A Phase 3 interventional study of JYNNEOS (Liquid Formulation) and JYNNEOS (Lyophilized Formulation) in Monkeypox Virus Infection, sponsored by Centers for Disease Control and Prevention. Completed at 1 site in Democratic Republic of the Congo. Open to participants aged 18 Years and older, including healthy volunteers. Per ClinicalTrials.gov, last updated 2026-04-23.
Sponsored by Centers for Disease Control and Prevention · Phase 3, Interventional, and Prevention
Mpox is a febrile rash illness caused by the monkeypox virus. Its natural occurrence in the DRC puts healthcare and frontline workers at high risk of acquiring monkeypox virus infections that can prevent them from performing work duties, compromise the overall healthcare delivery in an already fragile system, and can result in death (case fatality estimates are approximately 10%).
This is an open-label prospective cohort study in up to 1,600 eligible healthcare workers at risk of mpox infection through their daily work. The study will document mpox exposure and infection in participants while concurrently evaluating the immunogenicity and safety of the vaccine, JYNNEOS (also known as MVA-BN, IMVAMUNE®, IMVANEX), in healthcare personnel in the DRC. Participation in the study is voluntary and open to male and female healthcare personnel ages 18 years and older in Tshuapa Province in The Democratic Republic of Congo who are at risk of monkeypox virus infection through their daily work or laboratory personnel performing diagnostic testing for monkeypox virus.
Orthopoxvirus infections produce antibody responses that are cross-protective against other viruses within the genus. It is this property of orthopoxviruses that allows a vaccine for vaccinia virus against smallpox to be used to provide protection against mpox. Studies performed during and in the immediate aftermath of smallpox eradication demonstrated that smallpox vaccination (with a first generation vaccine) could confer protection against infection with monkeypox virus. Newer, third generation vaccines such as JYNNEOS, an attenuated (replication deficient) strain of vaccinia virus may offer an alternative safer source of vaccine-derived protection.
The clinical presentation of mpox infection is similar to smallpox, although it is less transmissible human-to-human than smallpox and less deadly (case fatality estimates for mpox are approximately 10%). Naturally-occurring human mpox is largely restricted to remote regions of the Congo Basin forest in Central Africa. This study is the first rigorous evaluation of JYNNEOS in a region where natural Orthopoxvirus transmission occurs at appreciable and predictable rates. Healthcare and frontline workers in the DRC are currently at high risk of acquiring monkeypox virus infection that prevents them from performing work duties, compromises healthcare delivery in an already fragile system, and can result in death.
This open-label prospective cohort study in up to 1,600 healthcare personnel at risk of mpox infection through their daily work will document monkeypox virus exposure and infection in vaccinated participants while concurrently evaluating the immunogenicity and safety of JYNNEOS vaccine. Study participation is voluntary and open to male and female healthcare personnel ages 18 years and older in Tshuapa Province in the DRC. Participants will receive two subcutaneous doses of JYNNEOS vaccine on days 0 and 28. A subset of study participants will receive a booster dose. Blood samples will be obtained on days 0, 14, 28, 42, 180, 365, 545, and 730 following primary vaccination for immunogenicity analysis. For individuals who receive a booster dose, blood samples will be obtained on Days 3, 7, 14, and > 1 year following booster vaccination. After each vaccination participants will be observed for at least thirty minutes. They will maintain an adverse event diary to record systemic and local adverse events for 7 days after each immunization. They will also record exposure to the monkeypox virus in an exposure diary that is reviewed at each follow-up visit.
The study will evaluate the proportion of participants who after being vaccinated 1) develop suspected or confirmed mpox infection, and 2) experience exposure to monkeypox virus. The study will also evaluate the safety and immunogenicity of JYNNEOS.
72 studies on the registry are indexed under Mpox, Monkeypox; 30 are open to participants now.
This study's enrollment of 1,600 is above the median of 305 across 36 interventional studies indexed under Mpox, Monkeypox.
Browse Mpox, Monkeypox studies →Centers for Disease Control and Prevention is the lead sponsor of 275 studies on the registry; 2 are open to participants now.
Of its 11 completed or terminated interventional studies of FDA-regulated products, 8 (73%) have results posted.
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Informed consent
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Up to 1000 male and female healthcare personnel ages 18 years and older in Tshuapa Province in The Democratic Republic of Congo at risk for mpox will receive two doses of attenuated live virus smallpox vaccine (JYNNEOS liquid formulation) administered on days 0 and 28 via subcutaneous injection (deltoid) (1 x 10\^8 Tissue Culture Infectious Dose 50 \[TCID50\] per 0.5 mL). A subset of participants will receive a booster dose.
Biological: JYNNEOS (Liquid Formulation)
Up to 600 male and female healthcare personnel ages 18 years and older in Tshuapa Province in The Democratic Republic of Congo at risk for mpox will receive two doses of attenuated live virus smallpox vaccine (JYNNEOS lyophilized formulation) administered on days 0 and 28 via subcutaneous injection (deltoid) (1 x 10\^8 Tissue Culture Infectious Dose 50 \[TCID50\] per 0.5 mL). A subset of participants will receive a booster dose.
Biological: JYNNEOS (Lyophilized Formulation)
Up to 400 male and female healthcare personnel ages 18 years and older who received primary vaccination as a previous study participant
Biological: JYNNEOS (Liquid Formulation)
Two doses of attenuated live virus smallpox vaccine (JYNNEOS liquid formulation) administered on days 0 and 28 via subcutaneous injection (deltoid) (1 x 10\^8 Tissue Culture Infectious Dose 50 \[TCID50\] per 0.5 mL). A subset of participants will receive a booster dose.
Also known as: Modified Vaccinia Ankara (MVA), MVA-BN, IMVAMUNE, IMVANEX
Two doses of attenuated live virus smallpox vaccine (JYNNEOS lyophilized formulation) administered on days 0 and 28 via subcutaneous injection (deltoid) (1 x 10\^8 Tissue Culture Infectious Dose 50 \[TCID50\] per 0.5 mL). A subset of participants will receive a booster dose.
Also known as: Modified Vaccinia Ankara (MVA), MVA-BN, IMVAMUNE, IMVANEX
Monkeypox virus infection
Proportion of participants who develop suspected or confirmed monkeypox virus infection following receipt of JYNNEOS
Time frame: 2 years following initial vaccination
Monkeypox virus exposure
Proportion of participants who experience exposure to monkeypox virus following receipt of JYNNEOS
Time frame: 2 years following initial vaccination
Orthopoxvirus Antibody Response
Proportion of participants who have orthopoxvirus antibody responses
Time frame: Days 0, 14, 28, 42, 180, 365, 545, and 730 days after the receipt of the first dose of vaccine; Days 3, 7, and/or 14, and through study completion (an average of 1 year) after receipt of single booster dose of vaccine.
Distribution of Geometric Means Titers (GMTs)
Distribution of geometric means titers (GMTs)
Time frame: Days 0, 14, 28, 42, 180, 365, 545, and 730 days after the receipt of the first dose of vaccine; Days 3, 7, and/or 14, and through study completion (an average of 1 year) after receipt of single booster dose of vaccine.
Adverse event and serious adverse event information
Number of participants with reported local or systemic vaccine-associated adverse events by dose, including Serious Adverse Events and Deaths
Time frame: 2 years following initial vaccination; Day 3, 7, and/or 14 after receipt of single booster dose of vaccine
Plan to share: No
Ran from Feb 2017 to primary completion in Oct 2025, 104 months.
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This study is completed, as verified in Apr 2026. You cannot join it, but the record below documents what was studied.
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Studies of the drug tested here: MVA-BN smallpox vaccine
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