144 www.thelancet.com Vol 366 July 9, 2005 Articles Introduction Before the widespread use of conjugate vaccines for Haemophilus influenzae type b (Hib), this organism was the most common cause of severe invasive childhood infections in developed countries. 1 The introduction of routine vaccination in developed countries has led to substantial declines in the incidence of Hib disease. 2,3 However, routine use of these vaccines in less developed countries has been delayed until recently 4,5 because there was little information available about the burden of Hib disease and because the high price of conjugate vaccines made them unaffordable. 6 Studies undertaken in The Gambia before the Hib conjugate vaccine was investigated showed that the epidemiology of Hib disease was different from that seen in more developed countries. Overall incidence was higher, infections occurred in children of younger ages, and pneumonia was more prevalent and related to a worse outcome in The Gambia than in more developed countries. 7,8 The case fatality rate of Hib meningitis was 30%, 9 sequelae were common, and less than half of patients recovered fully from Hib disease. 10 However, epiglottitis was not reported in The Gambia or any other developing countries. 11 A successful trial of a Hib conjugate vaccine was undertaken in 42 000 infants in The Gambia during 1993 to 1995. 12 The vaccine used was a Hib polysaccharide-tetanus toxoid conjugate (PRP-T; Act-Hib, supplied by Pasteur Mérieux, Lyon, France). The vaccine showed 95% efficacy against culture- confirmed invasive Hib disease, 21% efficacy against radiologically defined pneumonia with definite alveolar consolidation, and 60% protection against carriage in the second year of life. 12,13 However, reports of vaccine efficacy under optimum trial conditions do not guarantee that a vaccine will be effective in routine use. This possible situation is especially relevant in developing countries where suboptimum storage and transport conditions can reduce vaccine efficacy. Indirect effects (herd immunity) can make vaccination more effective than suggested by an individually randomised efficacy trial. Additionally, national immunisation could change the epidemiology of Hib disease in developing countries, leading to the emergence of epiglottitis. Lancet 2005; 366: 144–50 Published online June 29, 2005 DOI:10.1016/S0140-6736(05) 66788-8 See Comment page 101 Medical Research Council Laboratories, The Gambia (R Adegbola FRCPath, O Secka MPhil, G Lahai BSc, S Usen FWACP, C Oluwalana MBBS, S Obaro FRCPCH, M Weber MD, Prof K McAdam FRCP, T Corrah FRCP); National Health Laboratory (N Lloyd-Evans MSc) and Department of State for Health, Banjul, The Gambia (A Njie MPH); Royal Children’s Hospital, Victoria, Australia (Prof K Mulholland FRACP); and Department of Infectious Diseases, London School of Hygiene and Tropical Medicine, London, UK (Prof B Greenwood FRCP, P Milligan PhD) Correspondence to: Dr Richard A Adegbola, MRC Laboratories, P O Box 273, Banjul, The Gambia radegbola@mrc.gm 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 Richard A Adegbola, Ousman Secka, George Lahai, Nellie Lloyd-Evans, Alpha Njie, Stanley Usen, Claire Oluwalana, Stephen Obaro, Martin Weber, Tumani Corrah, Kim Mulholland, Keith McAdam, Brian Greenwood, Paul J M Milligan Summary 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.