A Study to Evaluate the Efficacy and Safety of Sefaxersen (RO7434656) in Participants With Primary Immunoglobulin A (IgA) Nephropathy at High Risk of Progression

Inclusion Criteria: * Primary IgAN, as evidenced by a kidney biopsy performed within 10 years prior to or during screening, without known secondary cause * Treatment with maximum tolerated doses of angiotensin-converting enzyme (ACE) inhibitors or angiotensin II receptor blockers (ARBs) for at least 90 days immediately prior to screening, and without an intent to modify the dose during the study, except for interruptions due to illness (not greater than 7 consecutive days), unless the potential participant is intolerant to these medications * Urine Protein-to-Creatinine Ratio (UPCR) ≥ 1 gram per gram (g/g) or urine protein excretion ≥ 1 gram per day (g/day) (with UPCR ≥ 0.8 g/g), all measured from a 24-hour urine collection during screening * eGFR ≥ 20 mL/min/1.73 m\^2, as calculated by the 2021 CKD-EPI creatinine equation (Inker et al. 2021a) * Vaccination against Neisseria meningitidis, Streptococcus pneumoniae and Haemophilus influenzae according to national vaccination recommendations * Female participants of childbearing potential must use adequate contraception Exclusion Criteria: * Pregnancy or breastfeeding, or intention of becoming pregnant during the study or within 12 weeks after the final dose of sefaxersen * Histopathologic or other evidence of another autoimmune glomerular disease * Presence of ≥ 50% crescents on kidney biopsy, sustained doubling of serum creatinine within 3 months prior to screening, or rapidly progressive glomerulonephritis in the opinion o…