Two acellular pertussis vaccines, which are usually combined with diph
theria and tetanus toxoids, are now licensed for use in the United Sta
tes. These vaccines are immunogenic and produce fewer adverse reaction
s than whole-cell pertussis vaccines. At present, acellular vaccines a
re to be used only for the fourth and fifth booster doses of diphtheri
a-tetanus-pertussis vaccine in children 15 months of age or older. Eff
icacy of the acellular vaccines in younger children is currently being
investigated.