Monkeypox is rapidly spreading in the United States in an outbreak the Biden administration has declared a public health emergency.

Unlike the early days of the novel coronavirus, the United States has a vaccine available to protect people against monkeypox — both before and after exposure to the virus. But supplies are limited, and public health authorities are prioritizing high-risk groups while awaiting additional shipments.

Here is more information about the vaccine, its effectiveness and who should be receiving it. You can learn more about monkeypox symptoms, how it spreads and treatments in this separate explainer.

More about monkeypox

— What is the monkeypox vaccine?

The only FDA-approved vaccine for monkeypox offered in the United States is called Jynneos, manufactured by Bavarian Nordic.

The two-dose regimen was approved by the Food and Drug Administration for smallpox and monkeypox in 2019 for adults determined to be at high risk for infection. The FDA recently issued an emergency use authorization allowing the vaccine for children deemed at high risk. Supplies are limited, but the United States has ordered additional doses to have nearly 7 million by mid-2023 and has authorized a new method of administering the shots to stretch out the supply by as much as fivefold.

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— What are the differences between the Jynneos and ACAM2000 vaccines? Which should I be getting?

The United States has a stockpile of the older generation smallpox vaccine, ACAM2000, believed to also be effective against monkeypox because the two viruses come from the same family. But public health authorities are not administering ACAM2000 because it relies on live virus, carrying a higher risk of severe side effects, particularly for people who are immunocompromised or have skin conditions such as eczema. Recipients of ACAM2000 could also infect others. Jynneos is a non-replicating vaccine, meaning it cannot make new viral particles and does not carry the same risks.

Public health officials are vigilant after the case of a child with severe eczema who developed a life-threatening infection after contact with live virus on a parent who received ACAM2000 before deploying overseas. Chicago Public Health Commissioner Allison Arwady said the risks of ACAM2000 do not justify administering it for monkeypox, even while Jynneos supply is low. Unlike smallpox with a 30% fatality rate, monkeypox has not caused fatalities in the United States.

— How effective is the monkeypox vaccine?

Experts and public health officials believe the Jynneos vaccine works. But there are outstanding questions on how well it works and for how long.

The FDA notes Jynneos’s effectiveness is inferred from studies of how the immune system responded and research on animals. That’s not the same as a study of clinical outcomes demonstrating that vaccinated people are less likely to contract monkeypox than the unvaccinated. Because monkeypox is a rare virus, it was not easy to measure such differences before this current outbreak like it was during clinical trials for coronavirus vaccines when COVID-19 was rampant.

Based on data from Africa where the virus is endemic, health authorities believe vaccination against smallpox is 85% effective against monkeypox. But it’s unclear how that differs with only one dose, with some public health agencies delaying second doses to stretch supplies.

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“There’s a lot of knowledge gaps,” said Anne Rimoin, a professor of epidemiology at the University of California at Los Angeles who has spent decades working on monkeypox in Africa. She said the outstanding questions include: How long after completing the vaccine series is someone considered fully protected? How long does protection last? And what impact does previous smallpox vaccination have?

— How will we learn more about the Jynneos vaccine’s effectiveness?

Rochelle Walensky, director of the Centers for Disease Control and Prevention, said the agency is developing “a portfolio” of projects to study vaccine effectiveness in “various locales, populations and timepoints.” The CDC declined to provide further details about that research.

“We recognize that some of these initial estimates are going to take some time,” Walensky said.

The world would have been better prepared had it conducted more research on vaccines in African countries where monkeypox is endemic, said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota

“What we are trying to do basically is build the aircraft, including the outside, at 30,000 feet right now,” Osterholm said.

In the early 2000s, Bernard Moss, a virologist at the U.S. National Institute of Allergy and Infectious Diseases, studied the efficacy of a single dose of a vaccine similar to Jynneos in monkeys, a test of the very circumstances we find ourselves in, with not enough vaccine supply to meet demand.

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The shot offered the monkeys protection for 30 days, a positive sign although the research differs from the present-day outbreak as the virus in experiments was more virulent and the vaccine was administered intramuscularly as opposed to intradermally. NIH is currently designing a randomized trial to evaluate the efficacy of a smaller dose for humans.

— Should I get the monkeypox vaccine? What if I’m not at risk?

While outstanding questions remain on how effective and long-lasting monkeypox vaccines are, experts still urge you to get them if you face higher risk of contracting the virus. They believe the benefits of protection against a nasty virus, with enough evidence to receive the FDA’s approval, outweigh a low risk of side effects.

Even though few fatalities have been reported, data suggests about 5%-10% of patients have been hospitalized and everyone must isolate for weeks. Some patients have reported excruciating pain from lesions around the genitals and anus.

While supplies are limited, eligibility is limited to known contacts of people with monkeypox and those in high-risk groups. The biggest one is sexually active men who have sex with men. Localities with smaller supplies may have more restrictive criteria.

You may have heard that monkeypox is not exclusively a sexually transmitted disease and that anyone can get monkeypox through close contact and contaminated items. That’s true. Right now, infections are overwhelmingly in men who have sex with men who appear to be contracting the virus through skin-to-skin contact during sex.

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While vaccines are limited, public health authorities are trying to prioritize those most likely to contract the virus based on how it’s currently spreading. The vaccine is also available for people with confirmed exposures, such as children at a day-care where an employee had monkeypox.

For those not in high-risk groups but clamoring to get immunized, medical ethicists point out that it would be in everyone’s interest to slow the virus down most effectively — and doing so means prioritizing vaccines for those at greatest risk.

“Even if you are worried, you need to be having a real long think about where that vaccine is going to be most effective,” said Nicholas Evans, a bioethicist at the University of Massachusetts Lowell. “And it turns out that it might be most beneficial to you in someone else’s arm.”

— I’m gay and monogamous, or not sexually active. Should I get vaccinated?

Viruses don’t know sexual orientation. This is why public health officials tend to emphasize the virus is primarily affecting men who have sex with men — to focus on the behavior, rather than the identity.

Eligibility for monkeypox vaccines usually specifies a man who has sex with men who has had multiple or anonymous recent sexual partners. This is because the virus is spreading in a small sexual network where opportunities for exposure are higher than for the average American.

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If you are a man who has sex with other men, but are not currently sexually active or are in a monogamous relationship, your risk for exposure is far lower. Remember the virus can spread in ways other than sex, such as hugging, sharing contaminated clothing and bedding, as well as dancing with clothes off and rubbing against an infected person. But those activities are considered lower risk than sex, which is why the sexually active are prioritized for vaccine.

“If you are comfortable in a genuinely monogamous relationship, you are at exceedingly low risk,” said William Schaffner, a professor in Vanderbilt University’s infectious diseases division, adding that they can wait until the vaccine is more widely available.

— Does the smallpox vaccine protect me against monkeypox?

Older Americans who received the smallpox vaccine in childhood before it was discontinued may have some level of protection against monkeypox, according to the World Health Organization.

But experts and public health officials are not sure how much protection smallpox shots are providing in this current outbreak and have warned people in high-risk groups not to count on immunity. The CDC says people exposed to monkeypox who have not received a smallpox vaccine in the past three years should consider getting vaccinated.

An observational study of 181 monkeypox patients in Spain during the current outbreak found 32 had previously been vaccinated for smallpox and did not have differences in clinical features, such as the number of lesions, than patients who were not vaccinated for smallpox. The study’s authors said this finding warranted further investigation.

— How do I get a monkeypox vaccine?

It depends where you live.

Because vaccines are administered at the local level, go to your local health department’s website to learn where and how the vaccine is being offered. Some private clinics in your area may also be offering vaccines. You can also check with local LGBT organizations that may be compiling resources on how to get vaccinated.

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— How far apart should I get the doses, and what happens if I just get one shot?

The CDC recommends that people get two doses, at least four weeks apart. But the current reality of vaccine wait times might mean people will go longer between doses. Some communities have delayed second doses to give more people their first doses.

Schaffner, the Vanderbilt University infectious-diseases expert, said infections could breakthrough the longer people wait after their first dose, as immunity wanes.

Research about the efficacy of the second dose is scant, so it is unclear how much protection a person loses from delaying that shot. But if data about other vaccines is any indication, it would be the second dose that offers long-term protection.

“We feel pretty strongly that two doses are necessary,” Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said Aug. 9. “We don’t want to give people a false sense of security if they resume activities that they had put a pause on because they erroneously think that they’re protected.”

— How does the Biden administration’s new plan to split vaccine doses impact immunization efforts?

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The Biden administration has taken steps to stretch the nation’s vaccine supply by authorizing a new injection method that only uses one-fifth of the original dose. Officials say this approach can turn 441,000 available doses of Jynneos into more than 2 million.

The alternate vaccination approach is known as an intradermal injection, using a thinner needle under the top layer of skin instead of delivering vaccine to the fatty tissue under the skin.

The FDA in its announcement authorizing the new approach cited a 2015 study of the two-dose vaccine that found similar immune responses after intradermal shots, although with more redness and itchiness at the injection site.

— What are the downsides of intradermal monkeypox vaccine injections?

Public health officials face several obstacles in vaccinating more people using the alternate intradermal injection strategy.

They have to procure new needles. They have to train staff who are not familiar with the injection method, which is performed routinely by some health care workers and similar to tuberculosis tests. If administered incorrectly, the dose could leak out or be insufficient.

And the study cited by the FDA to justify the alternate injection method is not a real world study that showed people are less likely to contract the virus or how long the protection would last. People who receive the pared-down dose may require additional shots if the new approach leads to an insufficient level of protection against the virus.

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Bavarian Nordic, the Jynneos vaccine manufacturer, raised safety concerns about the FDA authorizing intradermal injections, saying it would have been prudent to conduct further studies first. Health and Human Services Secretary Xavier Becerra said, “We wouldn’t have moved forward unless we thought it was safe and effective, and if FDA hadn’t dotted its I’s and crossed its T’s.”

— Do I still need to take precautions after getting vaccinated?

The Centers for Disease Control and Prevention has recommended limiting sexual partners until two weeks after the second dose, emphasizing that behavioral change can be temporary until vaccines are widely available.

But officials acknowledge there is still some risk for the vaccinated with outstanding questions about the vaccine’s protection.

Walensky, the CDC director, says precautions are prudent while vaccine effectiveness studies are underway, especially for those who only received one dose. She suggested “avoiding close skin-to-skin contact, including intimate contact, with somebody who has monkeypox because we don’t yet know how well these vaccines work.”

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The Washington Post’s Dan Diamond contributed to this report.