A Randomized, Double-Blind, Placebo-Controlled, Parallel-Group Study to Evaluate the Efficacy and Safety of VX-661 in Combination With Ivacaftor

Relative Change From Baseline (Day 1) in ppFEV1 Through Week 24

FEV1 is the volume of air that can forcibly be blown out in one second, after full inspiration.

Time frame: Day 1, Through Week 24

Number of Pulmonary Exacerbations Per Year

Pulmonary exacerbation was defined as a new event or change in antibiotic therapy for greater than or equal to 4 sinopulmonary signs/symptoms. Pulmonary exacerbation events per year (48 weeks) were reported.

Time frame: Day 1 through Week 24

Absolute Change From Baseline (Day 1) Body Mass Index (BMI) at Week 24

BMI was defined as weight in kilograms (kg) divided by height in square meter (m\^2).

Time frame: Day 1, Week 24

Absolute Change From Baseline (Day 1) in Cystic Fibrosis Questionnaire-Revised (CFQ-R) Respiratory Domain Score Through Week 24

The CFQ-R is a validated participant-reported outcome measuring health-related quality of life for participants with cystic fibrosis. Respiratory domain assessed respiratory symptoms, score range: 0-100; higher scores indicating fewer symptoms and better health-related quality of life.

Time frame: Day 1, Through Week 24

Number of Participants With Treatment-Emergent Adverse Events (AEs) and Serious Adverse Events (SAEs)

AE: any untoward medical occurrence in a participant during the study; the event does not necessarily have a causal relationship with the treatment. This includes any newly occurring event or previous condition that has increased in severity or frequency after the informed consent form is signed. AE includes serious as well as non-serious AEs. SAE (subset of AE): medical event or condition, which falls into any of the following categories, regardless of its relationship to the study drug: death, life threatening adverse experience, inpatient hospitalization/prolongation of hospitalization, persistent/significant disability or incapacity, congenital anomaly/birth defect, important medical event. Any AE that increased in severity or newly developed at or after initial dosing of study drug to Week 28 was considered treatment-emergent.