The primary objective of the study is to determine whether titration of Avonex reduces the severity of flu-like symptoms (FLS). Secondary objectives of this study are to determine whether titration of Avonex reduces the incidence of FLS and to evaluate the overall safety and tolerability of Avonex intramuscular (IM) injections.

Key Inclusion Criteria:

• Must have a Body Mass Index of 19 to 30 kg/m2, and a minimum body weight of 50.0 kg at Screening.
• Female participants of childbearing potential must practice effective contraception during the study and be willing and able to continue contraception for 30 days after their last dose of study treatment.

Key Exclusion Criteria:

• Known history of or positive test result for human immunodeficiency virus (HIV), hepatitis C virus (test for hepatitis C virus antibody [HCV Ab]) or hepatitis B virus (test for hepatitis B surface antigen [HBsAg] and hepatitis B core antibody [HBcAb])
• Known history of chronic fatigue syndrome or fibromyalgia
• Within one month, flu-like illnesses (e.g., gastroenteritis, upper respiratory infection, common cold)
• History of severe allergic reactions to any drug or anaphylactic reactions
• Known allergy to Avonex or any of its components
• Serious infection (e.g., pneumonia, septicemia) within the 3 months prior to Screening or active bacterial or viral infection
• History of alcohol or substance abuse (as defined by the Investigator)
• Female participants who are pregnant or currently breastfeeding
• Any previous treatment with any interferon product.10. Vaccinations within 2 weeks or 5 half-lives, whichever is longer, prior to Day 1
• Blood donation within 30 days prior to Screening
• Use of any tobacco product more than 5 times within 30 days prior to Screening

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