BACKGROUNDA resurgence of pertussis has increased the demand for low-cost vaccines. The aim of this study was to test the immunogenicity of a booster acellular monovalent pertussis vaccine containing reduced-dose (2 μg) recombinant pertussis toxin (PT) and 5 μg filamentous haemagglutinin (FHA; apgen) against a version of apgen containing tetanus and reduced-dose diphtheria toxoids (Tdapgen) and a licensed vaccine containing chemically detoxified PT and FHA combined with tetanus toxoid and reduced-dose diphtheria toxoid (Tdapchem).METHODSThis phase 2/3, observer-blinded, randomised, controlled, non-inferiority trial was done in adolescents aged 9-17 years at two clinical research centres in Bangkok and Pathum Thani, Thailand. Eligible participants were screened and randomly assigned (1:1:1) to receive one booster dose of apgen, Tdapgen, or Tdapchem vaccine. Participants were followed up until day 336 post-immunisation. The primary endpoint was non-inferior seroconversion rates in Tdapgen and Tdapchem vaccine groups, with seroconversion rate defined as the proportion of participants with at least a four-fold increase on day 28 post-immunisation relative to baseline of anti-PT and anti-FHA IgG. The non-inferiority for seroconversion rates of anti-PT and anti-FHA IgG was defined as the lower bound of the two-sided 95% CI of the seroconversion rate for Tdapgen compared with Tdapchem exceeding -10%. Immunogenicity was analysed in the per-protocol population. All safety data were collected, and the prevalence of adverse events was analysed in the intention-to-treat population. This trial was registered on the Thai Clinical Trial Registry (TCTR20181031001).FINDINGSBetween June 18, and Aug 3, 2019, 450 adolescents (mean age 12·1 years, SD 2·5) were enrolled and randomly assigned (150 participants in each group). Day 28 anti-PT IgG seroconversion rates were 141 (94%) of 150 participants who received Tdapgen (95% CI 88·8-97·0) and 105 (71%) of 149 participants who received Tdapchem (62·7-77·2; p<0·0001). Day 28 anti-FHA IgG seroconversion rates were 144 (96%) of 150 participants who received Tdapgen (91·4-98·3) and 124 (83%) of 149 participants who received Tdapchem (76·4-88·4; p<0·0001). The difference in seroconversion rates was 23·5% (95% CI 15·3-31·8) for anti-PT IgG and 12·8% (6·0-19·6) for anti-FHA IgG, when comparing the Tdapgen versus the Tdapchem vaccine group. No vaccine-related serious adverse events were reported.INTERPRETATIONRecombinant Tdapgen vaccine showed non-inferior immunogenicity compared with Tdapchem at day 28 in terms of seroconversion rate of anti-PT IgG and anti-FHA IgG relative to baseline. The reduced-dose approach for Tdapgen vaccines thus presents as a potentially cost-saving booster strategy to protect adolescents against pertussis.FUNDINGOffice of National Higher Education Science Research and Innovation Policy Council (Programme Management Unit Competitiveness), Thailand, and BioNet-Asia.