Long 2006a.

Methods	Factorial design, individually randomised trial conducted in La Magdalena Atlicpac, Mexico
Participants	Eligibility: children aged 6‐15 months were eligible for inclusion in the review. Children who were suffering from diseases causing immunosuppression and any congenital or acquired alteration of the digestive tract that could alter the absorption of micronutrients were excluded. Children who were taking vitamin supplements were also excluded from the study. Sample: 786 children were randomised to 4 intervention groups. Mean age of participants was 9.8 months; proportion of boys in study population was 51.7%
Interventions	The 4 intervention groups were as follows: Group I: vitamin A group that received 20,000 IU retinol every 2 months for children aged < 1 year or 45,000 IU for children aged > 1 year Group II: Zn group that received a daily dose equivalent to 20 mg elemental Zn as zinc methionine Group III: zinc supplement plus vitamin A as above Group IV: placebo Interventions were delivered every 2 months for 12 months
Outcomes	Diarrhoea and respiratory disease morbidity
Notes	We have included data of this factorial design trial in 2 sets. The first data set gives comparisons for vitamin A vs placebo, and the second set includes data for vitamin A + zinc vs zinc only. Data on respiratory morbidity was given with three definitions. We have pooled the data for "cough + difficulty breathing" under the heading of lower respiratory tract infection.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "The randomisation sequence was generated by using a random‐number table by project personnel from CENSIA, a division of the Mexican Ministry of Health."
Allocation concealment (selection bias)	Low risk	Quote: "These solutions were packaged in consecutively numbered, colour‐coded, opaque plastic droplet bottles to ensure that field personnel and the principal investigator were blinded."
Blinding (performance bias and detection bias) Blinding of Participants	Low risk	Quote: "The vitamin A, zinc, and vitamin A + zinc supplements were prepared by personnel at the National Institute of Nutrition in 5‐mL solutions that were similar in taste and appearance."
Blinding (performance bias and detection bias) Blinding of provider	Low risk	Quote: "This double‐blind randomised trial . . . These solutions were packaged in consecutively numbered, color‐coded, opaque plastic droplet bottles to ensure that field personnel and the principal investigator were blinded."
Blinding (performance bias and detection bias) Blinding of outcome assessor	Low risk	Quote: "This double‐blind randomised trial . . . These solutions were packaged in consecutively numbered, color‐coded, opaque plastic droplet bottles to ensure that field personnel and the principal investigator were blinded." Comment: probably done.
Incomplete outcome data (attrition bias)	Low risk	Comment: lost to follow‐up data given along with reasons for lost to follow‐up. 93 children were lost to follow‐up or excluded.
Selective reporting (reporting bias)	Unclear risk	Comment: study protocol not available so cannot assess or make any judgement
Other bias	Low risk	Comment: this study appears to be free of other bias.