Where is improvement of vitamin A status most likely to affect morbidity and mortality?
The obvious answer to this question is: Where vitamin A deficiency is now a serious problem. For the mortality trials, all of which were conducted in settings where it was assumed vitamin A was a public health problem under the WHO definition, we attempted to ask about population-level predictors of the relative effect. Statistical power was very low for these analyses of the predictors of response (mortality of control group, xerophthalmia, stunting, wasting) since study was the unit of analysis (n = 8). Individual-level data might have uncovered more subtle effects had they been accessible. We found no relationship between the baseline prevalence of xerophthalmia and the relative effect of vitamin A. Thus we have to conclude that whereas the existence of clinically apparent deficiency was a marker for all programmes, the actual prevalence added little additional information in predicting outcome. One important question is unanswered. No studies were conducted in populations with bioc
