Seroprevalence of SARS-CoV-2 in slums and non-slums of Mumbai, India, during June 29-July 19, 2020

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Estimate seroprevalence in representative samples from slum and non-slum communities in Mumbai, India, a mega-city in a low or middle-income country and test if prevalence is different in slums.


After geographically-spaced community sampling of households, one individual per household was tested for IgG antibodies to SARS-CoV-2 N-protein in a two-week interval.


Slum and non-slum communities in three wards, one each from the three main zones of Mumbai.


Individuals over age 12 who consent to and have no contraindications to venipuncture were eligible. 6,904 participants (4,202 from slums and 2,702 from non-slums) were tested.

Main outcome measures

The primary outcomes were the positive test rate for IgG antibodies to the SARS-CoV-2 N-protein by demographic group (age and gender) and location (slums and non-slums). The secondary outcome is seroprevalence at slum and non-slum levels. Sera was tested via chemiluminescence (CLIA) using Abbott Diagnostics Architect TM N-protein based test. Seroprevalence was calculated using weights to match the population distribution by age and gender and accounting for imperfect sensitivity and specificity of the test.


The positive test rate was 54.1% (95% CI: 52.7 to 55.6) and 16.1% (95% CI: 14.9 to 17.4) in slums and non-slums, respectively, a difference of 38 percentage points (P < 0.001). Accounting for imperfect accuracy of tests (e.g., sensitivity, 0.90; specificity 1.00), seroprevalence was as high as 58.4% (95% CI: 56.8 to 59.9) and 17.3% (95% CI: 16 to 18.7) in slums and non-slums, respectively.


The high seroprevalence in slums implies a moderate infection fatality rate. The stark difference in seroprevalence across slums and non-slums has implications for the efficacy of social distancing, the level of herd immunity, and equity. It underlines the importance of geographic specificity and urban structure in modeling SARS-CoV-2.

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  1. Our take

    This cross-sectional study, available as a preprint and thus not yet peer reviewed, found that participants randomly sampled in slum communities in Mumbai, India had a 54.1% seroprevalence of IgG antibodies to SARS-CoV-2 infection compared to 16.1% in non-slum communities. These findings suggest that those in slum communities have had greater exposure to the virus than those in non-slum communities, potentially due, in part, to higher population density in slum areas. However, those in non-slum areas were less likely to participate, and thus results may be subject to sampling bias.

    Study design


    Study population and setting

    Investigators sampled 6,904 individuals 12 years or older from three wards in Mumbai, India (Matunga, Chembur West, and Dahisar) to test the positivity rate of IgG antibodies to SARS-CoV-2 infection which would indicate prior infection. Investigators sampled individuals by randomly sampling households and testing one member of a sampled household in order to ensure a representative sample of the surveyed areas. They aimed to compare the seroprevalence of antibodies between slums (i.e. communities of people living on land for which they do not have legal rights) and non-slums. There were 4,202 participants in slums and 2,702 participants in non-slums. Investigators estimated the average prevalence over a two-week period (June 29 to July 14, 2020 in slums and July 3 to July 19, 2020 in non-slums).

    Summary of main findings

    The positive test rate of IgG antibodies in those sampled from slums was 54.1% compared to 16.1% among those in non-slums, for a prevalence difference of 38%. When adjusting for potential test imperfections (i.e. having a sensitivity of 0.90 and a specificity of 1.00) the positive test rate was 58.4% in slums and 17.3% in non-slums.

    Study strengths

    This study is one of few studies to provide seroprevalence estimates in from Mumbai, India and to compare results between slum and non-slum areas.


    The denominator for the seroprevalence was estimated through using 2011 Census data, which may not accurately reflect the current population estimates. Additionally, these data may not be representative to the general Mumbai population if the specific wards are not reflective of Mumbai demographics. Thus, the figures can be an over or underestimation of the prevalence of IgG antibodies among slums or non-slums. The research team couldn’t gain access to all buildings in non-slum areas and therefore some of the participant sampling was out of their control and may not have been random. Additionally, the authors note that participants in non-slums were less willing to participate due to fear of being exposed to the virus when getting tested.

    Value added

    This study shows a significantly higher burden of SARS-CoV-2 exposure among slums compared to non-slums, and thus highlights underpinning issues (e.g. high population density and poor infrastructure) that may contribute to higher infection rates. This has the potential to lead to higher morbidity and mortality in these areas as it would be harder to control transmission.

  2. SciScore for 10.1101/2020.08.27.20182741: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    Institutional Review Board StatementConsent: ) Individuals were eligible if they were age 12 years or older and excluded if they refused informed consent or had a contraindication to venipuncture.
    Randomizationnot detected.
    Blindingnot detected.
    Power AnalysisTo balance statistical power and bias, we stopped sampling either when we hit sample-size targets or the sampling period lapsed.
    Sex as a biological variablenot detected.

    Table 2: Resources

    At Kasturba Hospital in Mumbai, plasma was separated and used to test for IgG antibodies via chemiluminescence (CLIA) using Abbott Diagnostics Architect™ N-protein based test.
    suggested: None
    Software and Algorithms
    At Kasturba Hospital in Mumbai, plasma was separated and used to test for IgG antibodies via chemiluminescence (CLIA) using Abbott Diagnostics Architect™ N-protein based test.
    Abbott Diagnostics Architect™
    suggested: None
    Abbott recommends a 1.4 cutoff for IgG score to label a test result as positive.
    suggested: (Abbott, RRID:SCR_010477)

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: An explicit section about the limitations of the techniques employed in this study was not found. We encourage authors to address study limitations.

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    About SciScore

    SciScore is an automated tool that is designed to assist expert reviewers by finding and presenting formulaic information scattered throughout a paper in a standard, easy to digest format. SciScore checks for the presence and correctness of RRIDs (research resource identifiers), and for rigor criteria such as sex and investigator blinding. For details on the theoretical underpinning of rigor criteria and the tools shown here, including references cited, please follow this link.