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Key Facts About Vaccines to Prevent Mpox Disease

Mpox is an orthopoxvirus that is related to the smallpox virus. It has been associated with sporadic outbreaks over the past decades. Since May 2022, an outbreak of mpox has been ongoing in several countries, including the United States.

People with mpox get a rash that may be painful or itchy and that may be located anywhere on the body. Some people also get flu-like symptoms. Mpox can spread until the rash has healed, all scabs have fallen off, and a fresh layer of skin has formed. The mpox virus can spread to anyone through close skin-to-skin contact. It can also spread through touching objects, fabrics, and surfaces that have been used by someone with mpox or by contact with respiratory secretions. The CDC provides additional information on how mpox can spread.

There are two FDA-approved vaccines for the prevention of smallpox and mpox disease: JYNNEOS and ACAM2000. These two vaccines are the only FDA-approved vaccines for the prevention of mpox disease.

JYNNEOS Vaccine

JYNNEOS is a live virus vaccine that contains Modified Vaccinia Ankara-Bavarian Nordic (MVA-BN), a weakened, non-replicating orthopoxvirus. JYNNEOS may be safely used in significantly immunocompromised individuals for whom certain live vaccines are not indicated or recommended for use.

JYNNEOS is approved for use in individuals 18 years of age and older who are determined to be at high risk for smallpox or mpox infection. JYNNEOS is approved for administration subcutaneously (beneath the skin), as a two-dose series, 4 weeks apart. JYNNEOS was originally developed for use in the event of a smallpox bioterrorist attack in certain populations (e.g., immunocompromised individuals) as an alternative to ACAM2000, which is associated with certain serious adverse reactions that have not been observed with JYNNEOS.

FDA Emergency Use Authorization of JYNNEOS Vaccine

On August 9, 2022, FDA issued an emergency use authorization (EUA) for JYNNEOS to allow healthcare providers to administer the vaccine intradermally (between the layers of the skin) for individuals 18 years of age and older who are determined to be at high risk for mpox infection. The EUA also allows for use of the vaccine in individuals younger than 18 years of age determined to be at high risk for mpox infection; in these individuals JYNNEOS is administered by subcutaneous injection. For all age groups, JYNNEOS is given as a two-dose series, 4 weeks apart.

In issuing this EUA, the FDA determined that the known and potential benefits of JYNNEOS outweigh the known and potential risks for the authorized uses.

A 2015 clinical study evaluated a two-dose series of JYNNEOS given intradermally compared to subcutaneously in individuals 18 years of age and older. Individuals who received the vaccine intradermally received a lower volume (one fifth) than individuals who received the vaccine subcutaneously. The results of this study demonstrated that intradermal administration produced a similar immune response to subcutaneous administration. Administration by the intradermal route resulted in more redness, firmness, itchiness and swelling at the injection site, but less pain.

To support the FDA’s authorization of two doses of JYNNEOS administered by the subcutaneous route of administration in individuals younger than 18 years of age, the FDA considered the available JYNNEOS safety and immune response data in adults as well as historical data with use of live vaccinia virus smallpox vaccine in pediatric populations. Additional information on FDA’s decision to authorize JYNNEOS for emergency use may be found here.

ACAM2000 Vaccine

ACAM2000 is approved for the prevention of smallpox and mpox disease in individuals determined to be at high risk for smallpox or mpox infection.

ACAM2000 is a live, replicating vaccinia virus vaccine which is administered as a single dose and is given “percutaneously” by puncturing the skin of the upper arm multiple times with a two-pronged needle holding a small drop of the vaccine. The live vaccine virus is shed from the vaccination site and can be spread to other parts of the body or to other people. Therefore, appropriate care of the vaccination site is required until it is completely healed, which may take up to six weeks.

The vaccine can cause serious complications in vaccinated individuals and their close contacts to whom the virus has spread – the risks for serious vaccine side effects are greater for certain people, including those who are immunocompromised.

ACAM2000 may cause myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of tissue surrounding the heart). In studies, about 1 in every 175 persons who got the vaccine for the first time may have experienced myocarditis and/or pericarditis. Other serious side effects of ACAM2000 include swelling of the brain or spinal cord, problems with the vaccination site becoming infected, and accidental infection of the eye with the vaccine virus.

Every person who receives ACAM2000 is required to receive the Medication Guide approved by FDA. A Medication Guide is necessary for safe and effective use of the vaccine because it could help prevent serious adverse events and inform the vaccine recipient of serious risks relative to benefit that could affect their decisions to be vaccinated.

FDA to Provide Updates as Developments Occur

The FDA will provide updates as developments occur and will continue to work with federal public health partners and industry to ensure timely access to all available medical countermeasures against mpox. More information can be found on the agency’s mpox webpage. 

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