Global Vaccine Safety

Immunogenic overload

Extract from report of GACVS meeting of 6-7 June 2006, published in the WHO Weekly Epidemiological Record on 14 July 2006

The GACVS acknowledges that supposed “immune overload” as a result of infant immunization is a parental and societal concern that may limit confidence in and thus affect immunization programmes. It noted that the concept of immune overload was ill defined – but the available evidence was reviewed and discussed. Vaccine interference (i.e. the limitation of specific vaccine-induced immune responses as a result of combined, simultaneous or successive vaccinations) is a recognized phenomenon that may result from a variety of immunological mechanisms. It is important to recognize when this phenomenon occurs and to adjust the recommended immunization schedule accordingly. The Committee specifically discussed issues such as the influence of vaccine schedules on the protective responses that may be induced (vaccine administration in early infancy or at a later age, schedules accelerated or extended over a longer period of time) and of the effect of factors such as malnutrition or exposure to environmental pathogens/antigens that may differ in various country settings.

The Committee recognizes the difficulty in communicating complex scientific knowledge and the usefulness of studies addressing public concerns. It concluded that additional epidemiological studies assessing the presence of an association between vaccination and recurrent infant infections or atopic dermatitis would be welcome. Demonstrating the absence of such risks would reinforce the confidence of health-care providers, if not the public at large, in infant immunization.

The available evidence reviewed by GACVS does not support the hypothesis that vaccines, as currently used, weaken or harm the immune system. Surveillance should continue, and changes in vaccine schedules or introduction of new vaccines may provide opportunities to perform randomized studies to identify any possible harm posed by infant vaccines or to strengthen the evidence indicating a lack of harm. This is of crucial importance in helping national authorities respond to public concerns and support immunization programmes.