Merck Sterile Diluent syringe experienced suction when twisting the needle onto the hub of syringe; causing cap of syringe to pop off; MMRII was administered to a patient that was reconstituted using the defective sterile diluent syringe; caller unsure if patient received full amount of dose as some of the diluent leaked out due to the suction.; unsure if patient received full amount of dose; No additional AE; This spontaneous report was received from a nurse, concerning to a 15-month-old. The patient’s medical history, concurrent conditions and concomitant therapies were not reported. On an unknown date, the syringe device of the sterile diluent (BAXTER STERILE DILUENT) solution for injection/infusion (lot # 1973242 has been verified for that product, expiration date reported and confirmed as 17-FEB-2026) experienced suction when twisting the needle into the hub of the syringe (Device malfunction), causing the cap to pop off (Syringe issue). Also, on an unknown date, using that defective syringe device to reconstituted, the patient was vaccinated with measles, mumps, and rubella (wistar ra 27-3) virus vaccine, live (recombinant Human albumin [rHA] (M-M-R II) injection, 0.5 milliliter, administered for prophylaxis (strength and route of administration were not provided; lot # Y010046 has been verified to be valid for this vaccine, expiration date reported and confirmed as 29-MAY-2026). (Poor quality device used). The reporter was unsure if the patient received the full dose as some diluent leaked out due to the suction (Device leakage) (Accidental underdose). No symptoms were reporter. No additional adverse event (No adverse event).