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Here's why scientists don't know the efficacy of the JYNNEOS monkeypox vaccine

JYNNEOS has never been tested for efficacy against monkeypox in humans. It has been tested for efficacy in animals, and safety and immunogenicity in humans.

WASHINGTON — For weeks, the Verify team has tried to figure out how well the JYNNEOS vaccine works at preventing monkeypox. But turns out it's not that simple.

There’s no conclusive measurement, because while the vaccine was tested for a lot of things, it was not tested to see how well it stopped people from getting sick with monkeypox.


Was the efficacy of JYNNEOS studied in humans against monkeypox?


  • Dr. Dan Barouch- director of the Center for Virology and Vaccine Research at Beth Israel Medical Center and professor of Medicine at Harvard Medical School
  • Dr. William Moss- executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health
  • Dr. Anthony Fauci- Chief Medical Advisor to the President and director of the National Institute of Allergy and Infectious Diseases at NIH
  • Centers for Disease Control and Prevention- "Monkeypox and Smallpox Vaccine Guidance" and "JYNNEOS vaccine"


This is false.

No. What was studied was efficacy in monkeys that were exposed to the virus and the antibody levels simulated from JYNNEOS relative to another smallpox/monkeypox vaccine known as ACAM2000. 


JYNNEOS was approved for the prevention of smallpox and monkeypox by the Food and Drug Administration (FDA) in 2019. Monkeypox is part of the same virus family as smallpox.

Experts say there’s never been a human trial quantifying how well the vaccine protects against monkeypox.

“We're usually looking for, you know, large scale... phase three clinical trials, as we saw with the COVID-19 vaccines, to demonstrate efficacy in humans," Dr. William Moss said. "We just can't do that with smallpox, because it's eradicated. And we couldn't do that previously with monkeypox, because the outbreaks were just too small and often in very remote areas.”

That's not to say that JYNNEOS hasn't been tested in people.

JYNNEOS has been tested on individuals with HIV, stem cell transplant recipients, older individuals and those with eczema. It’s also been studied head to head against an older smallpox vaccine called ACAM2000 and analyzed to see if a smaller dose would create antibodies. 

RELATED: US will stretch monkeypox vaccine supply with smaller doses

While it hasn’t been tested in a human trial to see how well it prevents people from getting sick, it has been tested in animals like prairie dogs and monkeys. That's known as the "animal rule." 

“When you show something is safe in the human and is effective in an animal model or two animal models, then you can give approval for that intervention," Dr. Anthony Fauci said. "And thus far that has the proof.”

So what we know, is that the vaccine stimulates an antibody response and it prevented 80-100 percent of monkeys in a lab, who were infected, from getting sick. What we don’t know is how many people infected with monkeypox would and would not have gotten sick in a trial. 

“I think that we are relatively confident that the vaccine will show efficacy in humans, however, that has not been formally shown,” Dr. Barouch said.

Dr. Moss acknowledged that there's very limited evidence of the vaccine's efficacy in humans so far. 

“Despite the very limited evidence, and I acknowledge [that], the vaccine is fairly safe," Dr. Moss said. "So I do think that the benefits outweigh the risks for this vaccine for high risk individuals.”

While there’s no number for efficacy—or how well something works in a controlled lab—we do have some indication of effectiveness, or how well something works in the real world.

When we asked Bavarian Nordic, the company that makes JYNNEOS, about effectiveness, a spokesperson said it is about 85%. That estimate is also used by the CDC and World Health Organization, but that number is based on studies from the 1980s of another smallpox vaccine.

A company spokesperson says a study is currently underway in the Democratic Republic of the Congo.

RELATED: VERIFY: Yes, health officials encourage people with monkeypox to isolate from pets, too

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