Assessing different measures of population-level vaccine protection using a case-control study

Ali Mohammad, Youngae You, Suman Kanungo, Byomkesh Manna, Jacqueline Deen, Anna Lena Lopez, Thomas Wierzba, S K Bhattacharya, Dipika Sur, John David Clemens

    Research output: Contribution to journalArticlepeer-review


    Background: Case-control studies have not been examined for their utility in assessing population-level vaccine protection in individually randomized trials. 
    Methods: We used the data of a randomized, placebo-controlled trial of a cholera vaccine to compare the results of case-control analyses with those of cohort analyses. Cases of cholera were selected from the trial population followed for three years following dosing. For each case, we selected 4 age-matched controls who had not developed cholera. For each case and control, GIS was used to calculate vaccine coverage of individuals in a surrounding "virtual" cluster. Specific selection strategies were used to evaluate the vaccine protective effects. 
    Results: 66,900 out of 108,389 individuals received two doses of the assigned regimen. For direct protection among subjects in low vaccine coverage clusters, we observed 78% (95% CI: 47-91%) protection in a cohort analysis and 84% (95% CI: 60-94%) in case-control analysis after adjusting for confounding factors. Using our GIS-based approach, estimated indirect protection was 52% (95% CI: 10-74%) in cohort and 76% (95% CI: 47-89%) in case control analysis. Estimates of total and overall effectiveness were similar for cohort and case-control analyses. 
    Conclusion: The findings show that case-control analyses of individually randomized vaccine trials may be used to evaluate direct as well as population-level vaccine protection. 
    Original languageEnglish
    Pages (from-to)6878-6883
    Number of pages6
    Issue number48
    Publication statusPublished - 2015


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