Serial household serosurvey for COVID-19 in low and high transmission neighborhoods of urban Pakistan

Serial household antibody sero-surveys informs the pandemic where testing is nonuniform. Young populations with intergenerational co-residence may have different transmission dynamics. We conducted two serial cross-sectional surveys in April and June 2020 in low- and high-transmission neighborhoods of Karachi, Pakistan, using random sampling. Symptoms were assessed and blood tested for antibody using chemiluminescence. Seroprevalence was adjusted using Bayesian regression and post stratification. CRI with 95% confidence intervals was obtained. We enrolled 2004 participants from 406 households. In June 8.7% (95% CI 5.1-13.1) and 15.1% (95% CI 9.4 -21.7) were infected in low- and high-transmission-areas respectively compared with 0.2% (95% CI 0-0.7) and 0.4% (95% CI 0 - 1.3) in April. Conditional risk of infection was 0.31 (95% CI 0.16-0.47) and 0.41(95% CI 0.28-0.52) in District Malir & District East respectively with overall only 5.4% symptomatic. Rapid increase in seroprevalence from baseline is seen in Karachi, with a high probability of infection within household.