administered yf-vax to a patient reconstituted using a sterile diluent for an mmr vaccine instead of the diluent supplied with the yf-vax with no reported ae; administered yf-vax to a patient reconstituted using a sterile diluent for an mmr vaccine instead of the diluent supplied with the yf-vax with no reported ae; Initial information received on 27-Jan-2025 regarding an unsolicited valid non-serious case received from a physician. This case involves an unknown age female patient who received yellow fever vaccine – US [YF-VAX] reconstituted using a sterile diluent for an MMR vaccine instead of the diluent supplied with the YF-VAX with no reported adverse event. The patient’s past medical history, medical treatment(s), concomitant medications, vaccination(s) and family history were not provided. On an unknown date, the patient received unknown dose of suspect yellow fever vaccine – US Solution for injection (Unknown strength, lot and expiry date) via unknown route in unknown administration site for Immunization, reconstituted using a sterile diluent for an MMR vaccine instead of the diluent supplied with the YF-VAX with no reported adverse event (product preparation error) (poor quality product administered) (Latency same day). Information on the batch number was requested corresponding to the one at time of event occurrence. Reportedly, the patient was a singer on a cruise ship. She was mandated to take a yellow fever vaccine, and since the dose was invalid, she needs to be re-vaccinated as soon as possible. This suspected adverse reaction report is submitted and classified as a medication error solely and exclusively to ensure the marketing authorization holder’s compliance with the requirements set out in the Directive 2001/83/EC and Module VI of the Good Pharmacovigilance Practices. The classification as a medical error is in no way intended, nor should it be interpreted or construed as an allegation or claim made by the marketing authorization holder that any third party has contributed to or is to be held liable for the occurrence of this medication error.