symptoms. The probability that an individual in a household is infected, given that another household member is infected is high in both the areas. These results emphasise the need to enhance surveillance activities of COVID-19 especially in low-transmission sites and provide insights to risks of household transmission in tightly knit neighbourhoods in urban LMIC settings.