Safety, immunogenicity and an open, retrospective study of efficacy of a monocomponent pertussis toxoid vaccine in infants.

One hundred forty-five infants were vaccinated with 25 micrograms of pertussis toxoid (NICHD-Ptxd) at 3, 5 and 7 or at 3, 5 and 12 months of age. One month after the third vaccination all had high serum IgG and neutralizing antibodies (antitoxin) against pertussis toxin. Vaccination at 3, 5 and 12 months resulted in higher antibody titers than vaccination at 3, 5 and 7 months. Sera obtained from 109 children at 3 years of age showed a decline of antibodies, but all had detectable antibodies. Adverse reactions were confined to local redness and swelling, which exceeded 2 cm after 17% of all injections. When the children were 3 years old, a comparison was made of the incidence of clinical pertussis in 142 of the 145 vaccinated children and in 284 age-matched controls living in the same areas. Information on symptoms of pertussis was obtained from the parents during telephone interviews. None of the vaccinated children had clinical pertussis, defined as a 6-week course of paroxysmal cough with whooping attacks or vomiting, whereas 57 controls (20%) had experienced these symptoms. Sixteen vaccinated children were exposed to pertussis in the household. Two of them had laboratory-verified Bordetella pertussis infections with cough of 2 and 4 weeks, respectively, without whooping attacks or vomiting, whereas 14 did not develop a cough. The study shows that NICHD-Ptxd is immunogenic in infants and that it most likely confers a high degree of protection against pertussis.