This study is designed to evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics, and immunogenicity of ALXN1720 administered subcutaneously (SC) or intravenously (IV).

Participants will be randomized in a 3:1 ratio to receive the active treatment or placebo.

The study will be conducted in healthy adult participants, including participants of Japanese descent.

Inclusion Criteria:

• Body weight within 50 to 90 kilograms (kg), inclusive, and body mass index within the range of 18 to 29.9 kg/meter squared, inclusive.
• Willing to follow protocol-specified contraception guidance while on treatment and for 6 months after the last dose of study treatment.
• Vaccination with tetravalent meningococcal conjugate vaccine and serogroup B meningococcal vaccine.
• No clinically significant or relevant abnormalities as determined by medical history, physical examination, vital signs, 12-lead electrocardiogram, and clinical laboratory evaluation.
• For the cohorts with Japanese participants, parents and grandparents must both be Japanese, and participants must have resided for less than 5 years outside of Japan.

Exclusion Criteria:

• Current or recurrent disease that could affect clinical assessments or clinical laboratory evaluations.
• History of complement deficiency or complement activity below the reference range.
• Female participants who are breastfeeding.
• Immunization with a live-attenuated vaccine 28 days prior to dosing on Day 1 or planned vaccination during the course of the study. Immunization with inactivated or recombinant influenza vaccine, or nucleoside-modified messenger ribonucleic acid or recombinant COVID-19 vaccine is permitted.
• Current tobacco smoking, history of illicit drug abuse, or history of significant alcohol abuse.