A Monkeypox Vaccine Already Exists—Here’s How It Works, According to Experts

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Since early May, monkeypox (also known as MPXV) has been spreading within the U.S. and across the globe at rates never seen before since the zoonotic virus was initially discovered in the late 1950s. According to the Centers for Disease Control and Prevention (CDC), there are currently 26,864 cases worldwide and 7,101 in the United States.

"Monkeypox is a zoonotic viral infection that may result in a rash similar to that observed in smallpox. Its clinical manifestations may include fevers, chills, lymph node swelling, aches, and pains. Deaths from Monkeypox are, fortunately, quite rare," says Martin Hirsch, MD, editor-in-chief of infectious diseases for Wolters Kluwer's UpToDate, senior physician at the Massachusetts General Hospital, and a professor of medicine at Harvard Medical School.

Experts In This Article
  • Linda Yancey, MD, infectious disease specialist at Memorial Hermann Health System in Houston
  • Martin S. Hirsch, MD, Martin S. Hirsch is the editor-in-chief of infectious diseases for Wolters Kluwer's UpToDate, senior physician at the Massachusetts General Hospital, and a professor of medicine at Harvard Medical School.
  • Peter A. Lio, MD, FAAD, Peter Lio, MD, FAAD, is a clinical assistant professor of dermatology and pediatrics at Northwestern University Feinberg School of Medicine.

The virus primarily spreads through direct contact with infected skin, sores, or body fluids, he says, or through indirect contact with objects that a sick person has used (like blankets or clothing). It can also be transmitted via the placenta from an infected person to their fetus.

As a result, the federal government finally announced this week that the virus is a national public health emergency. The declaration opens up resource allocation and funds to manage the virus's spread, including distributing vaccines.

The good news is that there are two different vaccines for the virus that have existed for years, and the allocation of these shots has been rolled out for high-risk populations in numerous cities across the country. Just this week, the New York City Department of Health released 25,000 first dose appointment slots for trans, gay, and bisexual men and their sexual networks.

The bad news: A monkeypox vaccine is hard to get right now, even for people who are eligible. "Vaccines for monkeypox are available, but on a limited basis, and the criteria for receiving a vaccine may vary based on your risk status and where you live," says Dr. Hirsch.

The instance of a new disease outbreak and confusion about vaccine access and impact on your health can be really confusing, so we have some experts break down exactly what's going on with the monkeypox vaccine.

How the monkeypox vaccine works

We currently have two vaccines against smallpox that are also effective against monkeypox: ACAM2000 and Jynneos, says Linda Yancey, MD, an infectious disease specialist at Memorial Hermann Health System in Houston. That’s because the monkeypox virus is a relative of the virus that causes smallpox disease; both are types of orthopoxvirus.

Jynneos is the newer of the two vaccines, and has been around in the U.S. since 2019. "The vaccine presents a non-replicating virus to the immune system so that antibodies can be produced against it.” Specifically, this vaccine uses a sample of a weaker, related orthopoxvirus. “Then, when the person is exposed to the virus itself, it will be able to quickly kill it and prevent an infection," says Dr. Yancey.

The older monkeypox vaccine is the ACAM2000, which is an iteration of the vaccine used to eradicate smallpox in the 20th century. This vaccine uses a sample of a live, replicating virus called vaccinia to inoculate someone against smallpox. (More on the implications of that in a minute.)

These vaccines are highly effective at preventing monkeypox—an estimated 85 percent, according to the CDC. They work best before exposure, but can be used for post-exposure vaccination as well, says Dr. Yancey. "If they are given within four days of exposure, they can prevent the disease. If they are given five to fourteen days after exposure, they can lessen the severity of the illness," she says. This is a great feature of the vaccine because it offers a mechanism to prevent the spread of the disease once someone contracts it.

Eventually, when the production of the vaccine is increased, cities and towns would be able to take a public health measure called "ring vaccination." This means that if someone comes down with monkeypox, everyone who lives with them or has been in close contact with them could get the vaccine to help stave off the spread.

What’s the difference between the two monkeypox vaccines?

While both Jynneos and ACAM2000 use a live sample of the weaker vaccinia virus to help inoculate people, the version used in the older ACAM2000 virus can replicate. That has some implications for how these vaccines are administered—and who receives them.

For starters, Jynneos is a two-dose shot in the arm, taken 28 days apart, while ACAM2000 is pricked into the skin with a two-pronged needle in a single dose. (The live virus causes a little pock to form in the arm, then blisters and falls off within a few weeks.) Per the CDC, it takes 14 days after the second dose of Jynneos for you to have a full immune response. It takes about four weeks for a person’s immune response to fully develop after getting ACAM2000.

They also have different side effects. People can have pain or swelling at the injection site after getting their Jynneos shots, and the most common side effects are fatigue, headache, and muscle pain. ACAM2000 also can cause pain and swelling at the injection site, but it's also associated with more serious (but rare) side effects like myocarditis and pericarditis, aka inflammation of the heart or heart lining.

But the biggest difference comes down to who can take which vaccine. If you have seen warnings about not getting a specific vaccine if you have psoriasis or eczema, ACAM2000 is the one that people are referring to, says Peter Lio, MD, FAAD, clinical assistant professor of dermatology and pediatrics at Northwestern University Feinberg School of Medicine and Gladskin advisor. That’s because the live, replicating virus in ACAM2000—while generally safe for most people—could harm people who have compromised immune systems (say, having cancer or HIV), are pregnant, or have inflammatory skin conditions like eczema. (Jynneos is generally considered safer for these populations.)

However, don’t bother getting too caught up in the nitty-gritty of these vaccine differences. Why? Jynneos is the only one that is approved specifically by the FDA to fight monkeypox. (ACAM2000 can be used for monkeypox due to a specific FDA protocol that requires patients to fill out informed consent paperwork.) The current vaccine being distributed in cities like New York, San Francisco, and Chicago is the Jynneos vaccine, Dr. Yancey explains.  It’s also safer to distribute to people with compromised immune systems (critical, given that 41 percent of confirmed global monkeypox cases are in people who also have HIV). Dr. Lio assures that Jynneos is safe for people with skin conditions, and that monkeypox poses a much more serious risk to folks with these conditions than the most recent vaccine.

How to get the monkeypox vaccine if you’re eligible

Unfortunately, this is the hardest question to answer right now. Currently, the demand for the vaccine has far outstripped our supply, says Dr. Yancey. (There is a large stockpile of ACAM2000 vaccine and only a small one of Jynneos—but the federal government is working on addressing that as we speak.) More vaccines are on the way, but it will be a while before we can get everyone a shot who wants or needs one.

The only way to get the vaccine, at the moment, is to be among at-risk populations (such as men who have sex with men and their close contacts) and acquire a dose through your local health department. "We anticipate that over time it will become more widely available, but right now, hospitals and doctor's offices do not have it," says Dr. Yancey. Over time, this will be expanded to include the immunocompromised and health care workers. Eventually, it will be made available to the rest of the population if the outbreak continues.

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