COVID 19 herd immunity in the Brazilian Amazon

The herd immunity threshold is the proportion of a population that must be immune to an infectious disease, either by natural infection or vaccination such that, in the absence of additional preventative measures, new cases decline and the effective reproduction number falls below unity[1][1]. This fundamental epidemiological parameter is still unknown for the recently-emerged COVID-19, and mathematical models have predicted very divergent results[2][2],[3][3]. Population studies using antibody testing to infer total cumulative infections can provide empirical evidence of the level of population immunity in severely affected areas. Here we show that the transmission of SARS-CoV-2 in Manaus, located in the Brazilian Amazon, increased quickly during March and April and declined more slowly from May to September. In June, one month following the epidemic peak, 44% of the population was seropositive for SARS-CoV-2, equating to a cumulative incidence of 52%, after correcting for the false-negative rate of the antibody test. The seroprevalence fell in July and August due to antibody waning. After correcting for this, we estimate a final epidemic size of 66%. Although non-pharmaceutical interventions, plus a change in population behavior, may have helped to limit SARS-CoV-2 transmission in Manaus, the unusually high infection rate suggests that herd immunity played a significant role in determining the size of the epidemic. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was supported by the Itau Unibanco Todos pela Saude program and CADDE/FAPESP (MR/S0195/1 and FAPESP 18/14389-0). NRF is supported by the Wellcome Trust and Royal Society Sir Henry Dale Fellowship (204311/Z/16/Z). We acknowledge the National Heart, Lung, and Blood Institute of the US National Institutes of Health Recipient Epidemiology and Donor Evaluation Study (REDS, now in its 4th phase, REDS-IV-P) for providing the blood donor demographic and zip code data for analysis (grant number HHSN268201100007I). This work received funding from the U.K. Medical Research Council under a concordat with the U.K. Department for International Development. We additionally acknowledge support from Community Jameel and the NIHR Health Protection Research Unit in Modelling Methodology. ### 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: This project was approved by the Brazilian national research ethics committee, CONEP CAAE - 30178220.3.1001.0068. 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 The data required to reproduce the results in this article will be deposited on the Figshare repository upon acceptance of the article (URL), where the raw data underlying the main figures will be provided. Also, upon acceptance, the custom code will be made available at the linked GitHub repository (URL). [1]: #ref-1 [2]: #ref-2 [3]: #ref-3