• 1-Incidence of solicited adverse events from administration of study vaccine through 28 days following each dose [ Time Frame: 28 days following each dose ]
• 3-Incidence of MA-LRIs from administration of study vaccine through 28 days following each dose [ Time Frame: 28 days following each dose ]
• 4-Incidence of SAEs from administration of study vaccine through 28 days following each dose [ Time Frame: 28 days following each dose ]
• 5-Description of SNMCs from administration of study vaccine through the end of the RSV season or 180 days after the final dose of vaccine, whichever is later [ Time Frame: 180 days after the final dose ]
• 2-Incidence of unsolicited adverse events from administration of study vaccine through 28 days following each dose [ Time Frame: 28 days following each dose ]

The overall objective of the MEDI-534 clinical development program is to evaluate the safety, efficacy and tolerability of MEDI-534 for the prevention of RSV and PIV3 disease in young infants.

The primary objective of this study is to describe the safety and tolerability of multiple doses of MEDI-534 at 10 to 4, 10 to 5, or 10 to 6 TCID50 when administered to RSV and PIV3 seronegative children 6 to <24 months of age.

• Respirovirus Infections
• Respiratory Syncytial Virus Infections
• Parainfluenza Virus 3, Human

• Active Comparator: MEDI-534 at 10:4 TCID50 at 0, 2, and 4 months (Nasal spray)
• Active Comparator: MEDI-534 at 10:5 TCID50 at 0, 2, and 4 months (Nasal Spray)
• Active Comparator: MEDI-534 at 10:6 TCID50 at 0, 2, and 4 months (Nasal Spray)

Inclusion Criteria:

• Male or female whose age on the day of randomization is 6 to <24 months (reached 6th month birthday and not yet reached 2nd year birthday)
• Subject is seronegative to both RSV and PIV3 at screening
• Subject was the product of normal full term pregnancy (defined as >36 weeks gestation)
• Subject is in general good health
• Subject's legal representative is available by telephone
• Written informed consent and HIPAA authorization (if applicable) obtained from the subject's legal representative
• Subject's legal representative is able to understand and comply with the requirements of the protocol as judged by the investigator
• Subject is available to complete the follow-up period, which will be through the end of RSV season (provisionally defined as 01/Apr for the United States) or 180 days after the final dose of study vaccine, whichever is later
• Subject's legal representative must be willing and able to bring the subject to the study site for evaluation of respiratory illness in accordance with the protocol

Exclusion Criteria:

• Any fever (equal to or greater than 100.4°F [equal to or greater than 38.0°C], regardless of route) or lower respiratory illness (Section 4.1.2) within 7 days prior to randomization
• Moderate or severe nasal congestion that in the investigator's opinion could prevent intranasal delivery of vaccine
• Any drug therapy (chronic or other) within 7 days prior to randomization or expected receipt through the protocol-specified blood collection 28 days after each study vaccine dosing, except that infrequent use of over-the-counter medications such as pain relievers are permitted according to the judgment of the investigator
• Any current or expected receipt of immunosuppressive agents including steroids (2 mg/kg per day of prednisone or its equivalent, or equal to or greater than 20 mg/day if the subject weighs >10 kg, given daily or on alternate days for equal to or greater than 14 days); children in this category should not receive study vaccine until immunosuppressive agents including corticosteroid therapy have been discontinued for equal to or greater than 30 days; the use of topical steroids is permitted according to the judgment of the investigator
• History of receipt of blood transfusion or expected receipt through 30 days following final study vaccine dosing
• History of receipt of immunoglobulin products or expected receipt through 30 days after study vaccine dosing
• Receipt of any investigational drug within 60 days prior to randomization or expected receipt through 30 days after final study vaccine dosing
• Receipt of any live virus vaccine (excluding rotavirus vaccine) within 28 days prior to randomization or expected receipt within a 28-day window around any study vaccine dose
• Receipt of any inactivated (i.e., non-live) vaccine or rotavirus vaccine within 14 days prior to randomization or expected receipt within a 14-day window around any study vaccine dose
• Known or suspected immunodeficiency, including HIV
• Living in the same home or enrolled in the same classroom at day care with infants <24 months of age (only one child per household may be enrolled into the study)
• Contact with pregnant caregiver
• A household contact who is immunocompromised; the subject should also avoid close contact with immunocompromised individuals for at least 30 days after any study vaccine dose
• A household contact who is a health care provider in contact with immunocompromised patients or who is a day care provider for infants under the age of 6 months
• History of allergic reaction to any component of the study vaccine
• Previous medical history, or evidence, of an intercurrent or chronic illness that, in the opinion of the investigator, may compromise the safety of the subject
• Known or suspected active or chronic hepatitis infection
• History of medical diagnosis of asthma, reactive airway disease, wheezing requiring medication, cystic fibrosis, bronchopulmonary dysplasia, chronic pulmonary disease, medically confirmed apnea, hospitalization for respiratory illness or mechanical ventilation
• Family member or household contact who is an employee of the research center or otherwise involved with the conduct of the study
• Any condition that, in the opinion of the investigator, might interfere with study vaccine evaluation