Just how effective are monkeypox vaccines? Here’s what we know—and don’t.

As monkeypox continues to spread worldwide, health officials have recommended high-risk individuals get vaccinated as soon as possible—but research is limited on just how effectively the available vaccines prevent infection, Randy Dotinga writes for MedPage Today.

How the US plans to vaccinate against monkeypox

The US boosts vaccine supply to combat monkeypox

CDC has confirmed 6,326 cases of monkeypox across 48 U.S. states, the District of Columbia, and Puerto Rico. The states most impacted by the outbreak so far are New York with 1,617 cases, California with 826 cases, and Illinois with 533 cases.

To reduce the risk of viral transmission, health officials have recommended high-risk individuals be vaccinated against monkeypox. So far, the United States has purchased 7 million doses of Bavarian Nordic‘s Jynneos vaccine, the preferred vaccine against monkeypox, and the entire supply is expected to be available by mid-2023. 

So far, the supply of vaccines has been limited, with many jurisdictions, including San Francisco and New York City, saying that they do not have enough to meet demand. Last week, FDA announced it had cleared an additional 786,000 vaccine doses for use.

How well does the monkeypox vaccine actually work?

While health officials continue to emphasize the value of vaccination, there is limited evidence to show that the monkeypox vaccine is effective at preventing infection from the virus. CDC has acknowledged that “no data are available yet on the effectiveness of these vaccines in the current outbreak.”

According to Jay Varma, director of the Cornell Center for Pandemic Prevention and Response, “It is absolutely critical that public health officials work on messaging this uncertainty to people about being vaccinated.”

So far, much of the data on the monkeypox vaccine is from a retrospective analysis published in 1988, which examined whether a smallpox vaccine could also prevent monkeypox. In the study, researchers followed household contacts of 209 people in Zaire who had been infected with monkeypox and found that those who had scars from prior smallpox vaccination were 85% less likely to be infected. 

According to Ira Longini, a biostatistician at the University of Florida, the study’s conclusions are limited, since its statistical analysis has no reported confidence interval and did not adjust for other factors, such as age. In addition, the data was based solely on physical signs of vaccination. 

The study “is the only shred of evidence we have (in regard to vaccine effectiveness), which is pretty weak,” he said. “In principle it should work, but we don’t know.”

Richard Kennedy, co-director of the Mayo Vaccine Research Group, agreed, saying that the Jynneos vaccine and an older smallpox vaccine ACAM2000 “have not been tested directly against smallpox or monkeypox” and that “[t]he immune responses they create are very close to first- or second-generation vaccines: A little weaker, but not much.”

However, Kennedy noted that the vaccines have been tested against monkeypox in several animal studies. “These data are also clear and consistent with very good protection against disease with animals showing very few or no symptoms of illness after challenge,” he said. “The animal data was strong enough that the FDA approved Jynneos to be licensed for prevention of monkeypox.”

How can people protect themselves against monkeypox?

In addition to vaccination, health officials have advised those who are most at risk, particularly men who have sex with men (MSM), to take steps to reduce their risk of infection, including adjusting their sexual behavior during the current outbreak. 

For example, World Health Organization Director-General Tedros Adhanom Ghebreyesus last week recommended “reducing your number of sexual partners, reconsidering … sex with new partners, and exchanging contact details with any new partners to enable follow up if needed.”

Similarly, CDC offered advice for lowering the potential risk of infection during sex, including not kissing and avoiding sharing towels and other items. 

“The prevention messaging should be that vaccinations will be an essential part of getting this outbreak under control, and we need everyone to get vaccinated as soon as possible,” said Michael Donnelly, a data scientist and LGBT health advocate. “But even if you’re vaccinated, you’re still at risk, and unprotected anal sex may be the highest risk.”

“Even after vaccination, you may want to consider reducing the number of your sex partners and using condoms or pursuing other safer sex approaches,” he added. (Dotinga, MedPage Today, 8/1)