The World Health Organization (WHO) announced earlier this week that breakthrough infections have been reported in people who had been vaccinated against monkeypox.

There is limited data available on the efficacy of the JYNNEOS shot, so it will take time to understand how effective it is at preventing infection.

Data from the 1980s has shown that the vaccine was at least 85 percent effective, however, some health experts expect that number to change in the current outbreak.

“The fact that we’re beginning to see some breakthrough cases is also really important information because it tells us that the vaccine is not 100% effective in any given circumstance, whether preventive or post-exposure. We cannot expect 100% effectiveness at the moment based on this emerging information,” Dr. Rosamund Lewis, WHO’s Technical Lead for monkeypox, said during a recent WHO press conference.

Oladele A. Ogunseitan, PhD, a professor of population health & disease prevention and University of California Presidential Chair at UC Irvine, says that past data found the shot to be around 85 percent effective, but it’s unclear how efficacious the shots will be in the current outbreak.

Dr. Carl Fichtenbaum, a clinical professor in the Division of Infectious Diseases at the University of Cincinnati College of Medicine, says that vaccine has not been tested in humans to a large extent, so 85 percent is really a ballpark estimate.

A recent report from France that tracked breakthrough monkeypox cases between May and July 2022 found that of the 276 vaccinated individuals involved, 12 people (4 percent) had a breakthrough case.

Ten got infected within five days after being vaccinated and two got infected 22 and 25 days after being vaccinated.

Symptoms of these breakthrough monkeypox cases included fever, swollen lymph nodes, sore throat, body aches, cough and skin lesions.

None of the individuals who developed a breakthrough case needed to be hospitalized.

Ogunseitan suspects vaccine efficacy may be lower since two doses are required for optimal protection and most eligible individuals have only received a single dose so far due to supply shortages.

The vaccine is also being administered to some people who have recently been exposed to someone with monkeypox. In these cases, the vaccine is designed to reduce the severity of infection, not prevent the infection altogether.

According to Ogunseitan, there are also reports that some local health departments are administering smaller doses of the vaccine to expand the number of people who can be vaccinated.

This could have an impact on efficacy, says Ogunseitan.

“It is important to know if breakthrough infections are for people who only received one or two doses,” Ogunseitan said.

Breakthrough infections have been reported for several viral diseases, including chicken pox, COVID-19 and the flu.

No vaccine can be 100 percent effective, although some get close. They may prevent disease in the vast majority of cases in addition they often reduce the severity of disease in people who do end up getting sick.

Furthermore, the strength of people’s immune systems varies and certain individuals may not mount a strong enough immune response to avoid getting sick even if they are fully immunized.

“As always, those with immunosuppression, cancer, transplant patients, and persons with HIV who have a lower CD4+ lymphocyte count below 350 cells/mm3 would be more likely to not respond as well to vaccine and have a breakthrough infection,” Fichtenbaum said.

People older than 60 are also likely to be less protected, Fichtenbaum added.

Changes in the virus can also allow it to evade part of our immunity generated from vaccinations.

“The second, more worrisome reason is that viruses do mutate, and mutants may not be sensitive to the control offered by specific vaccines,” Ogunseitan said.

Monkeypox primarily spreads through close contact including but not limited to intimate and sexual contact.

Health experts recommend avoiding riskier behaviors until two weeks after the second dose.

“People who are vaccinated should continue to practice preventive behavior and not expose themselves to situations that will likely lead to exposure to the virus,” Ogunseitan said.

Avoiding skin-to-skin contact with people who have a rash, avoiding contact with objects and materials that have been exposed to someone with monkeypox, and practicing good hand hygiene can help prevent transmission, too.

Fichtenbaum says we are still learning about how common breakthrough infections are, and we don’t have the best track record for preventing the spread of infectious diseases during public health emergencies.

“Time will tell how many breakthroughs occur,” Fichtenbaum said.

The World Health Organization (WHO) announced earlier this week that breakthrough infections have been reported in people who had been vaccinated against monkeypox. There is limited data available on the efficacy of the monkeypox vaccine, so it will take time to understand how effective it is at preventing infection. In the meantime, health experts recommend that at-risk individuals avoid riskier behaviors until they are fully vaccinated.

Monkeypox is a rare viral disease. It’s a zoonotic condition, which means it spreads from animals to humans. It can also spread between two humans.

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