Two studies on the diphtheria, tetanus, pertussis vaccine found that introducing it too early or with other vaccines can decrease its effectiveness.

One study, which administered the DTP vaccine concurrently with the haemophilus influenzae type b Polysaccharide Tetanus Protein (or PTP, instead of that mouthful) used to protect against a form of meningitis, found that giving the two together, in the same syringe or at different sites, interfered with the whooping cough element of the vaccine. "These data underscore the caution required in decisions to add new vaccines to existing immunization regimens," concluded the authors, from the Center for Vaccine Development, University of Maryland School of Medicine in Baltimore, Maryland (Journal of American Medical Association, 5 Feb. 1992).

Another study conducted by the Departments of Paediatrics and Immunology at the John Radcliffe Hospital in Oxford, found that giving the DTP to babies at 2, 3 and 4 months, resulted in a significantly lower uptake than in children at 3, 5 and 9 months of age, the usual schedule now.

In the younger group, the lower response was thought to be due to the presence of maternal antibodies which have an "inhibitory effect" on responses to immunization, said the report.

This means that vaccines are less effective during the window period when whooping cough is most life threatening to young children.