Optimising the use of conjugate vaccines: a concept paper

Background Conjugate vaccines have been developed to provide protection against diseases caused by Haemophilus influenzae type b, Neisseria meningitidis and Streptococcus pneumoniae. The vaccines are safe and effective, and offer excellent prospects for disease control. In countries where these vaccines have been introduced, the schedules and strategies have varied. ‘Optimal’ vaccine strategies have not been defined. Strategic options Important factors to consider regarding the introduction of conjugate vaccines include: vaccine effectiveness, the age at which vaccine is administered, the number and timing of doses required, co-administration of other antigens, the effect of the vaccine on carriage and the importance of herd immunity, the impact of catch-up campaigns and serotype/ serogroup replacement. To facilitate the introduction of conjugate vaccines in less developed countries, the costs and programmatic feasibility of different vaccine strategies must also be considered. Plan of action To inform WHO’s position on the use of conjugate vaccines the available evidence on the immunogenicity, effectiveness and impact of meningococcal, pneumococcal and Hib conjugate vaccines is being reviewed. This review will identify the strengths and weaknesses of this evidence and highlight areas where further study would be of value. Progress and future plans A SAGE sub-group has been formed and preliminary reviews conducted. The review will be completed and presented to SAGE in April 2007. Discussion An important property of the Hib, pneumococcal and meningococcal conjugate vaccines used in national vaccine programmes to date is their ability to reduce carriage, resulting in herd immunity. The public health impact of vaccine programmes will be enhanced if vaccine strategies are designed to exploit these herd effects, e.g. by targeting the age groups with highest transmission. Further work is needed to describe the epidemiology of carriage, particularly meningococcal carriage in the African meningitis belt. Experience with conjugate vaccines so far suggests that protection in infancy is likely to be induced with fewer than 3 doses, but that direct protection may wane rapidly in the absence of a booster dose in the second year of life. More work is needed to assess the duration of protection from vaccination and to define long term correlates of protection.