Vaccination of Adults 65 Years of Age and Older with Tetanus Toxoid, Reduced Diphtheria Toxoid, and Acellular Pertussis Vaccine (Boostrix™): Results of a Randomized Clinical Trial

Background: Adults aged ≥65 years are at risk of both contracting pertussis and transmitting the disease to young infants and children. Data regarding the immunogenicity and safety of acellular pertussis-containing vaccines in adults ≥65 years are limited.

Objectives: To evaluate the immunogenicity and safety of tetanus toxoid, reduced diphtheria toxoid and acellular pertussis (Tdap) vaccine compared to licensed tetanus-diphtheria (Td) vaccine in adults ≥65 years.

Methods: This phase III, randomized, observer-blind study was conducted at 24 US centers between February-October 2009 (NCT00835237). Healthy adults ≥65 years of age were randomized (2:1) to receive either Tdap (Boostrix™; GSK Biologicals) or Td (Decavac™; Sanofi Pasteur). Antibodies against vaccine antigens were measured by ELISA. Reactogenicity and safety were assessed using diary cards.

Results: 1332 subjects [median age 71 (range 65-93) years] were enrolled. Geometric mean concentrations (GMCs) for anti-diphtheria and anti-tetanus antibodies were increased ~4-fold and ~6-fold, respectively, following Tdap vaccination, and anti-pertussis antibody GMCs were increased ~7-14-fold relative to pre-vaccination values.  Tdap was noninferior to Td with respect to seroprotection rates for diphtheria and tetanus (LL of 95%CI for between-group differences ≥-10%). Pertussis antibody GMCs following Tdap were non-inferior to a 3-dose primary DTaP series in a study where efficacy was demonstrated (LL of 95%CI for between-group GMC ratios ≥0.67). Injection site pain, redness and swelling within 4 days were comparable between Tdap (21.5%, 10.8%, and 7.5% of subjects), and Td (27.7%, 12.6%, and 11.7%, respectively). Grade 3 solicited symptoms were reported by ≤0.9% subjects in either group. There were no clinically relevant differences between groups with respect to reporting of solicited or unsolicited adverse events.

Conclusions: Tdap vaccine is immunogenic in adults ≥65 years of age and is expected to provide protection against diphtheria, tetanus and pertussis in this age group, with a safety profile comparable to that of Td vaccine.