Acellular
pertussis vaccines combined with diphtheria and tetanus toxoids were
demonstrated to be safe, immunogenic and efficacious in multiple phase 2 and 3
studies during the last decade. Additional combination products that include
inactivated poliovirus vaccine, Haemophilus influenzae type b-conjugate
vaccine and hepatitis B vaccine have been developed, tested, and licensed for
use in children. These broadened combination vaccines have a safety profile
which is not substantially different from the original DTaP products. Antibody
responses to the component antigens elicited with the expanded combination
vaccines are, in general, equivalent to levels achieved when the antigens are
administered separately. However, for most (but not all) acellular pertussis
combination vaccines, antibody responses to the capsular polysaccharide of Haemophilus
influenzae type b are lower with the combination product than when the
antigens are given separately. The clinical significance of these lower
antibody responses has not been established; several studies have demonstrated
both antibody and cellular immune priming, despite the lower antibody levels.
In Canada, where acellular pertussis vaccine has been used exclusively since
1998, the virtual elimination of invasive Haemophilus influenzae type b
disease has been maintained using the Aventis Pasteur 5-component acellular
pertussis vaccine combined with inactivated poliovirus and Haemophilus
influenzae b-conjugate vaccine which does not demonstrate any diminished
antibody responses. Subsequent to the change from whole-cell to acellular
pertussis vaccine, there have been clearly demonstrable decreases in serious
vaccine-associated adverse events including febrile seizures and
hypotonic-hyporesponsive episodes. Although too early for a definitive
conclusion about the effect of acellular pertussis vaccine on the epidemiology
of pertussis in Canada, there has been a relative decrease in reported cases in
the age cohorts immunized with the new combination vaccine. Acellular pertussis
combination vaccines provide a safe, effective, and convenient method of
simultaneous immunization against multiple infectious diseases in children.