Open-label, Follow-up of Doravirine/Islatravir (DOR/ISL 100 mg/0.75mg) for Participants With Human Immunodeficiency Virus-1 (HIV-1) Infection (MK-8591A-033)

The safety and tolerability of MK-8591A, a 2-drug fixed dose combination (FDC) of doravirine (100mg) and islatravir (0.75mg) \[DOR/ISL 100 mg/0.75 mg) will be evaluated in participants with Human Immunodeficiency Virus -1 (HIV-1) who were treated with DOR and ISL in earlier clinical studies.

As of Amendment 12, participants will have the option to continue receiving DOR/ISL (100 mg/0.75 mg) until treatment alternatives are identified, or up to Week 288 given that participants are still deriving benefit, based on individual benefit/risk assessment.

Inclusion Criteria: * Is currently receiving DOR 100 mg/ISL 0.75 mg adult FDC tablet in an MSD-sponsored clinical study and has completed the last treatment visit. * Is considered by the investigator to have derived clinical benefit from receiving DOR/ISL and for whom further treatment with DOR/ISL is considered clinically appropriate. * Female is not pregnant or breastfeeding, and at least one of the following conditions applies: Is not a woman of childbearing potential (WOCBP); or is a non-pregnant WOCBP who agrees to the following during the intervention period and for at least 6 weeks after the last dose of study intervention: Not be sexually active, or if sexually active, to use an acceptable method of contraception; or is pregnant and continues to receive study intervention (where allowed by local regulations and as appropriate based on available data/local standard-of-care guidelines) Exclusion Criteria: * Is taking or is anticipated to require any prohibited therapies.