Monkeypox, now often referred to as mpox, is a viral disease caused by the monkeypox virus, a member of the Orthopoxvirus genus. This virus is related to the variola virus, which causes smallpox. Vaccines play a significant role in preventing the spread of mpox and reducing disease severity. These vaccines work by preparing the body’s immune system to recognize and fight off the virus if an exposure occurs.
Available Monkeypox Vaccines
There are two primary vaccines utilized for mpox prevention: JYNNEOS and ACAM2000. JYNNEOS, also known by its brand names Imvamune or Imvanex, is a live, non-replicating vaccine derived from the Modified Vaccinia Ankara (MVA) virus. This means the vaccinia virus in JYNNEOS is weakened and cannot multiply in human cells, preventing disease. JYNNEOS is administered as two doses, given 28 days apart, either subcutaneously (injected into the fatty tissue below the skin, usually in the upper arm) or intradermally (injected into the top layer of the skin, the dermis). The intradermal route uses a smaller dose, about one-fifth of the subcutaneous dose.
ACAM2000 is a live, replication-competent vaccinia virus vaccine. The vaccinia virus in ACAM2000 can replicate at the vaccination site, leading to a localized infection. This vaccine is administered using a bifurcated (two-pronged) needle, which is used to prick the skin several times, a method known as scarification. A successful vaccination with ACAM2000 results in a visible lesion or “pock” at the site, which scabs over and eventually falls off, leaving a small scar. ACAM2000 is given as a single dose.
How Monkeypox Vaccines Work
Both JYNNEOS and ACAM2000 vaccines stimulate the immune system to build defenses against orthopoxviruses, including the mpox virus. JYNNEOS, containing a live but non-replicating vaccinia virus, introduces viral antigens to the body. The immune system recognizes these antigens and begins to produce antibodies and specialized immune cells, such as T cells, which are capable of neutralizing the virus and clearing infected cells. This process builds immunological memory, allowing for a rapid and robust response upon future exposure to the actual mpox virus.
ACAM2000, being a live, replicating vaccinia virus vaccine, establishes a localized infection at the vaccination site. This replication allows for a broader and more sustained presentation of viral antigens to the immune system. The body then mounts a strong immune response, generating both neutralizing antibodies and cellular immunity against the vaccinia virus, which provides cross-protection against other orthopoxviruses like mpox. The development of a visible “take” or lesion at the vaccination site indicates a successful immune response has been initiated.
Eligibility and Vaccination Strategies
Mpox vaccination strategies encompass both pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP). This is recommended for those at high risk of occupational exposure, such as certain laboratory personnel working with orthopoxviruses. It is also advised for individuals with certain risk factors, including those who exchange goods or services for sex, individuals living with HIV, especially with uncontrolled or advanced disease, or those taking pre-exposure prophylaxis for HIV.
For optimal effectiveness, the vaccine should be given as soon as possible after exposure, ideally within four days. Vaccination between four and fourteen days after exposure may still offer some protection against disease onset or lessen the severity of symptoms.
Safety Information and Potential Side Effects
Both JYNNEOS and ACAM2000 vaccines have known safety profiles, with varying types and frequencies of side effects. For JYNNEOS, common side effects include pain, redness, swelling, and itching at the injection site. Systemic reactions like fatigue, headache, and muscle pain can also occur. While rare, severe allergic reactions, such as anaphylaxis, are a possibility with JYNNEOS. The vaccine may contain trace amounts of chicken protein, benzonase, gentamicin, and ciprofloxacin, which should be considered for individuals with allergies to these components.
ACAM2000 carries a higher risk of more significant adverse reactions due to its live, replicating nature. Common side effects at the vaccination site include bleeding, blistering, pain, redness, and scarring. More serious adverse events, though less common, can include myocarditis and pericarditis (inflammation of the heart muscle and its surrounding sac). Other rare but serious complications include encephalitis, severe skin infections like eczema vaccinatum, and accidental eye infections that could lead to vision impairment. ACAM2000 is contraindicated in individuals with weakened immune systems, certain skin conditions such as eczema, or cardiac disease.