2. Cohort 1 only: Participant is seronegative to HPIV3 at screening as determined by ELISA; or the legal representative is willing to provide access to data documenting that the participant was screened for another MedImmune trial after written informed consent was obtained, and that the participant is seronegative to HPIV3 within 21 days prior to randomization into MI-CP150 as determined by ELISA at MedImmune