Comparing the immunogenicity and safety of healthy infants who receive a delayed third dose of pertussis vaccine to those vaccinated according to the standard Australian vaccination schedule.

The 2 +1 acellular pertussis combination vaccine schedule was safe and immunogenic. Pertussis antibody levels were lower after the second dose in the 2+1 schedule when measured at 6 months of age compared to the 3+0 schedule. This could be related to the earlier administration of the second dose at 12 weeks of age in the 2+1 schedule, and the fact that there was longer interval post dose 2 where the blood sample was collected (3 months after dose 2 in the 2+1 schedule compared to 2 months after dose 2 in the 3+0 schedule). As is known, the presence of pertussis antibodies at baseline (due to maternal transfer) negatively impacts the infant response to acellular pertussis vaccines in infancy. This was seen in both the 2+1 and 3+0 vaccine schedules. Delaying the 3rd dose to 12 months of age (as seen in the 2+1 schedule) results in higher antibodies compared to receipt of the 3rd dose at 6 months (3+0) schedule. A limitation is the fact that antibody levels for pertussis, diphtheria and tetanus were measured in different laboratories using different techniques: MIA at Telethon Kids Institute for the 2+1 schedule and ELISA at GSK laboratories for the 3+0 schedule. The assay at the Telethon Kids Institute is able to quantify absolute values above (no lower limit of detection).In the context of maternal Tdap vaccination the 2 +1 vaccine schedule is a potential safe and immunogenic vaccine schedule which could avoid the need for an additional DTPa booster in the second year of life.