BHP has a demographic surveillance system (DSS) which has followed a population of now more than 150,000 individuals for almost 30 years. Children will be randomised to receive VAS or placebo with their first vaccine after 6 months of age, and will be followed through the DSS to assess mortality and morbidity. Based on previous observations, the effects of VAS might differ according to sex and season. The interaction between VAS, sex, and season will also be studied in the present trial. By identifying situations where VAS may be beneficial, ineffective, or even harmful the study may contribute importantly to optimising the VAS policy for low-income countries.