Syphilis; Case reference number US-BN-2022-002040 is a spontaneous case report initially received on 23-Aug-2022 from a pharmacist via Communications (reference number: USBAV22-1139) and concerns a patient of unknown demographics.  The patient's medical history and concomitant medication details were not reported.  On an unspecified date, the patient was vaccinated with Jynneos (smallpox and monkeypox vaccine, live, nonreplicating; batch number: unknown), administered subcutaneously at a dose of 0.5 mL, for monkeypox/smallpox.  On an unspecified date, an unspecified amount of time after the vaccination, a syphilis test was performed and the result was positive. The provider believed that the patient had a false positive syphilis test, as reported.  At the time of this report, outcome of the event 'syphilis' was unknown.  The reporter assessed the event 'syphilis' as non-serious and related to Jynneos. However, 'syphilis' was assessed as serious due to seriousness criteria of medical significance.   Additional information received from Bavarian Nordic on 24-Aug-2022 included the local BN case ID.; Reporter's Comments: A patient of unknown demographics was vaccinated with Jynneos, administered subcutaneously at a dose of 0.5 mL, for monkeypox/smallpox and an unspecified time after the vaccinatio a syphilis test was performed and the result was positive however, the provider believed that the patient had a false positive syphilis test. Syphilis is unlisted and unexpected according to Jynneos. At this point, the outcome was unknown. The patient's medical history and concomitant medication details were not reported. Serologic testing to diagnose syphilis should include the use of both nontreponemal and treponemal tests. The use of only one test is insufficient for diagnosis since serologic testing (especially nontreponemal tests) can be associated with false positive results however either test can be used as the initial screening test. Confirmatory testing is necessary due to the potential for a false positive screening test result. Since syphilis is a sexually transmitted disease caused by the spirochete Treponema pallidum (which requires appropriate use and interpretation of diagnostic testing) the event is assessed as not related to suspect product. The case is considered serious due to criteria of medical significance.; Sender's Comments: A patient of unknown demographics was vaccinated with Jynneos, administered subcutaneously at a dose of 0.5 mL, for monkeypox/smallpox and an unspecified time after the vaccinatio a syphilis test was performed and the result was positive however, the provider believed that the patient had a false positive syphilis test. Syphilis is unlisted and unexpected according to CCDS and USPI for Jynneos. At this point, the outcome was unknown. The patient's medical history and concomitant medication details were not reported. Serologic testing to diagnose syphilis should include the use of both nontreponemal and treponemal tests. The use of only one test is insufficient for diagnosis since serologic testing (especially nontreponemal tests) can be associated with false positive results however either test can be used as the initial screening test. Confirmatory testing is necessary due to the potential for a false positive screening test result. Since syphilis is a sexually transmitted disease caused by the spirochete Treponema pallidum (which requires appropriate use and interpretation of diagnostic testing) the event is assessed as not related to suspect product. The case is considered serious due to criteria of medical significance.