Local reactions included pain at injection site, redness and swelling and were recorded by participants in an electronic diary (e-diary). Redness and swelling were measured and recorded in measuring device units. 1 measuring device unit =0.5 centimeter (cm) and graded as mild: greater than (>) 2.0 to 5.0 cm, moderate: >5.0 to 10.0 cm and severe: >10.0 cm. Pain at injection site was graded as mild: did not interfere with daily activity, moderate: interfered with daily activity and severe: prevented daily activity. The maximum severity was defined as highest grading of each local reaction within 7 days of vaccination.

Maternal safety population included all randomized maternal participants who received investigational product and were analyzed according to the investigational product they received. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure.

Systemic events included fever, fatigue, headache, nausea, muscle pain, joint pain, vomiting, diarrhea and were recorded by participants in an e-diary. Fever was categorized as: grade 1: mild (>=38.0 to 38.4 degrees [deg] Celsius [C]), grade 2: moderate (>38.4 to 38.9 deg C), grade 3: severe (>38.9 to 40.0 deg C) and grade 4 (>40.0 deg C). Fatigue, headache, nausea, muscle pain and joint pain were graded as mild: did not interfere with activity, moderate: some interference with activity and severe: prevented daily routine activity. Vomiting was graded as mild: 1 to 2 times in 24 hours(h), moderate: >2 times in 24h and severe: required intravenous hydration. Diarrhea was graded as mild: 2 to 3 loose stools in 24h, moderate: 4 to 5 loose stools in 24h and severe: 6 or more loose stools in 24h. The maximum severity was defined as highest grading of each systemic event within 7 days of vaccination.

Maternal safety population included all randomized maternal participants who received investigational product and were analyzed according to the investigational product they received. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure.

An AE was any untoward medical occurrence in a participant who received investigational product without regard to possibility of causal relationship. AEs included both serious and non-serious adverse events. Serious adverse event (SAE) was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly/birth defect.

Maternal safety population included all randomized maternal participants who received investigational product and were analyzed according to the investigational product they received.

MAE was defined as a non-serious AE that results in an evaluation at a medical facility. SAE was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly/birth defect. Obstetric complications were determined as per the study clinician's judgement.

Maternal safety population included all randomized maternal participants who received investigational product and were analyzed according to the investigational product they received.

Specific birth complications included clavicle fracture, torticollis, cephalhematoma, premature baby, acute respiratory failure, meconium aspiration syndrome, neonatal pneumothorax, neonatal respiratory depression, neonatal respiratory distress, neonatal respiratory distress syndrome, neonatal respiratory failure, pneumothorax, respiratory distress, transient tachypnea of the newborn and subgaleal hemorrhage. Percentage of participants with any specific birth complications were reported in this outcome measure.

Infant safety population included all infant participants who were born to vaccinated maternal participants and were analyzed according to the investigational product their mothers received.

An AE was any untoward medical occurrence in a participant who received investigational product without regard to possibility of causal relationship. AEs included both serious and non-serious adverse events. SAE was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly/birth defect.

Infant safety population included all infant participants who were born to vaccinated maternal participants and were analyzed according to the investigational product their mothers received.

MAE was defined as a non-serious AE that results in an evaluation at a medical facility. SAE was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly/birth defect.

Infant safety population included all infant participants who were born to vaccinated maternal participants and were analyzed according to the investigational product their mothers received.

AEs of special interest for infant participants included congenital anomalies and developmental delays. Congenital anomalies were defined as structural or functional anomalies that occurred during intrauterine life and could be identified prenatally, at birth or later in life. Severity was assessed based on the study investigator's judgement and graded as grade 1= mild (does not interfere with participant's usual function); grade 2= moderate (interferes to some extent with participant's usual function); grade 3= severe (interferes significantly with participant's usual function) and grade 4= life-threatening (life-threatening consequences; urgent intervention indicated). Percentage of infant participants with AEs of special interest of at least moderate severity were presented.

Infant safety population included all infant participants who were born to vaccinated maternal participants and were analyzed according to the investigational product their mothers received.

GMT of the 50% RSV A and RSV B neutralizing antibody were calculated by exponentiating the mean logarithm of the titers and the corresponding 95% confidence interval (CI) was based on the Student t distribution. Geometric mean ratios (GMRs) of the RSV vaccine group to the placebo group for the RSV A and RSV B neutralizing antibody titers at each time point was calculated and reported in statistical analysis.

Maternal evaluable immunogenicity population: eligible maternal participants who received randomized vaccine, had blood collection within protocol-specified time points, had at least 1 valid and determinate assay result after vaccination and had no other major protocol deviations. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure and "Number Analyzed" signifies participants evaluable for this outcome measure at specified time points.

GMFRs for RSV A and RSV B neutralizing antibody titers from before vaccination to each available time point after vaccination were calculated by exponentiating the mean logarithm of the fold rises. Corresponding 95% CI was based on the Student t distribution. Data for this outcome measure was planned to be analyzed for maternal participants only.

Maternal evaluable immunogenicity population: eligible maternal participants who received randomized vaccine, had blood collection within protocol-specified time points, had at least 1 valid and determinate assay result after vaccination and had no other major protocol deviations. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure and "Number Analyzed" signifies participants evaluable for this outcome measure at specified time points.

GMT of the 50% RSV A and RSV B neutralizing antibody were calculated by exponentiating the mean logarithm of the titers and the corresponding 95% CI was based on the Student t distribution. GMRs of the RSV vaccine group to the placebo group for the RSV A and RSV B neutralizing antibody titers at each time point was calculated and reported in statistical analysis.

Infant evaluable immunogenicity population: eligible infant participants born to maternal participants who received randomized vaccine and had no protocol deviation before delivery, had blood collection within protocol-specified time point at birth, had at least 1 valid, determinate assay result at birth and had no other major protocol deviations. Here, 'Number Analyzed' signifies participants evaluable for this outcome measure at specified time points.