Analysis
death, diarrhea
REGRESSION, CHOLERA
To evaluate the interplay between preexisting household WASH and OCV in preventing all episodes of cholera as well as severe cholera, we included only the OCV-only and control clusters of the trial since the OCV-WASH clusters had received an external WASH intervention. We approached the problem in two ways, following our original analysis of the trial We analyzed the fixed cohort present at the time of OCV dosing. Following our original analysis of the trial (9), we defined the start date of follow-up 14 days after the second dose of OCV, the median date of 14 days after the second dose in OCV clusters for residents who did not receive two doses of the vaccine, and 14 days after the median date of second dose of OCV in the nearest OCV cluster for residents of the control clusters. Follow-up of subjects was censored at migration out of the cluster, death, or two years of follow-up, whichever came first. Two years was selected as the maximum duration of follow-up because the population was highly mobile (ca. 25% migration per year), and a low fraction of the baseline cohort was still under follow-up thereafter.To evaluate protective effectiveness (PE), we used survival analysis. We fitted unadjusted and adjusted Cox proportional hazards regression models after verifying that the proportionality assumptions were fulfilled for independent variables. We then estimated the hazard ratios by exponentiating the coefficient of the group variable in these models. PE was calculated as (1–hazard ratio) × 100%, and robust sandwich variance estimates were used to account for the design effect of cluster randomization, allowing inferences for vaccine protection at the individual level. Several baseline variables were considered as potential confounders for inclusion as independent variables in the models: age, gender, religion, average household expenditure, residence in a house owned by the family, residence in a household with only one room, residence in a household with a concrete roof, a history of diarrhea within the previous 6 months, residence in the study area for<1 year, and residence in a household that knew about cholera vaccine. After forcing the variable for stratified randomization in the models, other variables were introduced via stepwise backward-elimination, starting with all potential covariates and sequentially removing the covariates until no further increase was observed in the Akaike Information Criterion. Two-way interactions between the OCV and WASH terms in the models were also considered as independent variables. The threshold of significance for individual estimates of protective effectiveness was p < 0.05 with a corresponding 95% CI (two-sided). All statistical analyses were done using R version 4.1.0.