CHICAGO — Monovalent acellular pertussis vaccine given at birth and 1 month results in immunogenicity in infants by 2 months of age, according to data presented in a late-breaking poster at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy.

The finding is important because most cases of severe pertussis disease occur in infants less than 4 months of age, before they are fully vaccinated. The currently recommended vaccination schedule for U.S. children aged 0–6 years calls for diphtheria, tetanus, and acellular pertussis (DTaP) vaccinations at 2, 4, and 6 months of age.

Two doses of acellular pertussis are thought to be crucial for protection, but are typically delayed until at least 6 weeks of age because of concerns that the newborns' immature immune systems may not recognize the vaccine and respond by making antibodies, and that maternal antibodies may interfere with vaccine efficacy, lead investigator Dr. Nicolas Wood told reporters at a press briefing at the conference, which was sponsored by the American Society for Microbiology.

“We think it's an important study because we were able to show that the early doses at birth and 1 month could get you higher antibody at a time [when] if you do get pertussis infection you are more likely to get severe disease,” said Dr. Wood, a pediatrician and clinical fellow at the Children's Hospital at Westmead, New South Wales, Australia.

Dr. Wood and his associates at the Women's and Children's Hospital, in Adelaide, Australia, examined the immunogenicity of birth and 1-month acellular pertussis (Pa) vaccination in a cohort of 76 children. Serology and pertussis vaccine were provided by GlaxoSmithKline. Each dose contained pertussis toxin 25 mcg, filamentous hemagglutinin 25 mcg, and pertactin 8 mcg. The children were divided into three groups. (See accompanying chart for vaccination schedules.)

At 2 months of age, infants who received pertussis vaccine at birth and 1 month had significantly higher antibody levels against pertussis toxin and pertactin (an antigenic protein in most pertussis vaccines) than infants who received pertussis just at birth or who did not receive any pertussis before 2 months of age.

The infants were followed for 4, 6, and 8 months, and at all time points, the infants who received pertussis vaccine at birth and 1 month had higher antibody levels against pertussis toxin and pertactin than infants on the other two schedules.

The difference in antipertussis response was significant at 2, 4, and 6 months for group 1 vs. groups 2 and 3 (P < .05), and at 2 and 4 months for group 2 versus group 3 (P < .05).

“By the end we were able to show that giving the early doses had given them earlier antibody protection, and by the time they'd finished the schedule, the antibodies were equivalent to those [in] babies [who] started later,” Dr. Wood said. “So it didn't result in the suppression of the immune system, which was of one of our concerns.”

Among the 75 evaluated children, there were no serious adverse reactions. One child was exposed to pertussis, which resulted in a mild case. Because there was only one exposure, Dr. Wood said no conclusions could be made about how protective the early schedule is against disease. Unlike hepatitis B, there is no specific antibody level cutoff point at which protection can be guaranteed.

Larger trials are needed to consider the combination of pertussis with hepatitis B vaccine at birth and different schedules, said Dr. Wood, who received research support from the Financial Markets Foundation for Children in Australia.

Dr. Claire Anne Siegrist of the Centre of Vaccinology and the Neonatal Immunology, University of Geneva, said in an interview that the findings confirm results reported previously by her and her colleagues. “What all of our studies have shown is that you can give the first dose of pertussis at birth and already have a significant immune response much earlier than if you wait until the age of 2 months, which is the regular timing,” she said.

Further studies are needed to define the best dosing schedule, she said, and to entice vaccine manufacturers to create a monovalent acellular pertussis vaccine, which currently is not commercially available.

A third study, reported by Dr. Natasha Halasa at the Interscience Conference on Antimicrobial Agents and Chemotherapy in 2005, supports the use of a monovalent pertussis vaccine, as antibody suppression was demonstrated when pertussis was given in combination with diphtheria. “One of the theories with the diphtheria added with it is that there may be some intolerance early on that is interfering with the antigen presentation with pertussis,” Dr. Halasa of Vanderbilt Children's Hospital, Nashville, Tenn., said in an interview.

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