What to Know About Monkeypox Vaccine Side Effects—Even With a Lower Dose

To stretch its supply, the U.S. is now offering one-fifth of a monkeypox vaccine dose. The injection may come with some slightly different side effects.

LOS ANGELES, CALIFORNIA - AUGUST 10: Licensed Vocational Nurse (LVN) Gabriela Solis prepares a dose of the Jynneos monkeypox vaccine at an L.A. County vaccination site in East Los Angeles on August 10, 2022 in Los Angeles, California. Los Angeles County reported 683 monkeypox cases as of August 9th, double the amount of cases from ten days earlier. California Governor Gavin Newsom declared a state of emergency on August 1st over the monkeypox outbreak which continues to grow globally. (Photo by Mario Tama/Getty Images)
Mario Tama/Getty Images

Fast Facts

  • The Centers for Disease Control and Prevention is urging people at risk of contracting monkeypox to get vaccinated.
  • The most common side effects from the vaccine being offered, Jynneos, are cold or flu-like symptoms, and pain and swelling at the injection site.
  • The Jynneos vaccine is also being offered at lower doses to stretch out supply—which can lead to minor changes in side effects.

Monkeypox continues to spread throughout the U.S., and the Centers for Disease Control and Prevention (CDC) is urging people who are at risk of the disease to get vaccinated.

Because U.S. supply of Jynneos—the vaccine recommended for use against monkeypox right now—is slim, the U.S. Food and Drug Administration (FDA) announced last week that healthcare providers can now stretch the vaccine supply, doling out one-fifth of the vaccine for each person.

That means a dose initially intended for one person can now inoculate five people via intradermal injection, or an injection directly under the top layer of skin. (Jynneos is typically given via subcutaneous injection, or into the tissue layer between the skin and the muscle.)

Although some experts are wary of the effectiveness of a lower dose of the vaccine, the move could help bolster a less-than-robust monkeypox vaccine supply—which would offer even more at-risk protection against the disease.

But if you are eligible for the monkeypox vaccine, and are able to get one, another question arises: What do the side effects of the Jynneos monkeypox vaccine look like? Here's what to know about the aftermath of a vaccine—and whether you can help reduce or relieve side effects.

Available Monkeypox Vaccines for At-Risk Populations

Though two vaccines are currently licensed in the U.S. to protect against both monkeypox and smallpox—Jynneos and ACAM2000—Jynneos is the preferred option due to a lesser risk of adverse events.

"The two vaccines are Jynneos and ACAM2000," Justin Kim, MD, MS, Infectious Disease and International Health expert at Dartmouth Health, Lebanon, New Hampshire, told Health. "Jynneos consists of a virus that is unable to replicate, in contrast to ACAM2000, which is able to replicate and theoretically cause disease."

It's the live vaccinia virus in the ACAM2000 vaccine that poses the greatest health risks.

"ACAM2000 was the vaccine used to eradicate smallpox," Linda Yancey, MD, infectious disease specialist, Memorial Hermann Health System in Houston, told Health. "It has more potential side effects, leaves a small scar at the vaccination site and carries the possibility of infecting immune-compromised household contacts, so it is not being used."

That leaves Jynneos as the safer option and the "workhorse of the current monkeypox response," said Dr. Kim. "The Jynneos vaccine contains the same virus but it has been rendered unable to replicate, so there are a lot fewer side effects," he added. It's also a newer technology and is easier to administer.

The Jynneos vaccine is a two-dose regimen given four weeks or 28 days apart. Those guidelines stay in place even with a smaller dosage, the FDA noted.

Potential Side Effects, Explained

All vaccines have the possibility of adverse reactions and side effects, but these are rare and only seen in a handful of patients for every million or so doses given, Dr. Yancey said. People who get any vaccine can expect cold or flu-like symptoms a few days after receiving one.

With the Jynneos vaccine specifically, the most common side effects in adults who had not previously received a smallpox vaccine were pain, redness, swelling, induration (a hard bump on the skin), and itching at the injection site. People also experienced muscle pain, headache, fatigue, nausea, and chills.

People who are allergic to any component of Jynneos are also at an increased risk of a severe allergic reaction to the vaccine. Those symptoms, though rare, include: face and throat swelling, difficulty breathing, increased heart rate, dizziness, and weakness, according to Cindy Williams, BSPharm, FASHP, vice president and chief pharmacy officer for Riverside Health System.

When given intradermally, as opposed to subcutaneously, patients may notice a spot on their skin at the injection site that is hard or raised for longer than they would if they received it subcutaneously, and more redness or itching, but less local pain, according to Kristen Nichols, PharmD, senior content management consultant for the clinical effectiveness business at Wolters Kluwer Health.

Though not currently being used, ACAM2000 has significantly more risks associated with vaccination, particularly for people with weakened immune systems, and people with skin conditions like eczema, dermatitis, or psoriasis.

The FDA notes that ACAM2000 may also cause myocarditis and pericarditis, or swelling and inflammation of the heart and surrounding tissues—as many as one in 175 adults who receive the vaccine for the first time may experience these reactions. Other more commonly observed side effects of ACAM2000 include: itching, sore arm, fever, headache, body aches, mild rash, and fatigue.

Recipients of an ACAM2000 vaccine also have to take special precautions to properly care for the vaccination site on the arm. If an unvaccinated person comes into contact with the vaccine site of another person, they can be infected with the vaccine virus, which can cause serious health problems. ACAM2000 recipients can also spread the virus from the vaccine from the vaccine site to other parts of their own body.

Managing Monkeypox Vaccine Side Effects

According to Dr. Kim, it can be difficult to tell who will get these side effects and to what degree, but there are some ways to reduce aches and pains after vaccination. For example, people can take acetaminophen and ibuprofen, which can be effective ways to reduce headaches, fever and muscle pain.

Another key way to reduce the side effects of the vaccine is to stay properly hydrated. "Drinking plenty of fluids before and after vaccination can help in the prevention of dehydration which can exacerbate any side effects that you may encounter," Williams said.

However, experts say if anything severe or concerning develops including chest pain, lower extremity swelling, rapid or irregular heartbeat, hives, difficulty breathing, dizziness, and/or weakness, reach out to a healthcare provider or take emergency action immediately.

"Severe allergic reactions will likely occur at the time of administration and can be addressed by healthcare personnel who are present at the time," Dr. Kim said. "Most of the other side effects can be treated supportively in consultation with your physician."

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