Serial population-based sero-surveys for COVID-19 in low and high transmission neighborhoods of urban Pakistan

Objective To determine population-based estimates of COVID-19 in a densely populated urban community of Karachi, Pakistan. Methods Three cross-sectional surveys were conducted in April, June and August in low- and high-transmission neighborhoods of Karachi. Participants were randomly selected to provide blood for Elecsys® immunoassay for detection of anti-SARS-CoV-2 antibodies. Bayesian regression model was used to estimate seroprevalence after adjusting for the demographic characteristics of each district. Conditional Risk of Infection (CRI) with 95% confidence interval was calculated using a non-parametric bootstrap of households. Infection fatality rates (IFR) were estimated. Results We enrolled 3005 participants from 623 households. In Phase 2, the adjusted seroprevalence was estimated as 8.7% (95% CI 5.1-13.1) and 15.1% (95% CI 9.4 -21.7) in low and high transmission areas respectively, compared to 0.2% (95% CI 0-0.7) and 0.4% (95% CI 0 - 1.3) in Phase 1. In Phase 3, it was 12.8% (95% CI 8.3 – 17.7) and 21.5% (95% CI 15.6-28) in low and high transmission areas, respectively. CRI was 0.31 (95% CI 0.16-0.47) and 0.41(95% CI 0.28-0.52) in low and high transmission neighborhoods respectively in Phase 2. Similar trends were observed in Phase 3. Only 5.4% of participants who tested positive for COVID-19 were symptomatic. IFR was 1.66% in phase 1, 0.37% in Phase 2 and 0.26% in Phase 3. Conclusion Initial rapid increase in seroprevalence was followed by a plateau in the later phase of the pandemic in Karachi. Continuing rounds of seroprevalence studies have the potential to fully characterize the pandemic in this geography. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was funded by Infectious Disease Research Lab at The Aga Khan University ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethical approval was obtained from Aga Khan University's Ethical review committee All necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes Data will be made available on request