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Lancet. 1992 May 30; 339(8805):1355-6.Researchers compared the immune responses of infants vaccinated at 2, 3, and 4 months of age with the diphtheria-pertussis-tetanus (DPT) conjugate vaccine with those of infants vaccinated at 3, 5, and 9 month intervals. The antibody titer for pertussis was the same for both the old and new schedules. Further, no reliable immunological correlates of protection existed for pertussis. Different batches of DPT vaccine and maternal antibodies may have accounted for the variation in immune responses. No one measured maternal antibodies in the infants in the 3, 5, and 9 month group, though. The protective level of antitoxin for tetanus and diphtheria (0.01 neutralizing units/ml) cannot be applied to ELISA assays, since the relationship between the 2 assays is inadequate at levels less than 0.1 IU/ml. 1964 research of 9 injection schedules showed that when infants received the first dose at 3 months rather than later, or when the intervals were 1 month long instead of 2 months, diphtheria and tetanus antitoxin titers were lower 2 weeks after series completion. Therefore, clinical researchers should do lengthy longitudinal studies of infants vaccinated at 2, 3, and 4 months before the clinicians can actually determine the persistence of immunity to school entry. The UK Public Health Laboratory Service is doing a longitudinal study of early and long term antibody responses to the new schedules. In fact, the accelerated vaccination schedule has improved coverage during the first year of life. 20-30% of pertussis cases are less than 6 months old in developing countries. WHO's Expanded Programme on Immunization encourages health workers to begin vaccine series as early in life as possible and to keep the intervals as short as possible. Even though no primary series of 3 doses of DPT protects an infant for a lifetime, health workers should achieve high coverage with early doses and shorter intervals. WHO already advocates giving women of reproductive age in developing countries 5 doses of tetanus toxoid to reduce neonatal deaths.