A Study of a Single Subcutaneous Dose of ALXN1210 in Healthy Adult Participants

Inclusion Criteria

• Body mass index from 18 through 29.9 kilogram (kg)/square meter, inclusive, and weight between 50 and 100 kg, inclusive.
• QT interval corrected using Fridericia's formula ≤ 450 milliseconds (msec) for males and ≤ 470 msec for females at Screening and prior to dosing on Day 1.
• Was willing and was able to give written informed consent and complied with the study visit schedule.
• Documented vaccination with tetravalent meningococcal conjugate vaccine at least 56 days and not more than 3 years prior to dosing. Documentation must have included a positive serum bactericidal antibody titer to confirm an immune response before study drug administration.
• Vaccination with serogroup B meningococcal vaccine at least 56 days prior to dosing on Day 1, with a booster administered at least 28 days prior to dosing on Day 1, with at least 28 days between the first and second injections.
• Female participants of childbearing potential, if heterosexually active, must use highly effective contraception.

Exclusion Criteria

• Participants who were in intimate and prolonged contact with (defined as living under the same roof or providing personal care to) people younger than 2 years of age or older than 65 years of age, or who were either immunocompromised or had 1 of the following underlying medical conditions: anatomic or functional asplenia (including sickle cell disease); congenital complement, properdin, factor D, or primary antibody deficiencies; acquired complement deficiencies (for example, those receiving eculizumab); or human immunodeficiency virus (HIV).
• Participants who were one of the following: professionals exposed to environments of greater risk for meningococcal disease; research, industrial, and clinical laboratory personnel routinely exposed to Neisseria meningitidis; military personnel during recruit training (military personnel could have been at increased risk of meningococcal infection when accommodated in close quarters); daycare center workers; those living on a college or university campus; and those who planned to travel during the course of the study to or had travelled to endemic areas for meningococcal meningitis (for example, India, Sub-Saharan Africa, or pilgrimage to Saudi Arabia for Hajj) within the past 6 months.
• History of any Neisseria infection.
• History of unexplained, recurrent infection, or infection requiring treatment with systemic antibiotics within 90 days prior to dosing.
• Evidence of HIV infection (HIV-1 or HIV-2 antibody titer).
• Acute or chronic hepatitis B virus infection. Hepatitis B surface antigen (HBsAg) testing was required for all participants prior to enrollment. Participants with positive HBsAg were not enrolled. For participants with negative HBsAg, the following testing algorithm was required: If hepatitis B core antibody (HBcAb) was negative, the participant was eligible to enroll and If HBcAb was positive, the hepatitis B surface antibody (HBsAb) was tested. (If both HBcAb and HBsAb were positive, the participant was eligible to enroll and If HBcAb was positive and HBsAb was negative, the participant was not enrolled.)
• Acute or chronic hepatitis C virus infection (evidenced by antibody titer).
• Active systemic viral or fungal infection within 14 days prior to dosing.
• Positive or indeterminate QuantiFERON-TB test which indicated possible tuberculosis (TB) infection.
• History of latent or active TB or exposure to endemic areas within 8 weeks prior to the Screening visit.
• Female participants who are breastfeeding or are heterosexually active and unwilling to practice contraception and are not postmenopausal.
• Positive serum pregnancy test at Screening or on Day -1.
• Serum creatinine greater than the upper limit of normal (ULN) of the reference range of the testing laboratory at Screening or on Day -1.
• Alanine aminotransferase or aspartate aminotransferase > ULN of the reference range of the testing laboratory at Screening or > 1.5*ULN of the