Part A: - Are HIV-1 and HIV-2 negative at Visit 0. - Starting at Visit 0 and continuously until the last planned visit in this study are individuals who: 1. Are assessed by the investigator as having a low likelihood of acquiring HIV and are committed to avoiding behaviors associated with a higher likelihood of acquiring HIV until the End of Study Visit. 2. Agree to discuss HIV disease risks; 3. Agree to HIV acquisition risk reduction counseling; Part B: - Are HIV-1 positive and HIV-2 negative at Visit 0. - Individuals who at Visit 0: 1. Are cART-naïve individuals who were diagnosed with HIV-1 infection ≤12 months prior to screening, OR are individuals who have discontinued cART and who were diagnosed with HIV-1 infection ≤12 months prior to screening or ≤18 months if this is found to be acceptable after discussion on a case-by-case basis with the sponsor's medical monitor. 2. If cART-experienced, have discontinued cART for at least 4 weeks before screening (if the individual was taking long-acting antiretroviral therapy [ART]), see the following bullet). For individuals who have discontinued cART: Are able to comply with study procedures and assessments in the investigator's judgment. 3. Have never received lenacapavir or ibalizumab or fostemsavir, and have not received other long-acting ARTs in the last 6 months (i.e., intramuscular cabotegravir, cabotegravir-rilpivirine). 4. Have a CD4+ T cell count of ≥500 cells/µL and plasma HIV-1 RNA levels between 5,000-100,000 copies/mL at screening. 5. Are willing to initiate cART at a protocol-defined timepoint (56 days post-dose, or earlier if meeting early cART start criteria or at investigator's discretion). 6. Are willing to undergo HIV transmission risk reduction counseling and to maintain low-risk behavior to protect their partners.

Parts A and B: - Have received an HIV vaccination or HIV broadly neutralizing antibody in another clinical study. - Have a known or suspected impairment/alteration of immune function or immunodeficiency (except for HIV infection, applicable to Part B only), including receipt of any immunostimulant, immunomodulator, immunosuppressive medication, immunoglobulin, blood product, or oral or parenteral steroid within 60 days prior to Day 1 or planned administration during the study. The following exception applies: Use of inhaled, intranasal, topical, or locally injected corticosteroids (e.g., intraarticular or intrabursal administration) is allowed. - Have a history of generalized urticaria or angioedema, or of allergy, anaphylaxis, hypersensitivity or intolerance to a human or humanized antibody or to BNT351 excipients. Part B only: - Are receiving ongoing therapy for Mycobacterium tuberculosis infection. - Have a history of opportunistic infections/AIDS-defining illnesses as defined in the protocol. - Have a history of multi-class drug resistant HIV-1 infection defined as resistance to three or more classes of HIV drugs. - Have a history of malignancy within 5 years before screening. Exceptions are squamous and basal cell carcinomas of the skin and carcinoma in situ of the cervix, or a malignancy which is considered in the investigator's judgment to have minimal risk of recurrence. Any malignancy that is an AIDS-defining illness (as defined in the protocol) is exclusionary regardless of the perceived risk of recurrence. NOTE: Other protocol defined inclusion/exclusion criteria apply.