Inclusion Criteria:  Diagnosis of severe congenital hemophilia A (intrinsic factor VIII [FVIII] level <1%) or moderate congenital hemophilia A (intrinsic FVIII level ≤5%) if previously prescribed prophylaxis A negative test for FVIII inhibitor (i.e., <0.6 Bethesda Units) within 8 weeks of enrollment Participants who completed successful immune tolerance induction (ITI) at least 5 years before screening are eligible, provided they have had no evidence of inhibitor recurrence (permanent or temporary) as may be indicated by detection of an inhibitor, FVIII half-life <6 hours, or FVIII recovery <66% since completing ITI Participants who were on standard FVIII prophylaxis, defined as the regular administration of FVIII to prevent bleeding, for at least the last 24 weeks, can be enrolled regardless of the number of bleeds during this period Adequate hematologic, hepatic and renal function For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception during the treatment period and for at least 24 weeks after the final dose of emicizumab  Exclusion Criteria:  Inherited or acquired bleeding disorder other than severe congenital hemophilia A (intrinsic FVIII level <1%) or moderate congenital hemophilia A (intrinsic FVIII level ≤5%) without FVIII inhibitors who were previously prescribed prophylaxis for at least 24 weeks Participants who have previously received emicizumab prophylaxis Participants who had joint replacement, joint procedure, synovectomy or synoviorthesis less than 5 years ago, or participants who had joint replacement, joint procedure, synovectomy or synoviorthesis more than 5 years ago but are still experiencing pain in the joint (only the specific joint will be excluded from the study), or participants that plan to have joint replacement, joint procedure, synovectomy or synoviorthesis, or participants that are deemed suitable candidates for joint replacement, joint procedure, synovectomy or synoviorthesis at screening Participants who have conditions other than hemophilia A that can affect joint health and structure (e.g., osteoarthritis) or with severely impaired mobility due to conditions other than hemophilia A Participants with reduced bone mineral density defined as clinically relevant vitamin D deficiency Participants with pre-existing cardiovascular disease not receiving controlled and targeted medication or in a stable condition Participants not eligible for MRI History of illicit drug or alcohol abuse within 48 weeks prior to screening Participants who are at high risk for thrombotic microangiopathy (TMA) Previous (within the last 12 months) or current treatment for thromboembolic disease (with the exception of previous catheter-associated thrombosis for which anti-thrombotic treatment is not currently ongoing) or signs of thromboembolic disease Other conditions (e.g., certain autoimmune diseases) that may currently increase the risk of bleeding or thrombosis History of clinically significant hypersensitivity associated with monoclonal antibody therapies or components of the emicizumab injection Planned surgery during the emicizumab loading dose phase. Surgeries in participants on emicizumab from Week 5 onwards are allowed Known HIV infection not controlled by medication Concomitant disease, condition, significant abnormality on screening evaluation or laboratory tests, or treatment that could interfere with the conduct of the study, or that would in the opinion of the investigator, pose an additional unacceptable risk in administering study drug to the participant Receipt of any of the following: An investigational drug to treat or reduce the risk of hemophilic bleeds within 5 half-lives of last drug administration at screening; A non-hemophilia-related investigational drug within last 30 days or 5 half-lives at screening, whichever is shorter; or, Any other investigational drug currently being administered or planned to be administered Inability to comply with the study protocol Pregnant or breastfeeding, or intending to become pregnant during the study