The primary objective of this study is to evaluate the efficacy of BG00011 compared with placebo in participants with Idiopathic Pulmonary Fibrosis (IPF). The secondary objectives of this study are: to evaluate the efficacy of BG00011 compared with placebo in participants with IPF as determined by change in percent predicted forced (expiratory) vital capacity (FVC); to assess progression-free survival in participants who receive BG00011 compared with placebo; to assess the occurrence of IPF exacerbation in participants who receive BG00011 compared with placebo; to assess the incidence of absolute decline in FVC ≥10% in participants who receive BG00011 compared with placebo; to assess the time to death or lung transplantation in participants who receive BG00011 compared with placebo, and the transplant-free survival rate at Week 26 and Week 52; to assess the time to non-elective hospitalizations in participants who receive BG00011 compared with placebo; to assess additional pulmonary function test (PFT) findings in participants who receive BG00011 compared with placebo; To assess performance on the 6 minute walk test (6MWT) in participants who receive BG00011 compared with placebo; to evaluate the safety and tolerability of BG00011; and to evaluate the serum concentration of BG00011.

Study was stopped because of safety concerns. End-of-study and safety follow-up visits are in progress.

Defined by a composite endpoint, including any of the following events: Absolute decline of 10% predicted in FVC (FVC percent predicted (baseline) - FVC percent predicted (progression) ≥10%); Non-elective hospitalization for respiratory events; Lung transplantation or death.

Measured in Days.

Specific criteria for acute exacerbation of Idiopathic Pulmonary Fibrosis (IPF) will be determined by the Investigator.

Measured in Days.

The FVC is the volume of air, measured in milliliters (mL), which can be forcibly exhaled from the lungs after taking the deepest breath possible.

Absolute Decline of 10% = FVC percent predicted (baseline) - FVC percent predicted (progression) ≥10%

Key Inclusion Criteria:

• Female subjects must be surgically sterile, postmenopausal (minimum 1 year without menses), or agree to use 1 or more forms of highly effective contraception from the time of signing of the informed consent form (ICF) until 3 months after the last injection of study medication. Male subjects must also agree to use 1 or more forms of highly effective contraception for either themselves or their partners from signing of ICF until 4 months after last injection of study medication.
• IPF diagnosed based on modified ATS/ERS/JRS/ALAT IPF guideline for diagnosis and management, within 3 years of Screening.
• Combination of high-resolution computed tomography (HRCT) pattern and, if one has been obtained, surgical lung biopsy pattern, consistent with diagnosis of IPF.
• Carbon monoxide diffusion capacity (DLco) (corrected for hemoglobin): 30% to 79% predicted of normal at Screening, with no clinically significant deterioration between the Screening Visit and randomization, as determined by the Investigator.
• Forced (expiratory) vital capacity (FVC) ≥50% predicted of normal at Screening, with no clinically significant deterioration between the Screening Visit and randomization, as determined by the Investigator.
• If a subject is taking nintedanib or pirfenidone, they must be on a stable dose for at least 8 weeks prior to randomization.

Key Exclusion Criteria:

• Unable to perform pulmonary functional tests (PFTs) or undergo HRCT procedure.
• Peripheral capillary oxygen saturation (SpO2) <90% at rest (if on oxygen supplementation, must be ≤2 L/min at rest).
• Airway obstruction (i.e., prebronchodilator FEV1/FVC <0.7) or evidence of a bronchodilator response as defined by an absolute increase of ≥12% and an increase of ≥200 milliliters (mL) in FEV1 or FVC, or both, after bronchodilator use, compared with the values before bronchodilator use at Screening.
• End-stage fibrotic disease likely requiring organ transplantation within 12 months, or if the subject has initiated active evaluation for organ transplantation.
• The extent of emphysema in the lungs exceeds fibrosis, based on central review of HRCT scans.
• Body weight <60 kg at Screening.
• History of or ongoing malignant disease, including solid tumors and hematologic malignancies, with the exception of basal cell carcinomas, squamous cell carcinomas, and carcinoma in situ of the cervix that have been completely excised and considered cured >2 years prior to Screening.
• Significant cardiac disease (e.g., New York Heart Association Class 3 or 4; myocardial infarction within the past 6 months; unstable angina; coronary angioplasty or coronary artery bypass graft within the past 6 months; uncontrolled atrial or ventricular cardiac arrhythmias; or pulmonary hypertension requiring pharmacologic treatment).
• Clinical diagnosis of any connective tissue disease (including but not limited to scleroderma, polymyositis/dermatomyositis, systemic lupus erythematosus, and rheumatoid arthritis) or a diagnosis of interstitial pneumonia with autoimmune features as determined by the Investigator.
• Other disease that may interfere with testing procedures or, in the judgment of the Investigator, may interfere with study participation or may put the patient at risk when participating in this study.
• Other unspecified reasons that, in the opinion of the Investigator or Biogen, make the subject unsuitable for enrollment.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.