Local reactions included redness, swelling and pain at injection site. Redness and swelling were measured and recorded in measuring device units. 1 measuring device unit=0.5 centimeter (cm). Redness and swelling were graded as mild: \> 2.0 cm to 5.0 cm, moderate: \> 5.0 cm to 10.0 cm, severe: \>10.0 cm. Pain at injection site was graded as mild: did not interfere with activity, moderate: interfered with activity, severe: prevented daily activity. Any local reaction: any redness, any swelling, or pain at the injection site of at least mild severity. Exact 2-sided 95% confidence interval (CI) was based on the Clopper and Pearson method.

Systemic events included fever, fatigue, headache, vomiting, nausea, diarrhea, muscle pain and joint pain. Fever defined as oral temperature \>=38.0 degrees Celsius (deg C) and categorized as mild: \>=38.0 to 38.4 deg C, moderate: \>38.4 to 38.9 deg C, severe: \>38.9 to 40.0 deg C, and Grade 4: \>40.0 deg C. Vomiting categorized as mild: 1-2 times in 24 hours (h); moderate: \>2 times in 24h; severe: required intravenous (IV) hydration. Diarrhea categorized as mild: 2-3 loose stools in 24h; moderate: 4-5 loose stools in 24h; severe: 6 or more loose stools in 24h. Headache, fatigue, nausea, muscle pain and joint pain were categorized as mild: didn't interfere with activity; moderate: some interference with activity; severe: prevented daily routine activity. Exact 2-sided 95% CI was based on the Clopper and Pearson method.

An AE was any untoward medical occurrence in a participant temporally associated with the use of study intervention, whether or not considered related to the study intervention. Exact 2-sided 95% CI was based on the Clopper and Pearson method. Only AEs collected by non-systematic assessment (i.e. excluding local reactions and systemic events) were reported in this outcome measure.

An SAE was defined as an AE that, at any dose met one of the following criteria: resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant disability/incapacity; was a congenital anomaly/birth defect; was a suspected transmission via a Pfizer product of an infectious agent, pathogenic or nonpathogenic; other significant medical events as judged by investigator. Exact 2-sided 95% CI was based on the Clopper and Pearson method.

A NDCMC was defined as a disease or medical condition that was not identified prior to study start and was expected to be persistent or otherwise long-lasting in its effects. Newly diagnosed chronic medical condition did not include illnesses considered to be temporary conditions. Exact 2-sided 95% CI was based on the Clopper and Pearson method.

GMTs and the corresponding 2-sided 95% CIs were calculated by exponentiating the mean logarithm of the titers and the corresponding CIs (based on the Student t distribution). Assay results below the lower limit of quantification (LLOQ) were set to 0.5\*LLOQ for analysis.

GMTs and the corresponding 2-sided 95% CIs were calculated by exponentiating the mean logarithm of the titers and the corresponding CIs (based on the Student t distribution). Assay results below the LLOQ were set to 0.5\*LLOQ for analysis.

Fold rises was defined as ratios of the results after vaccination to the results before vaccination. GMFRs and 2-sided 95% CIs were calculated by exponentiating the mean logarithm of fold rises and the corresponding CIs (based on the Student t distribution).

Systemic Reactions: fever, fatigue, headache, nausea, muscle pain, joint pain, vomiting, diarrhea and any systemic event recorded based on e-diary and participant reported reactogenicity events.

Local reactions included pain at injection site, redness and swelling recorded based on e-diary and participant reported reactogenicity events.

An adverse event was any untoward medical occurrence in a participant who received investigational product without regard to possibility of causal relationship. Adverse events included both serious and non-serious adverse events.

AESIs include preterm delivery, diagnosis of GB syndrome, Diagnosis of Acute polyneuropathy without underlying etiology, hypertensive disorders of pregnancy, atrial fibrillation.

SAE is 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.

Describe specific birth outcomes for infant participants

An AE is any untoward medical occurrence in a participant who received investigational product without regard to possibility of causal relationship. AEs include both serious and non-serious adverse events.

SAE is 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. NDCMC is defined as a disease or medical condition, not previously identified, that is expected to be persistent or otherwise long-lasting in its effects.

Local reactions were collected at the RSVpreF or placebo injection site after Vaccination 1 and Vaccination 2 and were recorded by participants using electronic diary (e-diary). Local reactions included redness, swelling and pain at injection site. Redness and swelling were measured and recorded in measuring device units where, 1 measuring device unit =0.5 centimeter (cm) and were 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 activity), moderate (interfered with activity), and severe (prevented daily activity). Percentage of participants reporting local reactions at injection site in Coadministration Group, and Sequential-Administration Group and associated 2-sided 95% confidence interval (CI) based on Clopper and Pearson method was presented in this outcome measure (OM). Safety population=all enrolled participants who received study intervention (RSVpreF, placebo).

Systemic events included fever, fatigue, headache, nausea, vomiting, diarrhea, muscle pain and joint pain and were recorded by participants using an e-diary. Fever was defined as an oral temperature \>=38.0 degree Celsius (deg C) and categorized as \>=38.0 to 38.4 deg C (mild), \>38.4 to 38.9 deg C (moderate), \>38.9 to 40.0 deg C (severe) and \>40.0 deg C (grade 4). 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 mild: 1 to 2 times in 24 hours (h), moderate: \>2 times in 24h, and severe: required intravenous hydration. Diarrhea was graded 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. Percentage of participants with systemic events within 7 days after each vaccination and the associated 2-sided 95% CI based on the Clopper and Pearson method was presented.

An AE was defined as any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Percentage of participants reporting AEs within 1 month after Vaccination 1 were reported in this outcome measure. Exact 2-sided CI was calculated using the Clopper and Pearson method. Only AEs collected by non-systematic assessment (i.e., excluding local reactions and systemic events) were reported in this outcome measure.

An AE was defined as any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Percentage of participants reporting AEs within 1 month after Vaccination 2 were reported in this outcome measure. Exact 2-sided CI was calculated using the Clopper and Pearson method. Only AEs collected by non-systematic assessment (i.e. excluding local reactions and systemic events) were reported in this outcome measure.

An SAE was defined as an AE that, at any dose: resulted in death; was life-threatening; required inpatient hospitalization or prolongation of existing hospitalization; resulted in persistent disability/incapacity; was a congenital anomaly/birth defect; was a suspected transmission via a Pfizer product of an infectious agent, pathogenic or nonpathogenic or that was considered to be an important medical event. Percentage of participants with SAEs and the associated 2-sided 95% CI based on the Clopper and Pearson method was presented. Percentage of participants reporting SAEs within 1 month after Vaccination 1 were reported in this outcome measure.

An SAE was defined as an AE that, at any dose: resulted in death; was life-threatening; required inpatient hospitalization or prolongation of existing hospitalization; resulted in persistent disability/incapacity; was a congenital anomaly/birth defect; was a suspected transmission via a Pfizer product of an infectious agent, pathogenic or nonpathogenic or that was considered to be an important medical event. Percentage of participants with SAEs and the associated 2-sided 95% CI based on the Clopper and Pearson method was presented. Percentage of participants reporting SAEs within 1 month after Vaccination 2 were reported in this outcome measure.

Geometric mean titer (GMT) of RSV A and RSV B neutralizing titers for the Coadministration Group and Sequential-Administration Group were reported in this outcome measure in descriptive data section. GMTs and the 2-sided 95% CI were calculated by exponentiating the mean logarithm of the titers and the corresponding CIs (based on Student's t distribution). Assay results below the lower limit of quantification (LLOQ) were set to 0.5\*LLOQ. Geometric mean ratio (GMR) was reported in the statistical analysis section and was calculated as ratio of GMTs in the Coadministration Group to the Sequential-Administration Group.

GMTs of strain-specific HAI titers for the Coadministration Group and Sequential-Administration Group were reported in this outcome measure in descriptive data section. GMTs and the 2-sided 95% CIs were calculated by exponentiating the mean logarithm of the titers and the corresponding CIs (based on Student's t distribution). Assay results below the LLOQ were set to 0.5\*LLOQ. GMR was reported in the statistical analysis section and was calculated as ratio of GMT in the Coadministration Group to the Sequential-Administration Group.