The purpose of this Phase 3, non-randomized, single-arm, open-label study is to evaluate the immune response, reactogenicity and safety of GSKs dTpa vaccine in Japanese pregnant women between 27 weeks and less than 37 weeks of pregnancy. Both the pregnant women and their neonates born during the study will be evaluated for specific analyses.

Inclusion Criteria:

1. Participants, who, in the opinion of the investigator, can and will comply with the requirements of the protocol.
2. Participants and Legally acceptable representative(s) [LAR(s)] who give physical or digital informed consent after the study has been explained according to local regulatory requirements, and before any study-specific procedures are performed. The informed consent given at screening should include consent for both the maternal participant's participation and participation of the infant after the infant's birth.
3. Healthy participants as established by medical history and clinical examination at screening.
4. Participants between and including 18 and 45 years of age at the time of the study intervention administration (Visit 1/Day 1).
5. Pre-pregnancy body mass index (BMI) (based on participant's report) between 17.0 and 39.9 kg/m^2, inclusive.
6. Pregnant female at 27,0/7 to 36,6/7 weeks of gestation (completed week 27 but not week 37) at the time of vaccination (Visit 1/Day 1), as established or confirmed by ultrasound examination.
7. No significant fetal abnormalities, as observed by the fetal morphological abnormality screening test conducted after 18 weeks of gestation and the most recent ultrasound testing (no more than 6 weeks before enrollment).
8. Nuchal translucency scan, serum testing and any other prenatal tests, if conducted, should suggest normal pregnancy.
9. Participants who are willing to provide cord blood and/or infant blood.
10. Participants who are willing to have them and their newborns followed-up until 1 month post-delivery.
11. Participants who do not plan to give their child for adoption.
12. Japanese ethnic origin.

Exclusion Criteria:

Medical conditions:

1. Participants diagnosed with multiple pregnancies.

2. Women with co-morbid medical or obstetric conditions that in the opinion of the investigator have the potential to complicate the pregnancy course and outcomes such as;

   • Gestational hypertension (defined as systolic blood pressure >=140 mmHg and/or diastolic blood pressure >=90 mmHg) at >=20 weeks of gestation in a woman with a previously normal blood pressure. Women with gestational hypertension who maintain blood pressure in the normal range (<140 mmHg and <90 mmHg) through diet and/or on antihypertensive medications would be eligible except for eclampsia/pre-eclampsia.
   • Hemodynamically significant cardiac disorders (previously corrected patent ductus arteriosis is allowed).
   • Gestational diabetes which is not controlled by medication, diet and/or exercise as determined by glucose challenge/tolerance test conducted after 20 weeks of gestation or as per local recommendations of the country (Gestational diabetes is defined as absence of pre-gestational diabetes and hyperglycemia during pregnancy, which is not due to other known causes).
   • Any other conditions that in the opinion of the investigator have the potential to complicate the pregnancy course and outcomes

3. Acute or unstable chronic conditions, chronic clinically significant abnormality, poorly controlled pre-existent co-morbidities or any other clinical conditions, as determined by physical examination and/or laboratory tests.

4. Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination.

5. Recurrent history or uncontrolled neurological disorders or any neuroinflammatory, congenital neurological conditions, encephalopathies, or seizures.

6. Condition that in the judgment of the investigator would make intramuscular injection unsafe.

7. Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant and/or to the unborn infant due to participation in the clinical study.

8. Prior major congenital anomalies or early onset (<34 weeks of gestation) of eclampsia/pre-eclampsia in previous pregnancy, or stillbirth or neonatal death, or multiple (>=2) spontaneous abortions, or pre-term delivery (<=34 weeks gestation) or having ongoing intervention (medical/surgical) in current pregnancy to prevent pre-term delivery.

9. Family history (first degree relatives only) of congenital anomalies, recurrent pregnancy losses (two or more consecutive losses) and unexplained neonatal death(s) in the participant.

10. History of an encephalopathy of unknown etiology, occurring within 7 days following previous vaccination with pertussis-containing vaccine.

11. History of transient thrombocytopenia or neurological complications (for convulsions or hypotonic-hyporesponsive episodes) following an earlier immunisation against diphtheria and/or tetanus.

12. History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention or having shown signs of hypersensitivity after previous administration of diphtheria, tetanus or pertussis vaccines.

13. History of physician-diagnosed or laboratory-confirmed pertussis within the past 5 years.

14. Lymphoproliferative disorder or malignancy within 5 years before the study dose administration (excluding effectively treated non melanoma skin cancer).

    Prior/Concomitant therapy:

15. Previous vaccination containing diphtheria, tetanus or pertussis antigens, or diphtheria and tetanus toxoids at any time during the current pregnancy and within 5 years from study participation.

16. Use of any investigational or non-registered product other than the study intervention(s) during the current pregnancy or their planned use during the study period.

17. Chronic administration of immune-modifying drugs (defined as more than 14 consecutive days in total) and/or planned use of long-acting immune-modifying treatments or planned administration through delivery.

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

    For corticosteroids, this will mean prednisone equivalent >=20 mg/day for adult participants. Inhaled, intra-articular/intra-bursal and topical steroids are allowed.

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

    long-acting immune-modifying drugs including among others immunotherapy monoclonal antibodies, antitumoral medication.

18. Planned administration/administration of a vaccine not foreseen by the study protocol in the period starting 30 days before the dose and planned administration through delivery (Visit 3/Day of delivery), except:

    • Seasonal influenza vaccines, RSV vaccines, Hepatitis B vaccines, and SARS-CoV-2, all of which may be administered according to standard of care >=14 days before or after study vaccination.

19. Administration of immunoglobulins or other blood products or plasma derivatives during the period starting 3 months before the study intervention or planned administration through delivery with the exception of anti-D (Rh)-immunoglobulin.

20. Planned administration of any prophylactic medication (e.g., analgesics, antipyretics) in the absence of any symptom and in anticipation of a reaction to the study intervention administration.

    Prior/Concurrent clinical study participation:

21. 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 drug/vaccine/invasive medical device.

    Other exclusion criteria:

22. History of chronic alcohol consumption and/or drug abuse, based on investigator's judgment.

23. Any study personnel or their immediate dependents, family, or household members.

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.

This is an open-label study.