CompletedNCT04491877VAD00001Updated Sep 9, 2025Results posted

Study of an Live-Attenuated Respiratory Syncytial Virus Vaccine in Infants and Toddlers

A Phase 1/2 interventional study of RSV vaccine formulation 1 and RSV vaccine formulation 2 in Respiratory Syncytial Virus Infection, sponsored by Sanofi Pasteur, a Sanofi Company. Completed at 29 sites in 3 countries. Open to participants aged 6 Months to 18 Months, including healthy volunteers. Per ClinicalTrials.gov, last updated 2025-09-09.

Sponsored by Sanofi Pasteur, a Sanofi Company · Phase 1/2, Interventional, and Prevention

Phase: Phase 1/2
Study type: Interventional
Enrollment: 259
Allocation: Randomized
Ages: 6 Months to 18 Months
Sex: All

Study summary

The primary objectives of the study were:

To assess the safety profile of each dose of the study product after each and any administration in all infants and toddlers regardless of baseline serostatus.
To characterize the Respiratory Syncytial Virus (RSV) A serum neutralizing antibody responses to the study product in each vaccine group after vaccination in RSV-naïve participants.

The secondary objectives of the study were:

To quantify the amount of vaccine virus shed by each participant by baseline serostatus.
To determine the proportion of vaccinated infants and toddlers in each vaccine group infected with the vaccine virus at D56 (56 days after vaccination 1) for Cohorts 1, 2, 3 and 4, and at Day 84 (28 days after vaccination 2) for Cohorts 2 and 4 by baseline serostatus.
To characterize the RSV A serum neutralizing antibody responses to the study product in each vaccine group after vaccination in RSV-experienced participants.
To characterize serum RSV anti-F immunoglobulin G (IgG) antibody responses to the study product in each vaccine group after vaccination by baseline serostatus.
To characterize serum RSV antibody responses (RSV A-neutralizing and anti-RSV F IgG) to the study product in each vaccine group after the RSV surveillance season or at least 5 months after the last vaccine administration by baseline serostatus.

Read the detailed description

Study duration per participant was maximum 12 months

Conditions studied

Respiratory Syncytial Virus Infection

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Respiratory Syncytial Virus Infections 293

In context

Respiratory Syncytial Virus Infections

293 studies on the registry are indexed under Respiratory Syncytial Virus Infections; 45 are open to participants now.

This study's enrollment of 259 is above the median of 90 across 213 interventional studies indexed under Respiratory Syncytial Virus Infections.

Browse Respiratory Syncytial Virus Infections studies →

Lead sponsor

Sanofi Pasteur, a Sanofi Company is the lead sponsor of 366 studies on the registry; 4 are open to participants now.

Of its 94 completed or terminated interventional studies of FDA-regulated products, 73 (78%) have results posted.

Counted across the registry records on this site, refreshed daily.

Who can participate

Ages eligible: 6 Months to 18 Months
Sexes eligible: All
Accepts healthy volunteers: Yes

Eligibility criteria

Inclusion Criteria:

Inclusion criteria :

Aged 6 through 18 months at Day 0.
Informed consent form has been signed and dated by the parent(s) or other legally acceptable representative (and by independent witness if required by local regulations).
Participant and parent / guardian / legally acceptable representative are able to attend all scheduled visits and to comply with all trial procedures

Exclusion Criteria:

Born at less than 34 weeks gestation
Born at less than 37 weeks gestation and less than 1 year of age at the time
Known or suspected congenital or acquired immunodeficiency; or receipt of immunosuppressive therapy, such as anti-cancer chemotherapy or radiation therapy, within the preceding 6 months; or long-term systemic corticosteroid therapy (prednisone or equivalent for more than 2 consecutive weeks within the past 3 months).
Probable or confirmed case of Coronavirus Disease 2019 (COVID-19).
Known systemic hypersensitivity to any of the vaccine components, or history of a life-threatening reaction to the vaccine used in the trial or to a vaccine containing any of the same substances
Any chronic illness
- Chronic illness may include, but is not limited to, cardiac disorders, lung disease (including any history of reactive airway disease, receipt of bronchodilator therapy, or medically diagnosed wheezing), renal disorders, auto-immune disorders, diabetes, psychomotor diseases, and known congenital or genetic diseases
Any history of medically diagnosed wheezing
Any acute febrile, respiratory or gastrointestinal illness in the past 24 hours that according to investigator judgment is significant enough to interfere with successful inoculation on the day of vaccination. A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided
Any previous anaphylactic reaction
Current suspected or documented developmental disorder, delay, or other developmental problem
Receipt of any of the following vaccines prior to enrollment:
- any influenza vaccine within 7 days prior, or
- any inactivated vaccine or live-attenuated rotavirus vaccine within the 14 days prior, or
- any live vaccine, other than rotavirus vaccine, within the 28 days prior, or
- another investigational vaccine or investigational drug within 28 days prior
Previous receipt of a licensed or investigational RSV vaccine or previous receipt or planned administration of any anti-RSV product (such as ribavirin or RSV immune immune globulins [IG] or RSV monoclonal antibody)
Receipt of immune globulins, blood or blood-derived products in the past 6 months prior to enrolment
Receipt of any of the following medications within 3 days prior to study enrollment (Day 0):
- systemic antibacterial, antiviral, antifungal, anti-parasitic, or antituberculous agents, whether for treatment or prophylaxis, or
- intranasal medications, or
- other prescription medication except as permitted concomitant medications (prescription or non-prescription) including nutritional supplements, medications for gastroesophageal reflux, eye drops, and topical medications, including (but not limited to) cutaneous (topical) steroids, topical antibiotics, and topical antifungal agents
Receipt of salicylate (aspirin) or salicylate-containing products within the 28 days prior to enrollment (Day 0)
Any previous vaccine-associated adverse reaction that was Grade 3 or above. Note: if grading is not possible, determine if the reaction was considered severe or life threatening; if so, it is exclusionary.
Scheduled administration of the following after planned inoculation:
- any influenza vaccine within 7 days after, or
- inactivated vaccine or live-attenuated rotavirus vaccine within the 14 days after, or
- any live vaccine other than rotavirus in the 28 days after, or
- another investigational vaccine or investigational drug in the 56 days after.
Any previous receipt of supplemental oxygen therapy in a home or hospital setting, except the temporary receipt of supplemental oxygen for transient tachypnea in newborn
Member of a household that contains an immunocompromised individual, including, but not limited to:
- a person who is HIV infected
- a person who has received chemotherapy within the 12 months prior to enrollment
- a person receiving immunosuppressant agents
- a person living with a solid organ or bone marrow transplant
Participation at the time of study enrollment (or in the 6 weeks preceding the first trial vaccination) or planned participation during the present trial period in another clinical trial investigating a vaccine, drug, medical device, or medical procedure
Member of a household that contains, or will contain, an infant who is less than 6 months of age at the enrollment date (or in the 6 weeks preceding the first trial vaccination) through Day 28
Member of a household that contains another child/other children who is/are, or is/are scheduled to be, enrolled in this study in the same year AND the date of enrollment will not be concurrent with the other participant(s) living in the household (i.e., all eligible children from the same household must be enrolled on the same date)
Attends a daycare facility and shares a daycare room with infants less than 6 months of age, and parent/guardian is unable or unwilling to suspend daycare for 28 days following inoculation
Deprived of freedom in an emergency setting or hospitalized involuntarily
Identified as a natural or adopted child of the Investigator or employee with direct involvement in the proposed study

The above information was not intended to contain all considerations relevant to a participant's potential participation in a clinical trial.

Study design

Phase: Phase 1 / Phase 2
Primary purpose: Prevention
Allocation: Randomized
Intervention model: Sequential assignment
Masking: Quadruple (Participant, Care provider, Investigator, Outcomes assessor)
Enrollment: 259 participants (actual)

Study arms

Experimental
Cohort 1 (RSV vaccine formulation 1)
1 administration of RSV vaccine formulation 1 on Day 0
Biological: RSV vaccine formulation 1
Placebo comparator
Cohort 1 (Placebo)
1 administration of placebo on Day 0
Biological: Placebo
Experimental
Cohort 2 (RSV vaccine formulation 1)
2 administrations of RSV vaccine formulation 1 on Day 0 and Day 56
Biological: RSV vaccine formulation 1
Placebo comparator
Cohort 2 (Placebo)
2 administrations of placebo on Day 0 and Day 56
Biological: Placebo
Experimental
Cohort 3 (RSV vaccine formulation 2)
1 administration of RSV vaccine formulation 2 on Day 0
Biological: RSV vaccine formulation 2
Placebo comparator
Cohort 3 (Placebo)
1 administration of placebo on Day 0
Biological: Placebo
Experimental
Cohort 4 (RSV vaccine formulation 1)
2 administrations of RSV vaccine formulation 1 on Day 0 and Day 56
Biological: RSV vaccine formulation 1
Experimental
Cohort 4 (RSV vaccine formulation 2)
2 administrations of RSV vaccine formulation 2 on Day 0 and Day 56
Biological: RSV vaccine formulation 2
Placebo comparator
Cohort 4 (Placebo)
2 administrations of placebo on Day 0 and Day 56
Biological: Placebo

Interventions

BiologicalRSV vaccine formulation 1
Pharmaceutical form: Suspension of virus Route of administration: Intranasal
BiologicalRSV vaccine formulation 2
Pharmaceutical form: Suspension of virus Route of administration: Intranasal
BiologicalPlacebo
Pharmaceutical form: Suspension Route of administration: Intranasal

What researchers measure

Primary outcomes

Number of Participants With Immediate Unsolicited Systemic Adverse Events (AEs)
An AE is any untoward medical occurrence in a participant or in a clinical investigation participant administered a medicinal product and which does not necessarily have a causal relationship with this treatment. An unsolicited AE is an observed AE that does not fulfill the conditions pre-listed in the case report book (CRB) in terms of diagnosis and/or onset window post-vaccination. Systemic AEs are all AEs that were not injection or administration site reactions. Immediate events are recorded to capture medically relevant unsolicited systemic AEs (including those related to the product administered) that occur within the first 30 minutes after vaccination.
Time frame: Cohorts 1 and 3: Within 30 minutes after vaccination on Day 0; Cohorts 2 and 4: Within 30 minutes after vaccination on Days 0 and 56
Number of Participants With Solicited Administration Site and Systemic Reactions
All noxious and unintended responses to a medicinal product related to any dose are considered adverse reactions (AR). A solicited reaction is an "expected" AR (sign or symptom) observed and reported under the conditions (nature and onset) pre-listed in the protocol and CRB. An administration site reaction is an AR at and around the administration site. Systemic ARs are all ARs that are not injection or administration site reactions.
Time frame: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Number of Participants With Unsolicited Adverse Events
An AE is any untoward medical occurrence in a participant or in a clinical investigation participant administered a medicinal product and which does not necessarily have a causal relationship with this treatment. An unsolicited AE is an observed AE that does not fulfill the conditions pre-listed in the CRB in terms of diagnosis and/or onset window post-vaccination.
Time frame: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Number of Participants With Adverse Events of Special Interest (AESIs)
An AESI is one of scientific and medical concern specific to the Sponsor's product or program, for which ongoing monitoring and rapid communication by the investigator to the sponsor can be appropriate.
Time frame: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Number of Participants With Medically Attended Adverse Events (MAAEs)
An MAAE is a new onset or a worsening of a condition that prompts the participant or participant's parent/guardian/legally authorized representative to seek unplanned medical advice at a physician's office or Emergency Department.
Time frame: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Number of Participants With Serious Adverse Events (SAEs)
An SAE is any untoward medical occurrence that at any dose results in death or is life-threatening or requires inpatient hospitalization or prolongation of existing hospitalization or results in persistent or significant disability/incapacity or is a congenital anomaly/birth defect or is an important medical event.
Time frame: From the first study vaccine administration (Day 0) up to end of the study, maximum of 12 months
Geometric Mean Titers Against RSV A Neutralizing Antibody in RSV-Naïve Participants
RSV A neutralizing antibody measured by microneutralization. RSV-naïve participants are defined as undetectable serum anti-RSV A IgA antibodies.
Time frame: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84

Secondary outcomes

Titer of Vaccine Virus Shedding Measured by Reverse Transcription Polymerase Chain Reaction (RT-PCR)
Shedding of the attenuated RSV vaccine strain in nasal swab samples was evaluated by RSV quantitative RT-PCR (qRT-PCR) assay, which specifically detected and quantified RSVt ΔNS2 vaccine strain (RSV ΔNS2/Δ1313/I1314L) in human nasal swab samples.
Time frame: Cohorts 1 and 3: Day 7; Cohorts 2 and 4: Days 7 and 63
Percentage of Participants Infected With Vaccine Virus at Days 56 and 84
Infection is defined as detection of vaccine virus in nasal swab by RT-PCR and/or a \>= 4-fold rise in RSV A serum neutralizing antibody titers, or in RSV serum anti-F immunoglobulin G (IgG) antibody titers.
Time frame: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84
Geometric Mean Titers Against RSV A Neutralizing Antibody in RSV-Experienced Participants
RSV A neutralizing antibody measured by microneutralization. RSV-experienced participants are defined as detectable serum anti-RSV A IgA antibodies. CI= confidence interval.
Time frame: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84
Geometric Mean Titers Against Serum Anti-F Immunoglobulin G (IgG) Antibody
The IgG antibodies to RSV F antigen was measured using the anti RSV F IgG ELISA. RSV-naïve and RSV-experienced participants are defined as undetectable or detectable serum anti-RSV A IgA antibodies, respectively.
Time frame: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84
Geometric Mean Titers Against RSV A Neutralizing Antibody After the RSV Surveillance Season
RSV A neutralizing antibody measured by microneutralization. RSV-naïve and RSV-experienced participants are defined as undetectable or detectable serum anti-RSV A IgA antibodies, respectively.
Time frame: Cohorts 1 and 3: Within 5 months after vaccination on Day 0; Cohorts 2 and 4: Within 5 months after vaccination on Day 56
Geometric Mean Titers Against Serum Anti-F Immunoglobulin G Antibody After the RSV Surveillance Season
The IgG antibodies to RSV F antigen was measured using the anti RSV F IgG ELISA. RSV-naïve and RSV-experienced participants are defined as undetectable or detectable serum anti-RSV A IgA antibodies, respectively.
Time frame: Cohorts 1 and 3: Within 5 months after vaccination on Day 0; Cohorts 2 and 4: Within 5 months after vaccination on Day 56

Results

Posted Jun 13, 2024

Participant flow

The study was conducted at 26 centers in the United States, Chile and Honduras between 17 September 2020 and 13 April 2023.

Participant flow — Overall Study
Milestone	Cohort 1: Respiratory Syncytial Virus (RSV) Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Started	18	18	11	10	10	12	61	58	61
Completed	17	16	9	8	10	10	56	51	57
Not completed	1	2	2	2	0	2	5	7	4
Withdrew: Protocol deviation	1	0	2	0	0	0	1	1	0
Withdrew: Withdrawal by parent/guardian	0	0	0	2	0	2	2	5	3
Withdrew: Lost to follow-up	0	2	0	0	0	0	2	1	1

Outcome measures

PrimaryNumber of Participants With Immediate Unsolicited Systemic Adverse Events (AEs)

An AE is any untoward medical occurrence in a participant or in a clinical investigation participant administered a medicinal product and which does not necessarily have a causal relationship with this treatment. An unsolicited AE is an observed AE that does not fulfill the conditions pre-listed in the case report book (CRB) in terms of diagnosis and/or onset window post-vaccination. Systemic AEs are all AEs that were not injection or administration site reactions. Immediate events are recorded to capture medically relevant unsolicited systemic AEs (including those related to the product administered) that occur within the first 30 minutes after vaccination.

Time frame:: Cohorts 1 and 3: Within 30 minutes after vaccination on Day 0; Cohorts 2 and 4: Within 30 minutes after vaccination on Days 0 and 56
Reported as:: Count of participants · Participants

Number of Participants With Immediate Unsolicited Systemic Adverse Events (AEs)
Participants	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Number of Participants With Immediate Unsolicited Systemic Adverse Events (AEs)	0	0	0	0	0	0	0	0	1

PrimaryNumber of Participants With Solicited Administration Site and Systemic Reactions

All noxious and unintended responses to a medicinal product related to any dose are considered adverse reactions (AR). A solicited reaction is an "expected" AR (sign or symptom) observed and reported under the conditions (nature and onset) pre-listed in the protocol and CRB. An administration site reaction is an AR at and around the administration site. Systemic ARs are all ARs that are not injection or administration site reactions.

Time frame:: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Reported as:: Count of participants · Participants

Number of Participants With Solicited Administration Site and Systemic Reactions
Participants	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Solicited administration site reactions	15	12	8	7	9	6	53	44	53
Solicited systemic reactions	14	16	9	9	9	9	50	46	51

PrimaryNumber of Participants With Unsolicited Adverse Events

Time frame:: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Reported as:: Count of participants · Participants

Number of Participants With Unsolicited Adverse Events
Participants	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Number of Participants With Unsolicited Adverse Events	6	2	2	5	0	2	43	35	46

PrimaryNumber of Participants With Adverse Events of Special Interest (AESIs)

An AESI is one of scientific and medical concern specific to the Sponsor's product or program, for which ongoing monitoring and rapid communication by the investigator to the sponsor can be appropriate.

Time frame:: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Reported as:: Count of participants · Participants

Number of Participants With Adverse Events of Special Interest (AESIs)
Participants	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Number of Participants With Adverse Events of Special Interest (AESIs)	3	0	0	0	0	1	20	11	20

PrimaryNumber of Participants With Medically Attended Adverse Events (MAAEs)

An MAAE is a new onset or a worsening of a condition that prompts the participant or participant's parent/guardian/legally authorized representative to seek unplanned medical advice at a physician's office or Emergency Department.

Time frame:: Cohorts 1 and 3: Within 28 days after vaccination on Day 0; Cohorts 2 and 4: Within 28 days after vaccination on Days 0 and 56
Reported as:: Count of participants · Participants

Number of Participants With Medically Attended Adverse Events (MAAEs)
Participants	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Number of Participants With Medically Attended Adverse Events (MAAEs)	3	1	0	4	0	1	36	29	36

PrimaryNumber of Participants With Serious Adverse Events (SAEs)

An SAE is any untoward medical occurrence that at any dose results in death or is life-threatening or requires inpatient hospitalization or prolongation of existing hospitalization or results in persistent or significant disability/incapacity or is a congenital anomaly/birth defect or is an important medical event.

Time frame:: From the first study vaccine administration (Day 0) up to end of the study, maximum of 12 months
Reported as:: Count of participants · Participants

Number of Participants With Serious Adverse Events (SAEs)
Participants	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Number of Participants With Serious Adverse Events (SAEs)	0	0	1	0	0	1	2	1	2

PrimaryGeometric Mean Titers Against RSV A Neutralizing Antibody in RSV-Naïve Participants

RSV A neutralizing antibody measured by microneutralization. RSV-naïve participants are defined as undetectable serum anti-RSV A IgA antibodies.

Time frame:: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84
Reported as:: Geometric mean · titer

Geometric Mean Titers Against RSV A Neutralizing Antibody in RSV-Naïve Participants
titer	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Day 56	75.9 (39.7 to 145)	32.8 (22.0 to 48.7)	180 (55.4 to 585)	27.5 (21.7 to 35.0)	99.2 (40.9 to 241)	17.9 (11.4 to 27.9)	83.7 (49.5 to 142)	79.4 (47.2 to 134)	20.6 (16.4 to 25.9)
Day 84	—	—	181 (65.2 to 500)	19.8 (12.1 to 32.1)	—	—	142 (86.4 to 232)	107 (70.0 to 163)	26.3 (18.8 to 37.0)

SecondaryTiter of Vaccine Virus Shedding Measured by Reverse Transcription Polymerase Chain Reaction (RT-PCR)

Shedding of the attenuated RSV vaccine strain in nasal swab samples was evaluated by RSV quantitative RT-PCR (qRT-PCR) assay, which specifically detected and quantified RSVt ΔNS2 vaccine strain (RSV ΔNS2/Δ1313/I1314L) in human nasal swab samples.

Time frame:: Cohorts 1 and 3: Day 7; Cohorts 2 and 4: Days 7 and 63
Reported as:: Mean · log10 copies/mL

Titer of Vaccine Virus Shedding Measured by Reverse Transcription Polymerase Chain Reaction (RT-PCR)
log10 copies/mL	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
After first vaccination	5.02 ± 1.03	—	6.27 ± 0.883	—	5.82 ± 0.666	—	5.16 ± 0.940	5.21 ± 0.985	—
After second vaccination	—	—	5.35 ± NA	—	—	—	5.10 ± 1.78	5.76 ± 1.21	—

SecondaryPercentage of Participants Infected With Vaccine Virus at Days 56 and 84

Infection is defined as detection of vaccine virus in nasal swab by RT-PCR and/or a \>= 4-fold rise in RSV A serum neutralizing antibody titers, or in RSV serum anti-F immunoglobulin G (IgG) antibody titers.

Time frame:: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84
Reported as:: Number · percentage of participants

Percentage of Participants Infected With Vaccine Virus at Days 56 and 84
percentage of participants	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
After first vaccination	82.4 (56.6 to 96.2)	12.5 (1.6 to 38.3)	100 (69.2 to 100)	0 (0 to 36.9)	100 (69.2 to 100)	0 (0 to 30.8)	63.3 (48.3 to 76.6)	58.3 (43.2 to 72.4)	4.0 (0.5 to 13.7)
After second vaccination	—	—	71.4 (29.0 to 96.3)	0 (0 to 41.0)	—	—	64.1 (47.2 to 78.8)	51.2 (35.1 to 67.1)	18.8 (8.9 to 32.6)

SecondaryGeometric Mean Titers Against RSV A Neutralizing Antibody in RSV-Experienced Participants

RSV A neutralizing antibody measured by microneutralization. RSV-experienced participants are defined as detectable serum anti-RSV A IgA antibodies. CI= confidence interval.

Time frame:: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84
Reported as:: Geometric mean · titer

Geometric Mean Titers Against RSV A Neutralizing Antibody in RSV-Experienced Participants
titer	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Day 56	134 (37.6 to 476)	52.1 (22.7 to 120)	672 (NA to NA)	—	409 (NA to NA)	15.0 (NA to NA)	120 (60.8 to 238)	96.0 (65.0 to 142)	73.5 (44.0 to 123)
Day 84	—	—	243 (NA to NA)	—	—	—	120 (65.1 to 221)	102 (70.5 to 148)	96.7 (44.9 to 208)

SecondaryGeometric Mean Titers Against Serum Anti-F Immunoglobulin G (IgG) Antibody

The IgG antibodies to RSV F antigen was measured using the anti RSV F IgG ELISA. RSV-naïve and RSV-experienced participants are defined as undetectable or detectable serum anti-RSV A IgA antibodies, respectively.

Time frame:: Cohorts 1 and 3: Day 56; Cohorts 2 and 4: Days 56 and 84
Reported as:: Geometric mean · titer

Geometric Mean Titers Against Serum Anti-F Immunoglobulin G (IgG) Antibody
titer	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
RSV-naïve participants: Day 56	32.8 (18.1 to 59.6)	15.0 (6.77 to 33.4)	58.8 (18.0 to 191)	8.63 (6.20 to 12.0)	48.7 (26.3 to 90.0)	7.50 (NA to NA)	43.2 (27.1 to 69.1)	45.6 (28.1 to 73.9)	10.4 (7.70 to 14.1)
RSV-naïve participants: Day 84	—	—	135 (55.5 to 331)	8.72 (6.11 to 12.4)	—	—	93.1 (49.0 to 177)	61.8 (35.1 to 109)	12.5 (8.30 to 18.7)
RSV-experienced participants: Day 56	205 (29.5 to 1421)	38.5 (7.60 to 195)	230 (NA to NA)	—	337 (NA to NA)	7.50 (NA to NA)	215 (93.8 to 491)	251 (139 to 455)	124 (50.7 to 304)
RSV-experienced participants: Day 84	—	—	165 (NA to NA)	—	—	—	250 (108 to 576)	287 (175 to 473)	221 (76.8 to 635)

SecondaryGeometric Mean Titers Against RSV A Neutralizing Antibody After the RSV Surveillance Season

RSV A neutralizing antibody measured by microneutralization. RSV-naïve and RSV-experienced participants are defined as undetectable or detectable serum anti-RSV A IgA antibodies, respectively.

Time frame:: Cohorts 1 and 3: Within 5 months after vaccination on Day 0; Cohorts 2 and 4: Within 5 months after vaccination on Day 56
Reported as:: Geometric mean · titer

Geometric Mean Titers Against RSV A Neutralizing Antibody After the RSV Surveillance Season
titer	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Geometric Mean Titers Against RSV A Neutralizing Antibody After the RSV Surveillance Season	73.2 (45.5 to 118)	24.4 (16.2 to 36.5)	303 (119 to 768)	16.6 (13.0 to 21.3)	294 (125 to 691)	108 (32.4 to 362)	112 (69.3 to 180)	123 (71.7 to 211)	56.6 (33.2 to 96.5)

SecondaryGeometric Mean Titers Against Serum Anti-F Immunoglobulin G Antibody After the RSV Surveillance Season

Time frame:: Cohorts 1 and 3: Within 5 months after vaccination on Day 0; Cohorts 2 and 4: Within 5 months after vaccination on Day 56
Reported as:: Geometric mean · titer

Geometric Mean Titers Against Serum Anti-F Immunoglobulin G Antibody After the RSV Surveillance Season
titer	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
Geometric Mean Titers Against Serum Anti-F Immunoglobulin G Antibody After the RSV Surveillance Season	63.7 (28.7 to 141)	14.2 (7.51 to 27.0)	388 (95.2 to 1581)	8.63 (6.12 to 12.2)	296 (92.6 to 948)	272 (35.8 to 2067)	222 (124 to 397)	300 (159 to 568)	94.4 (43.6 to 204)

Adverse events

Collected over From the first study vaccine administration (Day 0) up to end of the study, maximum of 12 months. Non-serious events are listed at a 5% frequency threshold.

Adverse event summary by group
Group	Deaths	Serious	Other
Cohort 1: RSV Low Dose	0/17 (0%)	0/17 (0%)	15/17 (88.2%)
Cohort 1: Placebo	0/18 (0%)	0/18 (0%)	17/18 (94.4%)
Cohort 2: RSV Low Dose	0/10 (0%)	1/10 (10%)	10/10 (100%)
Cohort 2: Placebo	0/10 (0%)	0/10 (0%)	9/10 (90%)
Cohort 3: RSV High Dose	0/10 (0%)	0/10 (0%)	9/10 (90%)
Cohort 3: Placebo	0/12 (0%)	1/12 (8.3%)	10/12 (83.3%)
Cohort 4: RSV Low Dose	0/61 (0%)	2/61 (3.3%)	55/61 (90.2%)
Cohort 4: RSV High Dose	0/57 (0%)	1/57 (1.8%)	49/57 (86%)
Cohort 4: Placebo	0/61 (0%)	2/61 (3.3%)	59/61 (96.7%)

Most frequent serious events

Most frequent serious events
Event	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
CelluliteSkin and subcutaneous tissue disorders	0/17	0/18	1/10	0/10	0/10	0/12	0/61	0/57	0/61
Respiratory Syncytial Virus InfectionInfections and infestations	0/17	0/18	0/10	0/10	0/10	1/12	0/61	0/57	0/61
Atypical PneumoniaInfections and infestations	0/17	0/18	0/10	0/10	0/10	0/12	0/61	1/57	0/61
DiarrhoeaGastrointestinal disorders	0/17	0/18	0/10	0/10	0/10	0/12	0/61	0/57	1/61
IntussusceptionGastrointestinal disorders	0/17	0/18	0/10	0/10	0/10	0/12	0/61	0/57	1/61
BronchiolitisInfections and infestations	0/17	0/18	0/10	0/10	0/10	0/12	1/61	0/57	0/61
Bronchial ObstructionRespiratory, thoracic and mediastinal disorders	0/17	0/18	0/10	0/10	0/10	0/12	1/61	0/57	0/61

Most frequent other events

Showing 10 of 27

Most frequent other events
Event	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo
RhinorrhoeaRespiratory, thoracic and mediastinal disorders	14/17	10/18	8/10	6/10	8/10	5/12	52/61	43/57	47/61
Nasal CongestionRespiratory, thoracic and mediastinal disorders	10/17	6/18	7/10	6/10	7/10	5/12	46/61	43/57	51/61
IrritabilityPsychiatric disorders	13/17	12/18	8/10	8/10	7/10	9/12	40/61	37/57	38/61
CryingGeneral disorders	9/17	8/18	7/10	6/10	7/10	3/12	31/61	28/57	29/61
Decreased AppetiteMetabolism and nutrition disorders	6/17	6/18	6/10	5/10	7/10	3/12	29/61	38/57	33/61
SomnolenceNervous system disorders	5/17	5/18	6/10	5/10	5/10	4/12	23/61	30/57	20/61
VomitingGastrointestinal disorders	5/17	3/18	3/10	4/10	2/10	1/12	22/61	23/57	28/61
PyrexiaGeneral disorders	1/17	5/18	1/10	1/10	1/10	2/12	21/61	19/57	22/61
NasopharyngitisInfections and infestations	1/17	0/18	1/10	3/10	0/10	0/12	19/61	20/57	18/61
Upper Respiratory Tract InfectionInfections and infestations	2/17	0/18	0/10	0/10	0/10	0/12	5/61	3/57	4/61

Baseline characteristics

Randomized participants included all participants allocated to a vaccine group.

Age, Continuous
Age, Continuous(months)	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo	Total
Mean	11.2 ± 4.51	11.8 ± 3.09	11.4 ± 4.01	10.8 ± 4.13	12.1 ± 3.11	12.8 ± 3.95	10.4 ± 3.17	10.6 ± 3.63	10.6 ± 3.78	10.9 ± 3.63

Sex: Female, Male
Sex: Female, Male(Participants)	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo	Total
Female	8	11	4	5	4	3	31	25	32	123
Male	10	7	7	5	6	9	30	33	29	136

Race/Ethnicity, Customized
Race/Ethnicity, Customized(Participants)	Cohort 1: RSV Low Dose	Cohort 1: Placebo	Cohort 2: RSV Low Dose	Cohort 2: Placebo	Cohort 3: RSV High Dose	Cohort 3: Placebo	Cohort 4: RSV Low Dose	Cohort 4: RSV High Dose	Cohort 4: Placebo	Total
American Indian or Alaska Native	0	0	0	0	0	0	10	11	9	30
Asian	0	0	0	0	0	0	0	0	0	0
Black or African American	1	2	0	0	0	2	1	3	1	10
Native Hawaiian or Other Pacific Islander	0	0	0	0	0	0	1	0	0	1
White	16	16	11	10	10	10	41	35	38	187
Mixed Origin	1	0	0	0	0	0	8	9	13	31
Not Reported	0	0	0	0	0	0	0	0	0	0
Unknown	0	0	0	0	0	0	0	0	0	0

Study locations

29 sites

Matrix Clinical Research-Site Number:8400012
Gardena, California 90247, United States
Paradigm Clinical Research-Site Number:8400026
La Mesa, California 91942, United States
Matrix Clinical Research-Site Number:8400032
Los Angeles, California 90057, United States
California Research Foundation-Site Number:8400016
San Diego, California 92123-1881, United States
Elite Clinical Trials, Inc.-Site Number:8400001
Blackfoot, Idaho 83221, United States
Leavitt Clinical Research-Site Number:8400036
Idaho Falls, Idaho 83404, United States
Snake River Research-Site Number:8400022
Idaho Falls, Idaho 83404, United States
The South Bend Clinic Center for Research-Site Number:8400024
South Bend, Indiana 46617, United States
Alliance for Multispeciality Research-Site Number:8400014
El Dorado, Kansas 67042, United States
AMR - Newton-Site Number:8400002
Newton, Kansas 67114, United States
Michael W. Simon, MD, PSC-Site Number:8400013
Lexington, Kentucky 40517, United States
Benchmark Research-Site Number:8400006
Covington, Louisiana 70433, United States
Nola Research Works-Site Number:8400017
New Orleans, Louisiana 70125, United States
Clinical Research Institute-Site Number:8400053
Minneapolis, Minnesota 55402, United States
Boeson Research-Site Number:8400011
Missoula, Montana 59804, United States
Be Well Clinical Studies-Site Number:8400054
Lincoln, Nebraska 68516, United States
Meridian Clinical Research - Norfolk-Site Number:8400005
Norfolk, Nebraska 68701, United States
MedPharmics Inc-Site Number:8400040
Albuquerque, New Mexico 87102, United States
East Carolina University/Brody Medical Sciences Building-Site Number:8400043
Greenville, North Carolina 27834, United States
Coastal Pediatric Research-Site Number:8400031
Charleston, South Carolina 29414, United States
Tribe Clinical Research-Site Number:8400027
Greenville, South Carolina 29607, United States
FMC Science-Site Number:8400042
Lampasas, Texas 76550-1820, United States
JBR Clinical Research-Site Number:8400041
Salt Lake City, Utah 84107, United States
Pediatric Associates of Charlottesville North-Site Number:8400007
Charlottesville, Virginia 22911, United States
National Clinical Research Inc-Site Number:8400004
Richmond, Virginia 23294, United States
Investigational Site Number :1520001
Santiago, Reg Metropolitana de Santiago 8380453, Chile
Investigational Site Number :1520004
Santiago, Reg Metropolitana de Santiago 8420383, Chile
Investigational Site Number :3400001
San Pedro Sula, 21104, Honduras
Investigational Site Number :3400002
Tegucigalpa, 11101, Honduras

References and documents

Publications

Idoko OT, Vargas SL, Bueso A, Rivera D, Edward H, Simon M, Banooni P, Berger S, Janicot S, Vercasson C, Pallardy S, Nteene R, Adhikarla H, Gerchman E, Gasparotto M, Gallichan S, Rivas E, Buchholz UJ, Collins PL, Sesay S, Gurunathan S, De Bruijn I, Dhingra MS. Live-Attenuated Intranasal RSV Vaccine in Infants and Toddlers. NEJM Evid. 2025 Sep;4(9):EVIDoa2500026. doi: 10.1056/EVIDoa2500026. Epub 2025 Aug 26. PubMed 40856556 ↗

Study documents

Study protocol · Jan 19, 2023
Statistical analysis plan · Mar 30, 2023

Documents are hosted by the registry — open the source record to download them.

Individual participant data

Plan to share: Yes — Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org

Updates

Tracking since Sep 25, 2026

No changes since tracking began. The registry record was last updated on Sep 9, 2025, before this site started recording changes on Sep 25, 2026. Its history is on ClinicalTrials.gov ↗

Registry details

Key details

Study ID: NCT04491877
Lead sponsor: Sanofi Pasteur, a Sanofi Company
Responsible party: Sponsor
First posted: Jul 29, 2020
Start date: Sep 17, 2020
Primary completion: Apr 13, 2023
Completion: Apr 13, 2023
Results posted: Jun 13, 2024
Last update: Sep 9, 2025

Study contacts

Clinical Sciences & Operations

study director · Sanofi Pasteur, a Sanofi Company

Oversight

Data monitoring committee: No
FDA-regulated drug: Yes
FDA-regulated device: No

View the source record on ClinicalTrials.gov ↗

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