MPOX — Mpox (formerly called monkeypox) is caused by infection with an orthopoxvirus historically transmitted by wild animals including rodents and primates. Unlike previous outbreaks, human-to-human transmission was responsible for the outbreak that began in May 2022; most cases occurred in men who have sex with men. Clinical manifestations of mpox infection include fever, headache, myalgia, chills, fatigue, lymphadenopathy, and characteristic skin lesions. Skin lesions can be present without systemic symptoms. In the recent cluster of cases reported in the US, all were in symptomatic men.1

THE VACCINE — Jynneos, a 2-dose orthopoxvirus vaccine, was licensed by the FDA in 2019 for prevention of smallpox and monkeypox infection in adults at high risk. Jynneos contains a live, replication-deficient, modified form of the vaccinia virus Ankara (modified vaccinia Ankara [MVA]), which is closely related to the variola and monkeypox viruses; MVA does not cause disease in humans. Immunocompromised persons, including those receiving immunosuppressive therapy, may have a diminished response to the vaccine. Jynneos is administered subcutaneously in two 0.5-mL doses or intradermally in two 0.1-mL doses, each given 28 days apart.2

CLINICAL STUDIES — Among 917 persons included in the vaccine effectiveness analysis of Jynneos, two doses of the vaccine were more effective in preventing mpox infection than one dose (85.9% vs 75.2%). Among partially vaccinated participants, vaccine efficacy was 77.0% for subcutaneous and 80.6% for intradermal administration. Among fully vaccinated participants, vaccine effectiveness was 88.9% for subcutaneous, 80.3% for intradermal, and 86.9% for heterologous administration. Two doses of the vaccine were also more effective than one dose in immunocompromised persons (70.2% vs 51%). The differences between one and two doses were not statistically significant in the overall population or in immunocompromised persons.3

In an electronic health record cohort study, patients with mpox infection were less likely than controls (patients with HIV infection or a prescription for preexposure HIV prophylaxis) to have received one or two doses of Jynneos vaccine. The estimated adjusted vaccine effectiveness was 35.8% for one dose and 66% for two doses of the Jynneos vaccine.4

CONCLUSION — The CDC is recommending a 2-dose series of the Jynneos vaccine for all persons at high risk for mpox (formerly monkeypox) infection. Two doses of the vaccine given 28 days apart offer greater protection than one dose.