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Inclusion Criteria:

* Healthy adults aged 18-50 years (inclusive)
* Fluent spoken English - to ensure a comprehensive understanding of the research project and their proposed involvement
* Capacity to provide written informed consent
* Females of childbearing potential with a negative urine pregnancy test at screening and willing to practice adequate birth control measures during the study.

Exclusion Criteria:

* Be currently involved in another study unless observational or non-interventional. Exceptions are the EHPC bronchoscopy study and COVID-19 observational and interventional trials. The exceptions will be applied at the discretion of the Chief Investigator to ensure no harm comes to the participants (e.g. excessive blood sampling)
* Be a participant in a previous EHPC trial within the last 3 years (at the discretion of the study team).

  * Vaccination (self-reported or confirmed from GP questionnaire \[GPQ\] or medical summary if deemed necessary at clinician discretion): Have had any previous pneumococcal vaccination (including in a research study).
  * Allergy:
* Have allergy to penicillin or amoxicillin.
* Have previous anaphylaxis or severe adverse reaction to any component/excipient of the vaccines or to any vaccine.

  • Health history (self-reported by the participant or confirmed in GPQ or medical summary if deemed necessary at clinician discretion): Ill health including but not limited to:
* Asplenia or dysfunction of the spleen.
* Chronic respiratory disease (e.g. asthma \[on medication\], COPD, emphysema, bronchiectasis).
* Chronic heart disease (e.g. angina, ischaemic heart disease, chronic heart failure) \[controlled stable hypertension may be included\].
* Chronic kidney disease (e.g. nephrotic syndrome, kidney transplant, on dialysis).
* Chronic liver disease (e.g. cirrhosis, biliary atresia, hepatitis).
* Chronic neurological conditions
* Connective tissue disease
* Dementia
* Diabetes mellitus (including diet controlled).
* Immunosuppression or history of receiving immunosuppressive therapy.
* Individuals with cochlear implants.
* Individuals with major cerebrospinal fluid leaks (e.g. following trauma, major skull surgery, or requiring CSF shunt).
* Recurrent otitis media.

  * Have any uncontrolled medical/surgical conditions (such as but not restricted to mental health conditions, epilepsy, narcolepsy or chronic pain) at the discretion of the study doctor.
  * Major pneumococcal illness requiring hospitalisation within the last 10 years
  * Other conditions considered by the clinical team as a concern for participant safety or integrity of the study.
  * Taking medication:
* That may affect the immune system e.g. steroids, inflammation altering (e.g. nasal steroids, Roaccutane) or disease-modifying anti-rheumatoid drugs.
* Long-term use of antibiotics (see also section 6.3 Temporary exclusion criteria).
* Nitroglycerin
* That affects blood clotting (any oral/injectable anticoagulants \[except aspirin\]).

  * Female participants who are pregnant, lactating or intending on becoming pregnant during the study.
  * Direct caring role or close contact with individuals at higher risk of infection (during the challenge period)
* Children under 5 years age.
* Chronic ill health or immunosuppressed adults.
* People that are part of the extremely vulnerable group as defined by Public Health England (PHE)

  • Smoker:
* Current or ex-smoker (regular cigarettes/cigars/e-cigarette/vaping/smoking of recreational drugs) in the last 6 months.
* Previous significant smoking history (more than 20 cigarettes per day for 20 years or the equivalent \[\>20 pack years\]).

  * History or current drug or alcohol abuse (frequently drinking alcohol): men and women should not regularly drink \>3 units/day and \>2 units/day respectively) at discretion of the clinician.
  * Significant mental health problems (uncontrolled condition or previous admission in a psychiatric unit, at the discretion of the clinician) that would impair the participant's ability to participate in the study
  * Overseas travel planned: Overseas travel during the follow-up period Part A or Part B.
  * Participants who meet STOP criteria at the time of screening, as detailed in Table 4.
  * Any other issue which, in the opinion of the study staff, may
* Put the participant or their contacts at risk because of participation in the study,
* Adversely affect the interpretation of the study results, or
* Impair the participant's ability to participate in the study.

• Liverpool, United Kingdom