The review found no eligible studies for this question.
Vitamin A supplementation leads to lower all‐cause mortality compared with placebo or usual care (high certainty).
Vitamin A supplementation probably leads to higher vomiting compared with placebo or usual care (moderate certainty).
Vitamin A supplementation leads to lower mortality due to diarrhoea compared with placebo or usual care (high certainty).
Vitamin A supplementation showed no clear difference in mortality due to measles compared with placebo or usual care (low certainty).
Vitamin A supplementation showed no clear difference in mortality due to LRTI compared with placebo or usual care (low certainty).
Vitamin A supplementation may leads to lower diarrhoea incidence Mean episodes per child per year compared with placebo or usual care (low certainty).
Vitamin A supplementation probably leads to lower measles incidence Mean episodes of measles per child per year compared with placebo or usual care (moderate certainty).
Vitamin A supplementation showed no clear difference in LRTI incidence Mean episodes per child per year compared with placebo or usual care (low certainty).
Vitamin A supplementation probably leads to lower bitot's spots incidence compared with placebo or usual care (moderate certainty).
Vitamin A supplementation probably leads to lower night blindness incidence compared with placebo or usual care (moderate certainty).
Vitamin D supplementation probably leads to lower pre‐eclampsia compared with placebo or no intervention (moderate certainty).
Vitamin D supplementation probably leads to lower gestational diabetes compared with placebo or no intervention (moderate certainty).
Vitamin D supplementation may leads to lower maternal adverse events: severe postpartum haemorrhage compared with placebo or no intervention (low certainty).
It is very uncertain whether vitamin D supplementation changes maternal adverse event: nephritic syndrome compared with placebo or no intervention (very low certainty).
Vitamin D supplementation showed no clear difference in preterm birth (less than 37 weeks' gestation) compared with placebo or no intervention (low certainty).
Vitamin D supplementation probably leads to lower low birthweight (less than 2500 g) compared with placebo or no intervention (moderate certainty).
Vitamin C supplementation showed no clear difference in major cardiovascular event Physicians compared with placebo (low certainty).
It is very uncertain whether vitamin C supplementation changes cardiovascular mortality Physicians compared with placebo (very low certainty).