Elimination of Haemophilus influenzae type b (Hib) disease from The Gambia after the introduction of routine immunisation with a Hib conjugate vaccine: a prospective study.

BACKGROUND: Routine immunisation of infants in The Gambia with a Haemophilus influenzae type b (Hib) polysaccharide-tetanus toxoid conjugate vaccine began in May, 1997. We investigated the effectiveness of the vaccine when delivered through the expanded programme on immunisation and the effect of national immunisation on incidence of Hib disease. METHODS: Surveillance for Hib disease was maintained in the western half of The Gambia using standard methods with an emphasis on meningitis. We estimated vaccine efficacy using the case control method, and vaccine coverage and population denominators for incidence rates using a cluster sample survey. Prevalence of Hib carriage in a sample of 1-2-year old children attending health centres for vaccination was ascertained with oropharyngeal swabs plated onto antiserum agar. FINDINGS: Between May, 1997, and April, 2002, a total of 5984 children were examined for possible Hib infections. 49 children had Hib disease, 36 of whom had meningitis. The annual incidence rates of Hib meningitis before any use of the vaccine (1990-93) dropped from over 200 per 100,000 children aged younger than 1 year to none per 100,000 in 2002, and from 60 to no cases per 100,000 in children younger than 5 years. The prevalence of Hib carriage decreased from 12% to 0.25% (p<0.0001). Two doses of vaccine were needed for direct protection from Hib disease (vaccine efficacy 94%, 95% CI 62-99). Since most children received a protective dose after the age of greatest disease risk, indirect effects were important in reducing disease incidence. INTERPRETATION: The Gambian Hib immunisation programme reduced the occurrence of Hib disease despite irregular vaccine supply. The effect of the programme in The Gambia has important implications for the introduction of the vaccine into routine immunisation programmes of other developing countries.

Comparison of two cluster sampling methods for health surveys in developing countries.

BACKGROUND: The Expanded Program for Immunization (EPI) random walk method has been widely used by the World Health Organization and others for rapid cluster sample surveys where an up-to-date household sampling frame is not available. However, it is not a probability sample, does not allow for population movement since the last census, and does not ensure objectivity in household selection or permit call-backs for non-response. Compact segment sampling avoids these problems and has been proposed as a slower but cleaner alternative. METHODS: We conducted two surveys, one using the EPI scheme and one using compact segment sampling, to estimate vaccination coverage in Western Region of The Gambia within 3 months of each other in 2000-2001. RESULTS: Point estimates for vaccination coverage from the two surveys rarely differed by more than 2%. Any differences were more likely to be due to household selection than to population movement. A simple mathematical model showed that even in extreme situations, ignoring population movement since the last census is unlikely to have any appreciable effect. Rates of homogeneity did not differ systematically between the surveys. CONCLUSIONS: In situations where quality of fieldwork can be guaranteed, the EPI random walk method can give accurate and precise results. However, compact segment sampling is generally to be preferred as it ensures objectivity in household selection and permits the estimation of population totals (such as those unvaccinated), which are helpful for planning service provision.