RSV related hospitalization included a specialized acute medical care unit within an assisted living facility or nursing home.

Modified Intent-to-Treat-Infected (mITT-infected) population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

Participants with RSV related visits in a hospital/urgent care or ED requiring no minimum duration of hospitalization were reported in this outcome measure. Investigators determined if a medical visit was related to RSV. RSV-related medical visits were those attendances that would not otherwise occur in the absence of the RSV infection. These may have included: deterioration or decompensation of the lung function that required supplemental oxygen; development of secondary respiratory tract infections that required antibiotic treatment; management of severe symptoms associated with RSV such as fever; worsening or decompensation of cardiac or renal function in participants with underlying cardiac or renal disease.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

Progression of LRTI was defined as development of LRTI or transition from non-severe LRTI-RSV at randomization to severe LRTI-RSV at any time up to and including Day 10. LRTI was defined as: >= 2 lower respiratory signs or symptoms for at least 24 hours including at least 1 lower respiratory sign; or 3 lower respiratory symptoms for at least 24 hours. Lower respiratory symptoms included following: new or increased sputum, new or increased cough, new or increased dyspnea (shortness of breath). Lower respiratory signs included: new or increased wheezing, new or increased crackles/ronchi based on chest auscultation, respiratory rate >= 20 respirations/minute, low or decreased oxygen saturation (O2 less than (<) 95 percent (%) or <= 90% if pre-season baseline is < 95%), need for new or increased oxygen supplementation.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection. Here "Overall Number of Participants Analyzed" signifies the number of participants evaluable for this outcome measure.

Development of LRTI was defined as transitioning from not having LRTI at randomization but having nsLRTI-RSV or sLRTI-RSV at any time up to and including Day 10. LRTI was defined as: >= 2 lower respiratory signs or symptoms for at least 24 hours including at least 1 lower respiratory sign; or 3 lower respiratory symptoms for at least 24 hours. Lower respiratory symptoms included following: new or increased sputum, new or increased cough, new or increased dyspnea (shortness of breath). Lower respiratory signs included: new or increased wheezing, new or increased crackles/ronchi based on chest auscultation, respiratory rate >= 20 respirations/minute, low or decreased oxygen saturation (O2 < 95 % or <= 90% if pre-season baseline is < 95%), need for new or increased oxygen supplementation.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection. Here "Overall Number of Participants Analyzed" signifies the number of participants evaluable for this outcome measure.

Resolution of LRTI was defined as transition from RSV-related non-severe LRTI (nsLRTI-RSV) or RSV-related severe LRTI (sLRTI-RSV) at randomization to not having nsLRTI and sLRTI-RSV. LRTI was defined as: >= 2 lower respiratory signs or symptoms for at least 24 hours including at least 1 lower respiratory sign; or 3 lower respiratory symptoms for at least 24 hours. Lower respiratory symptoms included following: new or increased sputum, new or increased cough, new or increased dyspnea (shortness of breath). Lower respiratory signs included: new or increased wheezing, new or increased crackles/ronchi based on chest auscultation, respiratory rate >= 20 respirations/minute, low or decreased oxygen saturation (O2 < 95 % or <= 90% if pre-season baseline is < 95%), need for new or increased oxygen supplementation.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection. Here "Overall Number of Participants Analyzed" signifies the number of participants evaluable for this outcome measure.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

Progression of LRTI was defined as development of LRTI or transition from non severe LRTI-RSV at randomization to severe LRTI-RSV at any time up to and including Days 3, 5, 15 and 28. LRTI was defined as: >= 2 lower respiratory signs or symptoms for at least 24 hours including at least 1 lower respiratory sign; or 3 lower respiratory symptoms for at least 24 hours. Lower respiratory symptoms included following: new or increased sputum, new or increased cough, new or increased dyspnea (shortness of breath). Lower respiratory signs included: new or increased wheezing, new or increased crackles/ronchi based on chest auscultation, respiratory rate >= 20 respirations/minute, low or decreased oxygen saturation (O2 < 95 % or <= 90% if pre-season baseline is < 95%), need for new or increased oxygen supplementation.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection. Here "Overall Number of Participants Analyzed" signifies the number of participants evaluable for this outcome measure.

Development of LRTI was defined as transitioning from not having LRTI at randomization but having nsLRTI-RSV or sLRTI-RSV at any time up to and including Days 3, 5, 15 and 28. LRTI was defined as: >= 2 lower respiratory signs or symptoms for at least 24 hours including at least 1 lower respiratory sign; or 3 lower respiratory symptoms for at least 24 hours. Lower respiratory symptoms included following: new or increased sputum, new or increased cough, new or increased dyspnea (shortness of breath). Lower respiratory signs included: new or increased wheezing, new or increased crackles/ronchi based on chest auscultation, respiratory rate >= 20 respirations/minute, low or decreased oxygen saturation (O2 < 95 % or <= 90% if pre-season baseline is < 95%), need for new or increased oxygen supplementation.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection. Here "Overall Number of Participants Analyzed" signifies the number of participants evaluable for this outcome measure.

Resolution of LRTI was defined as transition from nsLRTI-RSV or sLRTI-RSV at randomization to not having nsLRTI and sLRTI-RSV. LRTI was defined as >= 2 lower respiratory signs or symptoms for at least 24 hours including at least 1 lower respiratory sign; or 3 lower respiratory symptoms for at least 24 hours. Lower respiratory symptoms included following: new or increased sputum, new or increased cough, new or increased dyspnea (shortness of breath). Lower respiratory signs included: new or increased wheezing, new or increased crackles/ronchi based on chest auscultation, respiratory rate >= 20 respirations/minute, low or decreased oxygen saturation (O2 < 95% or <= 90% if pre-season baseline is < 95%), need for new or increased oxygen supplementation.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection. Here, "Overall Number of Participants Analyzed" signifies the number of participants evaluable for this outcome measure.

Improvement in LRTI status was defined as LRTI resolution or transition from sLRTI-RSV at randomization to nsLRTI-RSV. LRTI was defined as: >= 2 lower respiratory signs or symptoms for at least 24 hours including at least 1 lower respiratory sign; or 3 lower respiratory symptoms for at least 24 hours. Lower respiratory symptoms included following: new or increased sputum, new or increased cough, new or increased dyspnea (shortness of breath). Lower respiratory signs included: new or increased wheezing, new or increased crackles/ronchi based on chest auscultation, respiratory rate >= 20 respirations/minute, low or decreased oxygen saturation (O2 < 95 % or <= 90% if pre-season baseline is < 95%), need for new or increased oxygen supplementation.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection. Here "Overall Number of Participants Analyzed" signifies the number of participants evaluable for this outcome measure.

RSV related hospitalization included a specialized acute medical care unit within an assisted living facility or nursing home.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

Improvement was defined as no new acute respiratory infection (ARI) signs or symptoms, and no worsening of existing signs or symptoms compared to the Day 1 visit. At least one sign or symptom (but not all) present at Day 1 was absent, improved or returned to pre-infection status. Resolution was defined as all ARI signs or symptoms were absent or returned to pre-infection status. Clinical response was evaluated by the investigator.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

Undetectable RSV viral load at a visit was defined as a central PCR laboratory result of target not detected (TND).

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

Undetectable viral load was considered to be 0 copies/mL for this analysis.

mITT-infected population included all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, and who had confirmed RSV infection.

An AE was 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. An adverse event was considered a TEAE if the event started on or after the study intervention start date (Day 1).

Safety analysis set (SAS) included all participants randomly assigned to study intervention and who took at least 1 dose of study intervention

An SAE was defined as any untoward medical occurrence that, at any dose, met one or more of the criteria listed below: 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, suspected transmission via a Pfizer product of an infectious agent, pathogenic or nonpathogenic and other important medical event.

SAS included all participants randomly assigned to study intervention and who took at least 1 dose of study intervention.

Pharmacokinetic (PK) concentration analysis set included all participants who received at least 1 dose of sisunatovir and in whom at least 1 concentration value was reported. Here, "Number Analyzed" signifies participants evaluable for the specified rows.