Immunologic response to Hib tetanus toxoid conjugated vaccine coadministered with DTPa either mixed or in two separate injections in toddlers not primed with Hib vaccine

This open randomized study compared the immunogenicity and safety of a diphtheria-tetanus-acellular pertussis (DTPa) and H. influenzae polyribosylribitol phosphate conjugated to the tetanus toxoid (Hib-PRP-T) vaccine (mixed prior to administration) with separate injections of DTPa and Hib vaccines in toddlers aged two years. A total of 119 children (60 mixed; 59 separate administration), primed with DTPw, but not with Hib vaccine were enrolled. Prior to immunization only 10.3% of toddlers had anti-PRP antibody titres > or =1.0 microg/ml, compared with all children on Days 7 and 30. The anti-PRP and anti-tetanus antibody geometric mean concentrations were lower after the combined DTPa/Hib vaccine compared to separately administered vaccines (47.16 microg/ml vs 78.36 microg/ml and 24.95 IU/ml vs 40.63 IU/ml, respectively). One month after vaccination all children had anti-tetanus and anti-diphtheria antibody titres above the protective level of > or =0.1 IU/ml. The rates of recorded adverse events were similar and mostly mild or moderate in intensity whether the vaccines were combined as a single injection or given separately. We conclude that in 2-year old children, previously not immunized against Hib, a single dose of DTPa and Hib was safe and highly immunogenic irrespective of whether it was given as a combined vaccine or separate injections. Although the increase in anti-T and early (7-10 days after) anti-PRP concentrations was greater when the vaccine components were given separately than after combined administration, the DTPa/Hib combined vaccine would provide an effective method of delivering primary Hib vaccination in unprimed toddlers.