A second dose of Tdap, a reduced-antigen-content tetanus-diphtheria-acellular pertussis vaccine, 9 years after initial vaccination was well tolerated and immunogenic, according to the results of a phase 3 multicenter extension study (ClincalTrials.gov NCT00489970) published in Vaccine.

Spikes in the number of pertussis cases appear to occur every 3 to 5 years despite adequate vaccine coverage with efficient vaccines. In order to investigate antibody persistence of vaccinated individuals at 5 and 9 years post-vaccination and to test the immunogenicity and safety of a second Tdap booster dose administered 9 years after the initial Tdap vaccination, 1257 participants were enrolled at 5 years after initial Tdap vaccination, 809 of whom were re-vaccinated at 9 years after initial vaccination. Patients re-vaccinated 9 years after initial vaccination received doses containing either 3 (Tdap-B group) or 5 pertussis components (Tdap-A group).

The seroprotection rate for tetanus and diphtheria in both groups was ≥98.4% and ≥98.0%, respectively, at 5 years after initial vaccination and ≥98.3% and ≥98.1%, respectively, at 9 years after initial vaccination. For pertussis antigens, antibody concentrations above assay cut-offs were observed in ≥76.6% of individuals at 5 years after initial vaccination and ≥ 84.9% at 9 years after initial vaccination. At 9 years after initial vaccination with Tdap and 1 month post-Tdap vaccination, comparable seroprotection, seropositivity rates and antibody geometric mean concentration were observed in both groups. Non-inferiority of immune responses in both Tdap groups was demonstrated compared to a control group for diphtheria and tetanus or a 3-dose combined diphtheria-tetanus toxoids-acellular pertussis vaccine series for pertussis antigens. The non-inferiority criteria in terms of booster response were not met for all antigens and no safety concerns were raised during the trial.

The results provided information on the immunogenicity, safety, and reactogenicity of the second dose to Tdap vaccine 9 years after the first vaccine, but changes in assays and cut-offs limited the comparisons that can be made with other studies. This study was also limited by low power to assess non-inferiority objectives with regard to booster responses. Investigators concluded that the second Tdap dose is immunogenic and well-tolerated, supporting “the administration of Tdap as a decennial booster dose.”

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Brandon D, Kimmel M, Kuriyakose SO, Kostanyan L, Mesaros N. Antibody persistence and safety and immunogenicity of a second booster dose nine years after a first booster vaccination with a reduced antigen diphtheria-tetanus-acellular pertussis vaccine (Tdap) in adults. Vaccine. 2018;36:6325-6333.

This article originally appeared on Infectious Disease Advisor