For the reasons indicated above, it is proposed to undertake a pre-vaccination carriage survey in Chad in 5,000 subjects in the 2011 rainy season instead of the 2,000 subjects proposed originally for the epidemiological study (REF. 5698). As the prevalence of carriage was higher in rural than in urban areas in the 2010 rainy season surveys conducted in Chad, we propose to study 4000 subjects in two rural areas and 1,000 subjects in an urban area. This will allow a comparison to be made with the results obtained in the 2010 rainy season survey when 1000 subjects were recruited in urban and rural areas respectively.