This study will evaluate the safety and tolerability of Efimosfermin Alfa for participants with known or suspected MASH with fibrosis consistent with stage F2 or F3.

Inclusion Criteria:

• Able and willing to understand and sign a written informed consent form (ICF) that must be obtained prior to the initiation of study procedures
• Age >=18 through <=75 years at enrolment
• History or presence of 2 or more of the 5 components of metabolic syndrome per American Heart Association definition
• History or presence of known or suspected MASH with evidence of fibrosis

Exclusion Criteria:

• ALT or AST >=5 × upper limit of normal (ULN)
• Total bilirubin (BILI) >=1.3 milligram per deciliter (mg/dL). Individuals with documented Gilbert's syndrome may be enrolled if they experienced an isolated increase in total BILI of >=1.3 mg/dL and direct BILI is <=20% of total BILI; otherwise, the individual will be excluded.
• Serum albumin <=3.5 grams per deciliter (g/dL)
• International normalized ratio (INR) >=1.3 not due to therapeutic anticoagulation. Individuals receiving chronic anticoagulant treatment with higher INR values may be enrolled at the discretion of the Investigator and Study Medical Monitor.
• Alkaline phosphatase (ALP) >=2 × ULN
• Platelet (PLT) count <140 000 per (/) cubic millimeter (mm^3); individuals with a PLT count between 110,000/mm^3 and 140,000/mm^3 may be enrolled after discussion with the Study Medical Monitor
• Serum creatinine >=1.5 mg/dL or creatinine clearance <=60 milliliter (mL)/minute (min)/1.73 square meter by Chronic Kidney Disease Epidemiology Collaboration equation.
• Alpha-fetoprotein >=20 nanogram per milliliter (ng/mL)
• HbA1c >=9.0%
• Model for End-Stage Liver Disease (MELD) 3.0 score >=12 unless the score is elevated in the absence of liver dysfunction (eg, Gilbert's syndrome)
• Phosphatidylethanol (PEth) >=80 nanogram per milliliter (ng/mL) at Screening
• Known co-infection with any of the following: a. Human immunodeficiency virus; b. Hepatitis B virus; c. Hepatitis C virus (HCV); d. Hepatitis D virus; or e. Hepatitis E virus.
• Chronic liver disease from any other cause including, but not limited to, alcoholic liver disease; evidence of portal hypertension; viral hepatitis, or any history or evidence of cirrhosis; or decompensated liver disease such as clinical ascites, bleeding gastroesophageal varices, hepatorenal syndrome, or hepatic encephalopathy prior to Screening or Day 1.
• Current or history of excessive alcohol intake for >=3 months within the 12-month period prior to Screening

Study Sponsor will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/About\_GSK\_Patient\_Level\_Data\_Sharing\_Final\_13July2023.pdf

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or asset(s) with development terminated across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months, but an extension may be granted, when justified, for up to 6 months.

This is a double blind study.