A Study to Evaluate the Safety and Efficacy of Pembrolizumab (MK-3475) Coformulated With Berahyaluronidase Alfa (MK-3475A) in Participants With Relapsed or Refractory Classical Hodgkin Lymphoma (rrcHL) or Relapsed or Refractory Primary Mediastinal Large B-cell Lymphoma (rrPMBCL)(MK-3475A-F65)

The primary purpose of the study is to assess the pharmacokinetics (PK) profile of pembrolizumab following subcutaneous (SC) injection of pembrolizumab coformulated with hyaluronidase, and to evaluate the objective response rate (ORR) of pembrolizumab (+) berahyaluronidase alfa SC in adult participants with Relapsed or Refractory Classical Hodgkin Lymphoma (rrcHL) or Relapsed or Refractory Primary Mediastinal Large B-cell Lymphoma (rrPMBCL). There is no formal hypothesis to be tested for this study.

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Histologically confirmed diagnosis of classical Hodgkin lymphoma (cHL) or primary mediastinal B-cell lymphoma (PMBCL) * Radiographically measurable cHL or PMBCL disease assessed by investigator as per Lugano classification * Have a life expectancy of \>3 months * Human immunodeficiency virus (HIV)-infected participants must have well controlled HIV on antiretroviral therapy (ART) * Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeks, and have undetectable HBV viral load before enrollment * Participants with history of hepatitis C virus (HCV) infection are eligible if they have completed curative antiviral therapy at least 4 weeks before enrollment and HCV viral load is undetectable at screening * Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 assessed within 7 days before or on the day of the first dose of study intervention Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Has clinically significant (i.e., active) cardiovascular disease * Has pericardial effusion or clinically significant pleural effusion * Has known additional malignancy that is progressing or has required active treatment within the past 2 years * Diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study intervention * Received prior monoclonal antibody within 4 weeks prior to first dose of study intervention or has not recovered (i.e., ≤Grade 1 or at baseline) from adverse events (AEs) due to agents administered more than 4 weeks earlier * Received prior therapy with an anti-programmed cell death 1 protein (anti-PD-1), anti-programmed cell death ligand 1 (anti-PD-L1), or anti-programmed cell death ligand 2 (anti-PD-L2) agent, or with an agent directed to another stimulatory or coinhibitory T-cell receptor * Received prior systemic anticancer therapy including investigational agents within 4 weeks before the first dose of study intervention * Received prior radiotherapy within 2 weeks of start of study intervention, or has radiation-related toxicities, requiring corticosteroids * Received a live or live-attenuated vaccine within 30 days before first dose of study intervention * Is receiving systemic antineoplastic chemotherapy, immunotherapy, or biological therapy not specified in this protocol * Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis * Active autoimmune disease that has required systemic treatment in the past 2 years * History of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease * Active infection requiring systemic therapy except certain protocol-specified therapies * Concurrent active hepatitis B and hepatitis C virus infection * Has undergone solid organ transplant at any time, or prior allogeneic hematopoietic stem cell transplant (SCT) within the last 5 years