It was just a few months ago that cases of monkeypox were first reported outside of the countries where it’s typically endemic. Since then, the virus has spread worldwide, with 26,208 cases reported across 87 countries so far—including 6,616 in the U.S. alone as of Aug. 4, according to data from the Centers for Disease Control and Prevention (CDC).
Fortunately, safe and effective vaccines for the virus have existed and been in use for decades, and health officials are now beginning to roll them out to select populations to help stem the outbreak. But what are the potential side effects of the monkeypox vaccine? As with any other shot, officials say to prepare for a few minor symptoms after getting the jab.
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According to the Food & Drug Administration (FDA), two approved vaccines currently used to treat smallpox are also effective against monkeypox—a genetically similar offshoot of smallpox that causes identical (but milder) symptoms, and is less often fatal.
Jynneos, which is now being distributed in the U.S., was developed in the early 2000s before the FDA approved it in 2019 for “the prevention of smallpox and monkeypox in adults.” It’s given in two doses, spaced 28 days apart. Similar to other vaccines, the shots can produce minor side effects such as “pain, swelling, and redness” at the injection site, with “fatigue, headache, and muscle pain” being the most common reaction recorded during clinical trials, according to the CDC.
Another version of the smallpox vaccine, ACAM2000, is currently approved by the FDA for use against smallpox and is allowed for use against monkeypox “under an Expanded Access IND (EA-IND) protocol,” according to the CDC. While it isn’t currently being widely used, it’s administered as a single dose in which the surface of the skin on the upper arm is pricked with a double-pronged needle. If successful, a lesion will form at the inoculation site, known as a “take.” It may need six weeks to heal fully, and could leave a small scar. The CDC says the most common side effects of ACAM2000 are “injection site pain, swelling, and redness; fever; rash; lymph node swelling; and complications from inadvertent inoculation,” which is when a patient accidentally touches the lesion and spreads the rash to another part of their body.
The CDC says the ACAM2000 vaccine can also cause swelling of the heart (myocarditis) or the lining of the heart (pericarditis). According to the agency, one in every 175 people who received the vaccine for the first time in clinical trials developed one of these conditions within three weeks of receiving the dose. The potentially serious reaction is part of the reason why the newer Jynneos was developed. Jynneos “was designed to be less reactogenic than traditional smallpox vaccines and is comparable to routine vaccines in terms of injection site reactions,” Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Prevention.
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Fortunately, it’s considered “pretty darn rare” for patients to have any kind of serious reaction to the Jynneos vaccine, William Schaffner, MD, infectious disease specialist and professor of medicine at the Vanderbilt University School of Medicine, tells Self. “Some people may feel fatigued, out of sorts, or get a headache—that’s pretty conventional after vaccines. Just don’t plan anything exciting for a day or so,” he advises.
As with any shot, doctors recommend keeping a close eye on your symptoms and calling your healthcare provider if you feel anything is amiss. “If a person has unremitting fevers, severe fatigue, chest pain, or shortness of breath, they should seek medical advice,” Adalja tells Self.