To evaluate the safety and tolerability of single ascending intravenous (IV) and subcutaneous (SC) doses and a multiple dose regimen.

Participants will be randomized to single IV or SC or multiple dose treatment with REGN3918 or matched placebo.

Key Inclusion Criteria:

A subject must meet the follow criteria to be eligible for inclusion in the study:

1. Males and females from 18 to 50 years of age.
2. Subject is judged to be in good health based on medical history, physical examination, vital sign measurements and laboratory safety tests performed at screening and/or prior to administration of initial dose of study drug
3. The subject has a body mass index less than 30 kg/m2
4. Willingness to undergo vaccination and antibiotic prophylaxis against N. meningitides.

Key Exclusion Criteria:

A subject who meets any of the following criteria will be excluded from the study:

1. Any clinically significant physical examination abnormalities observed during the screening visit.
2. Hospitalization for any reason within 30 days of the screening visit
3. Persistent chronic or active recurring infection requiring treatment with antibiotics, antivirals, or antifungals.
4. Subject has a history of meningococcal infection.
5. Carriers of Neisseria meningitides based on cultures from naso-pharyngeal swabs during screening.
6. Subject has had a splenectomy.
7. Known allergy to penicillin class antibiotics
8. Known or suspected complement deficiency, or abnormal complement C3, C4 or CH50 during screening.
9. Recent (within the previous 2 months) bacterial, protozoal, viral or parasite infection requiring systemic treatment.
10. History of tuberculosis or systemic fungal diseases
11. HIV infection or HIV seropositivity at the screening visit
12. Positive HBsAg, HBcAb, or hepatitis C antibody at the screening visit
13. History of malignancy (except non-melanoma skin cancer or cervical in-situ)
14. Pregnant or breastfeeding women
15. Women of childbearing potential and men who are unwilling to practice highly effective contraception prior to the initial dose/start of the first treatment, during the study, and for at least 6 months after the last dose

Note: Other protocol defined Inclusion/Exclusion criteria may apply

Double-blinded