The main purpose of the proposed study is to evaluate the efficacy of efgartigimod PH20 SC in patients with moderate-to-severe Primary Sjögren's Disease (pSjD). The study consists of a double-blinded placebo-controlled treatment period and an open-label treatment period. The maximum study duration for participants in both study parts is approximately 105 weeks.

Inclusion Criteria:

• Is at least 18 years of age and the local legal age of consent for clinical studies when signing the ICF.
• Meets the following criteria at screening: fulfilled ACR/EULAR classification criteria 2016 PSjD criteria before screening; clinESSDAI ≥ 6 at screening; Anti-Ro/SS-A positive at central laboratory; Unstimulated residual salivary flow (≥ 0.01 mL/min)

Exclusion Criteria:

• Secondary (also referred to as associated) Sjögren's disease, defined as overlap with another autoimmune rheumatic or systemic inflammatory condition (eg, rheumatoid arthritis, systemic lupus erythematosus, scleroderma, or idiopathic inflammatory myopathy
• Active fibromyalgia which is not adequately controlled in the judgment of the investigator, or participant is receiving fibromyalgia treatment that has not been stable treatment for at least 12 weeks before screening.
• Any severe systemic pSjD manifestation that is not adequately controlled at screening or baseline that may put the participant at undue risk based on the investigator's opinion.
• Use of cyclophosphamide ≤ 24 weeks prior to screening
• Anti-CD20 or anti-CD19 antibody received < 6 months before screening