Vaccination of infants with a four-dose and a three-dose vaccination schedule.

Swedish infants were vaccinated with diphtheria, tetanus and pertussis toxoids, inactivated poliovirus vaccine and a Haemophilus influenzae type b - tetanus toxoid conjugate vaccine at 2, 4, 6 and 15 months (US vaccination program, 'US arm', n=118) or at 3, 5 and 12 months of age (Swedish vaccination program, 'Swedish arm', n=103). The antigen amounts in the diphtheria and tetanus vaccines were higher in the Swedish than in the US arm while the amounts in the other vaccines were the same in both arms. There were no serious side effects. Local reactions increased with the numbers of doses but did not differ significantly between the groups. Serum was obtained at 2, 7, 15 and 16 months in the US arm and at 3, 6, 12 and 13 months of age in the Swedish arm. A fifth serum was obtained in both groups at 4 yr of age. For vaccines with the same antigen amount the following was observed: a. three doses at 2, 4 and 6 months were more immunogenic than two doses at 3 and 5 months; b. the third dose in the Swedish arm was more immunogenic than the third dose in the US arm; c. the fourth dose in the US arm induced higher antibodies than the third dose in the Swedish arm (except for pertussis toxin antibodies that were similar in both groups) and the differences tended to remain at the age of 4 yr. Children in the Swedish arm received a higher diphtheria toxoid dose (25 Lf) than in the US arm (15 Lf) which led to higher diphtheria toxin antibodies in the Swedish arm at comparable ages. Children in the Swedish arm received a higher tetanus toxoid dose (7 Lf) than in the US arm (6 Lf). Tetanus antibodies were similar at comparable ages. In conclusion, the immunogenicity of vaccines in infancy can be improved by increasing the number of doses, by prolonging the intervals between doses and by increasing the antigen amount in the vaccine.