The goal of this clinical trial is to examine immune responses to the BCG vaccine in healthy adults who have, or who have not, taken antibiotics to deplete their gut bacteria prior to vaccination.

The main question it aims to answer is: does depletion of the gut microbiota lead to impaired BCG-induced protection against specific and non-specific to challenges to the immune system?

The study is divided into two sub-studies. The first sub-study (BCG re-challenge) is an experimental medicine study in 168 healthy participants to determine if depletion of the gut microbiota leads to impaired BCG-induced protection against a subsequent Mycobacterium bovis BCG intradermal challenge.

The second sub-study (Yellow Fever vaccine) has a very similar experimental design to the first but will determine if depletion of the gut microbiota leads to impaired BCG-induced protection against other infections. To assess this, participants in this sub-study (n=180) will be re-challenged after 3 months with a live attenuated viral vaccine, the Yellow Fever vaccine, which induces a mild viremia.

In both sub-studies, participants will initially be randomised to receive a 3 day course of antibiotics or none (comparator group). The two groups in each sub-study will be randomised again to receive either BCG vaccine or 0.9% NaCl placebo injection in the left arm.

BCG re-challenge sub-study (Sub-study 1): Six months following randomisation, all participants will receive a BCG vaccine challenge in the right arm. A punch skin biopsy will be taken of this challenge site 2 weeks after the challenge to assess M. bovis BCG bacterial load in the skin.

Yellow Fever vaccine sub-study (Sub-study 2): Three months following randomisation, all participants will receive a Yellow Fever vaccine challenge in the right arm. Blood samples will be collected from Yellow Fever vaccinated participants at day 3, 5 and 7 following Yellow Fever vaccine challenge to quantify Yellow Fever viral load in blood.

All participants in both sub-studies will have blood samples collected at randomisation, before each vaccination, 2 weeks after each BCG vaccination and in the Yellow Fever vaccine sub-study at day 3, 5 and 7 following Yellow Fever vaccination. Stool samples will be collected prior to randomisation, and prior to each vaccination.

Inclusion Criteria:

• 18-35 years old
• Provided a signed and dated informed consent form
• BCG naïve (Arm 1) and BCG and YF vaccine naïve (Arm 2)
• Willing to take short antibiotic course
• Willing to undergo a punch biopsy (Arm 1)
• Willing to have up to 7 blood samples and 3 stool samples collected over 5-7 months
• Not pregnant or intending to get pregnant for the duration of the study (a pregnancy test will be offered to females)

Exclusion Criteria:

• Previous BCG or YF vaccination

• Previous YF infection

• Evidence of latent TB infection (LTBI) (assessed through a questionnaire) (IGRA to confirm if needed)

• People with contraindications for BCG vaccination:

  • malignancies involving bone marrow or lymphoid systems, primary or secondary immunodeficiencies, HIV infection
  • moderate/severe skin disease including eczema, dermatitis or psoriasis
  • requiring immunosuppressive drugs or other immune modifying drugs e.g. corticosteroids, non-biological immunosuppressants, biological agents (such as monoclonal antibodies against tumour necrosis factor (TNF)-alpha)

• People with contraindications to YF vaccination:

  • History of thymus disease, including myasthenia gravis, thymoma, thymectomy, DiGeorge syndrome, thymic damage from chemoradiotherapy or graft-versus-host disease
  • YF vaccination is contraindicated in immunocompromised individuals, including individuals who have HIV infection, primary immunodeficiencies (including inherited IFNAR1 deficiency), or are taking corticosteroids or other immunosuppressive agents and haematopoietic stem cell transplant recipients
  • People who have had a haematopoietic stem cell transplant
  • Individuals with history of severe allergic reactions to egg or chicken proteins

• Pregnant or breastfeeding or planning to become pregnant

• History of renal disease/insufficiency

• Tattoo obscuring BCG vaccination site(s)

• Any history of severe allergic reaction or anaphylaxis to vaccination or antibiotics

• People with chronic serious underlying illness

• Have received any prescribed oral or intravenous antibiotic in the 28 days prior to study visits 1 and 4 (including isoniazid, rifampicin, streptomycin and ethambutol as these particular antibiotics have activity against M. bovis)

Non-identifiable individual participant data from the study may be made available long-term for use by future researchers from a recognised research institution whose proposed use of the data has been ethically reviewed and approved by an independent committee and who accept SAHMRI's conditions, under a collaborator agreement, for accessing:

• Individual participant data that underlie the results reported in our articles after deidentification (text, tables, figures and appendices)
• Study protocol, Statistical Analysis Plan, PICF

Molecular data generated from this study (i.e. transcriptomic, epigenomic and metagenomic) will be labelled with a unique identification code and stored on an appropriate data repository (e.g. Gene Expression Omnibus, Sequence Read Archive) with demographic information (age and sex). No identifying participant information will be shared.

At time of publication. Data will be available indefinitely.

Researchers from a recognised research institution can approach SAHMRI for access of data. The researcher will need to provide evidence that the proposed use of the data has been ethically reviewed and approved by an Institutional Review Board (IRB)/ Human Research Ethics Committee(HREC), and accept SAHMRI's conditions, under a collaborator agreement.

Participants and Investigator are blinded to BCG vaccine or placebo.