The purpose of this research study is to evaluate whether the investigational, live attenuated, intranasally delivered vaccine MV-012-968 ('study vaccine') may have prophylactic efficacy against symptomatic RSV infection when administered to adults 18-45 years of age in the Human Viral Challenge model.

Key Inclusion Criteria:

1. An informed consent document signed and dated by the participant and the Investigator.
2. Aged between 18 and 45 years old on the day of signing the consent form.
3. In good general health with no history, or current evidence, of clinically significant medical conditions (including respiratory, cardiac and immunodeficiency), and no clinically significant test abnormalities that will interfere with participant safety, as defined by medical history, physical examination (including vital signs), electrocardiogram (ECG), and routine laboratory tests as determined by the Investigator.
4. A documented medical history prior to enrolment.
5. Females of child bearing potential must have a negative pregnancy test prior to enrollment.
6. Females and Males must agree to adhere to the highly effective contraceptive requirements of the study from 2 weeks prior to the first study visit until 90 days after the date of study vaccination.
7. Sero-suitable to the challenge virus, as defined in the study Analytical Plan.

Exclusion Criteria:

1. History of symptoms or signs suggestive of upper or lower respiratory tract infection within 4 weeks prior to the first study visit
2. Rhinitis which is clinically active
3. History of moderate to severe rhinitis
4. Acute sinusitis during Screening
5. Use of corticosteroid in respiratory tract (e.g. nasal or inhaled steroid) in the 30 days leading up to study vaccination
6. Females who are breastfeeding or have been pregnant within 6 months prior to the study
7. Participants who have smoked ≥ 10 pack years at any time [10 pack years is equivalent to one pack of 20 cigarettes a day for 10 years])
8. Positive human immunodeficiency virus (HIV), active hepatitis A (HAV), B (HBV), or C (HCV) test
9. Those employed or immediate relatives of those employed at hVIVO or the Sponsor
10. Participants may not live or work in direct close contact with, or live with anyone whose work brings them into direct close contact with, children under 2 years of age
11. Any other finding that, in the opinion of the Investigator, deems the participant unsuitable for the study