Assessed solicited administration site events were pain, erythema and swelling. Any = occurrence of the symptom regardless of intensity grade. Any erythema and swelling symptom = symptom reported with a surface diameter greater than 0 millimeters.

Assessed solicited systemic events were fatigue, headache, nausea, vomiting, diarrhea, abdominal pain and fever \[temperature equal to or above (≥) 38 degrees Celsius (°C)\]. Any = occurrence of the symptom regardless of intensity grade or relation to study intervention.

Hematological parameters assessed were Eosinophils (EOS), Erythrocytes (ERY), Hematocrit (HEM), Lymphocytes (LYMP), Mean Corpuscular Volume (MCV), Neutrophils (NEU), Platelets (PLA), and White Blood Cells (WBC) count. The increase and/or decrease of these parameters were evaluated at Day 8. Abnormal laboratory values refer to range indicator at Day 8 (D8) categorized as Missing, Below, Within and Above normal values and compared to the baseline (B) range indicator of the same parameter, at Screening (up to 15 days before vaccination) i.e. Missing, Below, Within and Above. E.g. 'WBC decrease Below (B) - Within (D8)' = WBC decrease in subjects with below normal values at baseline and within normal values at Day 8.

Biochemical parameters assessed were Alanine Amino-Transferase (ALT), Aspartate Amino-Transferase (AST), Creatinine (CRE) and Urea nitrogen (URN). The increase was evaluated only for AST and ALT parameters at Day 8. Abnormal laboratory values refer to range indicator at Day 8 (D8) categorized as Missing, Below, Within and Above normal values and compared to the baseline (B) range indicator of the same parameter, at Screening (up to 15 days before vaccination) i.e. Missing, Below, Within and Above. E.g. 'AST increase Below (B) - Within (D8)' = AST increase in subjects with below normal values at baseline and within normal values at Day 8.

An unsolicited AE covers any untoward medical occurrence in a clinical investigation subject temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. Unsolicited AE is any AE reported in addition to those solicited during the clinical study and that was spontaneously communicated by a maternal subject. Also, any solicited symptom with onset outside the specified period of follow-up for solicited symptoms is to be reported as an unsolicited AE. Any was defined as the occurrence of any unsolicited AE regardless of intensity grade or relation to vaccination.

SAEs assessed included any untoward medical occurrences that resulted in death, were life-threatening, required hospitalization or prolongation of hospitalization or resulted in disability/incapacity, was a congenital anomaly/birth defect in the offspring of a study subject or abnormal pregnancy outcomes (spontaneous abortion, foetal death, stillbirth, congenital anomalies, ectopic pregnancy), other situations (medical events that might jeopardize the participant or required medical/surgical intervention to prevent one of the other SAEs listed above: e.g. invasive/malignant cancers, intensive treatment in an emergency room or at home for allergic bronchospasm, blood dyscrasias or convulsions that did not result in hospitalization). Any = occurrence of the symptom regardless of intensity grade or relationship to vaccination.

An AE is any untoward medical occurrence in a patient or clinical investigation subject, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. AEs leading to study withdrawal = AEs identified by investigators to cause subject(s) withdrawal until the resolution of the event. These subject withdrawals were considered different from subject withdrawals for other reasons.

MAEs were defined as adverse events with medically-attended visits that were not routine visits for physical examination or vaccination, such as visits for hospitalization, an emergency room visit, or an otherwise unscheduled visit to or from medical personnel (medical doctor) for any reason. Also, for instances where, due to the special circumstances, the subject could not seek medical advice for symptoms/an illness by visiting a medical facility or arranging for a home visit, the subject sought this advice instead via telephone, SMS, email, videotelephony or telemedicine, or other means. Any = occurrence of the symptom regardless of intensity grade or relationship to vaccination.

Pregnancy outcomes were: live birth with no congenital anomalies, live birth with congenital anomalies, Fetal death/still birth with no Congenital Anomalies (CA) - Antepartum and Unknown (Subjects withdrew from the study before delivery and pregnancy outcome information was not available for them).

Pregnancy-related AESIs were: Non-Reassuring Fetal Status, Hypertensive Disorders of Pregnancy (HDP), Oligohydramnios, Pathways to Preterm Birth (PPB), Chorioamnionitis, Fetal Growth Restriction, Gestational Liver Disease (GLD), Postpartum Haemorrhage and Gestational Diabetes Mellitus.

Neonatal AESIs, reported up to 6 weeks after birth were: Respiratory Distress In The Neonate, Macrosomia, Low Birth Weight, Small For Gestational Age, Preterm Birth, Large For Gestational Age, Neonatal Invasive Blood Stream Infections (NIBSI) and Congenital Anomalies (CA).

SAEs assessed included any untoward medical occurrences that resulted in death, were life-threatening, required hospitalization or prolongation of hospitalization or resulted in disability/incapacity or is a congenital anomaly/birth defect, other situations (medical events that might jeopardize the participant or required medical/surgical intervention to prevent one of the other SAEs listed above: e.g. invasive/malignant cancers, intensive treatment in an emergency room or at home for allergic bronchospasm, blood dyscrasias or convulsions that did not result in hospitalization). Any = occurrence of the symptom regardless of intensity grade or relationship to vaccination.

An AE is any untoward medical occurrence in a patient or clinical investigation subject, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. AEs leading to study withdrawal = AEs identified by investigators to cause subject(s) withdrawal until the resolution of the event. These subject withdrawals were considered different from subject withdrawals for other reasons.

MAEs were defined as adverse events with medically-attended visits that were not routine visits for physical examination or vaccination, such as visits for hospitalization, an emergency room visit, or an otherwise unscheduled visit to or from medical personnel (medical doctor) for any reason. Also, for instances where, due to the special circumstances, the subject could not seek medical advice for symptoms/an illness by visiting a medical facility or arranging for a home visit, the subject sought this advice instead via telephone, SMS, email, videotelephony or telemedicine, or other means. Any = occurrence of the symptom regardless of intensity grade.

Serological assays for the determination of IgG antibodies against RSV MAT were performed by Enzyme-linked immunosorbent assay (ELISA). The corresponding antibody concentrations were expressed in ELISA units per milliliter (EU/mL) and were measured on blood samples collected from vaccinated maternal subjects.

Serological assays for the determination of antibodies against RSV-A were performed by neutralization assay. The corresponding antibody titers were expressed in Estimated Dilution 60 (ED60) and were measured on blood samples collected from vaccinated maternal subjects.

Serological assays for the determination of IgG antibodies against RSV MAT were performed by ELISA. The corresponding antibody concentrations were expressed in EU/mL. The antibodies were measured on the cord blood sample collected at delivery, or on a blood sample collected from the infant within 3 days after birth (if no cord blood sample could be obtained).

Serological assays for the determination of antibodies against RSV-A were performed by neutralization assay. The corresponding antibody titers were presented as GMTs, expressed in ED60. The antibodies were measured on the cord blood sample collected at delivery, or on a blood sample collected from the infant within 3 days after birth (if no cord blood sample could be obtained).

The placental transfer ratio of IgG specific antibody concentration was determined from cord blood (or blood sample collected within 3 days after birth from infants if cord blood was not collected) over that of the blood sample from mother at delivery if blood sample was not collected during delivery). Serological assays for the determination of IgG antibodies against RSV MAT were performed by ELISA.