Mpox, formerly known as monkeypox, is a viral illness that has garnered global attention due to recent outbreaks involving human-to-human transmission. The virus belongs to the Orthopoxvirus genus, the same family as the virus that causes smallpox. While the symptoms are often milder than smallpox, Mpox can still lead to severe outcomes, particularly in immunocompromised individuals. Vaccination has emerged as a primary public health measure to limit the spread of the virus and protect those at the highest risk of exposure or severe disease.
Defining High-Risk Exposure Groups
Vaccination for Mpox is recommended for certain individuals who face an ongoing, elevated risk of exposure, a strategy known as Pre-Exposure Prophylaxis (PrEP). This preventative approach is prioritized for people whose activities or occupation place them in a high-risk category. The largest group identified for PrEP includes gay, bisexual, and other men who have sex with men, as well as transgender or nonbinary people.
Eligibility is further specified for those who have had multiple or anonymous sexual partners within the past six months. Individuals who have recently received a new diagnosis of a sexually transmitted disease (STD), such as chlamydia, gonorrhea, or syphilis, also meet the criteria for PrEP vaccination. People who have sex at commercial sex venues or large public events where anonymous sexual contact is common are similarly considered to be at high risk.
Beyond behavioral risk factors, certain occupational groups are also advised to receive the vaccine as a preventative measure. This includes laboratory workers who handle orthopoxviruses or perform diagnostic testing for Mpox. Healthcare workers who are part of response teams or who frequently care for patients infected with the virus are also eligible for preventative vaccination.
Vaccination After Known Exposure
The Mpox vaccine is also used in a Post-Exposure Prophylaxis (PEP) strategy for individuals who have recently been in contact with someone confirmed to have the virus. This approach is designed to prevent the onset of illness or significantly lessen the severity of the symptoms. For PEP to be most effective, the vaccine should be administered as soon as possible after the last known or presumed exposure.
Ideally, the vaccine should be given within four days of exposure, as this timeline offers the highest likelihood of preventing the disease entirely. Receiving the vaccine between four and fourteen days after exposure may still be beneficial, as it is expected to attenuate the severity of the illness. Known exposure typically involves close, sustained physical contact with an infected person, often occurring through sexual activity or household contact.
Clinicians may also consider vaccination for those who have certain risk factors and recent experiences that make exposure likely, even without a confirmed direct contact. It is important to note that vaccination is not expected to provide benefit if a person has already developed signs or symptoms of Mpox.
Understanding the Available Vaccines
Two vaccines developed for smallpox are currently utilized to prevent Mpox: Jynneos and ACAM2000. Jynneos is the preferred and most commonly used vaccine for Mpox due to its favorable safety profile. It is a live, non-replicating viral vaccine, meaning the virus in the shot cannot copy itself within human cells.
The Jynneos vaccine is a two-dose series, with the injections typically spaced 28 days apart to achieve optimal protection. Because the viral components are non-replicating, Jynneos is safe for use in most populations, including those who are immunocompromised or have a history of atopic dermatitis.
ACAM2000, in contrast, is an older, live, replicating viral vaccine that carries a higher risk of adverse effects. This vaccine can cause a localized skin lesion that must be carefully covered to prevent the virus from spreading to others. The use of ACAM2000 is generally restricted to situations where Jynneos is unavailable or when a person has a specific occupational risk that outweighs the potential for serious side effects, such as myocarditis. Due to these risks, ACAM2000 is contraindicated for people with specific medical conditions, including a weakened immune system, pregnancy, and certain heart issues.
Safety Profile and Administration Details
The Jynneos vaccine is administered as a two-dose series, with the second dose given four weeks following the first for maximum effectiveness. The standard, FDA-approved method of administration involves a subcutaneous injection, which places the vaccine beneath the skin.
During the outbreak response, an alternative intradermal injection method was authorized for adults, which involves injecting a smaller dose between the layers of the skin. This intradermal route was adopted as a dose-sparing strategy to maximize the available vaccine supply, and it has been shown to provide an immune response comparable to the subcutaneous route.
Common side effects following Jynneos vaccination are generally mild and temporary, indicating that the immune system is responding to the vaccine. These often include localized pain, redness, and swelling at the injection site, sometimes accompanied by systemic symptoms like headache, fatigue, or muscle aches.
The intradermal injection can sometimes result in more noticeable injection-site reactions, such as increased itching, swelling, and a small area of skin thickening that may last for several weeks. Severe allergic reactions, such as anaphylaxis, are exceedingly rare, but medical monitoring is standard practice following any vaccination. The Jynneos vaccine is not recommended for anyone who has had a severe allergic reaction to a previous dose or to a component of the vaccine.