The purpose of the study is to assess the immune response, reactogenicity and safety of a booster dose of dTpa vaccine 1 month after vaccination in healthy Japanese participants aged 11 to <13 years.

Inclusion Criteria:

• Participants and/or participants' parent(s)/ Legally acceptable representative(s) [LAR(s)], who, in the opinion of the investigator, can and will comply with the requirements of the protocol.
• Physical or digital informed assent obtained from the participant prior to performance of any study-specific procedure.
• Physical or digital informed consent obtained from the parent(s)/LAR(s) of the participant prior to performance of any study-specific procedure.
• Healthy participants as established by medical history and clinical examination at screening.
• Male/female participant between and including 11 and <13 years of age at the time of the study intervention administration (Visit 1/Day 1).
• Previously completed all routine childhood vaccinations to the best knowledge of the participant's parent(s)/LAR(s). Participants with documented previous diphtheria, tetanus and pertussis vaccination (primary series and first booster) as per routine vaccination in Japan prior to study enrolment.
• Participants did not receive an additional diphtheria, tetanus with or without pertussis vaccination within 5 years prior to enrolment in the study.
• Japanese ethnic origin.
• Participants of non-childbearing potential may be enrolled in the clinical study.
• Participant of childbearing potential may be enrolled in the study if the participant:
• has practiced adequate contraception for at least 30 days prior to study intervention administration, and
• has a negative pregnancy test within 24 hours prior to the study intervention administration, and
• has agreed to continue adequate contraception during the entire treatment period and for 8 weeks after completion of the study intervention administration series.

Exclusion Criteria:

Medical conditions

• History of physician-diagnosed or laboratory-confirmed diphtheria, tetanus or pertussis diseases within the past 5 years.
• History of encephalopathy after administration of a previous dose of pertussis vaccine that could not be attributed to another identifiable cause, progressive neurologic disorder, uncontrolled epilepsy or progressive encephalopathy: pertussis vaccine should not be administered to individuals with these conditions until a treatment regimen has been established and the condition has stabilized.
• History of transient thrombocytopenia or neurological complications following an earlier immunization against diphtheria and/or tetanus.
• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention(s) or having shown signs of hypersensitivity after previous administration of diphtheria, tetanus or pertussis vaccines.
• Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination
• Acute or unstable chronic conditions clinically significant pulmonary, cardiovascular, hepatic, or renal functional abnormality, as determined by physical examination and/or laboratory screening tests.
• Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant due to participation in the clinical study.
• Condition that in the judgment of the investigator would make intramuscular injection unsafe.

Prior/Concomitant therapy

• Use of any investigational or non-registered product other than the study intervention(s) during the period beginning 30 days before the dose of study intervention(s), or their planned use during the study period.

• Administration of immunoglobulins or other blood products or plasma derivatives during the period starting 90 days before the study intervention or planned administration during the study period.

• Chronic administration of immune-modifying drugs and/or planned use of long-acting immune-modifying treatments at any time up to the end of the study.

  • Up to 6 months prior to the study intervention administration:

For corticosteroids, this will mean prednisone equivalent ³0.5 mg/kg/day with maximum of 20 mg/day. Inhaled, intra-articular/intra-bursal and topical steroids are allowed.

• Up to 6 months prior to study intervention administration: long-acting immune-modifying drugs including among others immunotherapy, monoclonal antibodies, antitumoral medication.

  • Planned administration/administration of a vaccine not foreseen by the study protocol in the period starting 30 days before the dose and ending 30 days after the dose of study intervention(s) administration*, with the exception of inactivated influenza vaccine and SARS-CoV-2 vaccine which can be given at any time during the study conduct.

    *If emergency mass vaccination for an unforeseen public health threat (e.g., a pandemic) is recommended and/or organized by public health authorities outside the routine immunization program, the time period described above can be reduced, provided it is used according to the local governmental recommendations and Sponsor is notified.

  • Planned administration of any prophylactic medication in the absence of any symptom and in anticipation of a reaction to the study intervention administration.

Prior/Concurrent clinical study participation

• Concurrently participating in another clinical study, at any time during the study period, in which the participant has been or will be exposed to an investigational or a non-investigational intervention vaccine/drug/invasive medical device.

Other exclusion criteria

• Pregnant or lactating participant.
• History of /current chronic alcohol consumption and/or drug abuse, based on investigator's judgment.
• Any study personnel or their immediate dependents, family, or household members.
• Child in care.

Study Sponsor will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/gsk-patient-level-data-sharing-july2025.pdf

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or asset(s) with development terminated across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months, but an extension may be granted, when justified, for up to 6 months.

Open-label study.