Long 2007.

Study characteristics
Methods	Individually randomised trial conducted in Mexico
Participants	Eligibility: children aged 5–15 months Excluded: children who were immunosuppressed; had any congenital abnormality or chronic diarrhoea; history of VAS Sample: 195 children; 97 in vitamin A group, 98 in placebo group. 49.7% boys
Interventions	Experimental group: vitamin A 20,000 IU for children aged < 12 months and 45,000 IU for children aged > 12 months Control group: placebo Study duration: intervention repeated every 2 months for 12 months
Outcomes	Incidence of diarrhoea and respiratory disease
Notes	Baseline sociodemographic characteristics of study children and households were similar between groups. Children received monthly visits and referrals to the doctor, which appeared to exceed normal treatment.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "The randomisation sequence was generated by project personnel based at the National Institute of Public Health". Comment: probably done.
Allocation concealment (selection bias)	Low risk	Comment: personnel at the National Institute of Nutrition prepared the supplements to assure that field personnel and the principal investigator were unaware of treatment regimen. Children in the vitamin A and placebo groups received a 5 mL solution, from identical opaque plastic droplet bottles numbered consecutively, administered by the field team.
Blinding (performance bias and detection bias) Blinding of participants	Low risk	Quote: "Testing had been carried out at the National Institute of Nutrition to assure that the placebo and vitamin A water miscible solution were similar in taste, viscosity and colour".
Blinding (performance bias and detection bias) Blinding of provider	Low risk	Quote: "Personnel at the National Institute of Nutrition carried out the preparation of the supplements to assure that field personnel and the principal investigator were unaware of treatment regimen".
Blinding (performance bias and detection bias) Blinding of outcome assessor	Low risk	Quote: "Personnel at the National Institute of Nutrition carried out the preparation of the supplements to assure that field personnel and the principal investigator were unaware of treatment regimen".
Incomplete outcome data (attrition bias)	Low risk	Comment: unclear what was done with data for 7 missing children, but dropout was small and similar between groups (4 in vitamin A group, 3 in control group).
Selective reporting (reporting bias)	Unclear risk	Comment: protocol not referenced, though the grant applications may be available.
Other bias	Low risk	Comment: study appeared free of other bias.