The World Health Organization (WHO) last week reported that breakthrough infections of monkeypox have occurred after vaccination—suggesting that more precautions may be necessary to reduce the spread of the virus in the current outbreak.

WHO reports breakthrough monkeypox infections

In a media briefing on Aug. 17, WHO announced preliminary reports of breakthrough monkeypox infections among people who had been vaccinated—suggesting that the vaccine is not 100% effective against infection from the virus.

According to Rosamund Lewis, WHO's technical lead for monkeypox, the breakthrough cases are not a surprise. "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 percent effective in any given circumstance, whether preventive or post-exposure," Lewis said

"We have known from the beginning that this vaccine would not be a silver bullet, that it would not meet all the expectations that are being put on it, and that we don't have firm efficacy data or effectiveness data in this context," she added.

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.

In addition, immunological studies in humans have shown that the vaccine generates monkeypox antibodies in people's blood—but the level of protection is not currently known.

"So we know that the vaccine does stimulate the immune system and people produce antibodies when they receive the vaccine," said Boghuma Titanji, an infectious disease specialist at Emory University, "but we don't have a clinical data in humans to actually tell us, 'Okay, that immune response translates to this level of protection against getting infected with monkeypox or reducing the severity of monkeypox disease if you do get infected.'"

Currently, WHO has called for international studies on the monkeypox vaccine to better determine its efficacy in the ongoing outbreak.

Health officials continue to encourage vaccination

Despite the monkeypox vaccine's overall efficacy being unclear, health officials have continued to stress the important of vaccination, particularly for those at high risk. So far, more than 38,000 monkeypox cases have been reported worldwide, including more than 13,500 in the United States, according to CDC.

However, early data from the United Kingdom, Germany, and areas of Canada suggest that some monkeypox outbreaks may now be on the decline—which some health officials have attributed to effective vaccination campaigns. For example, the number of new cases reported each day in the United Kingdom has steadily decreased since late July, going from 50 daily cases to around 25.

"Over 25,000 have been vaccinated with the smallpox vaccine, as part of the strategy to contain the monkeypox outbreak in the UK.," said the U.K. Health Security Agency. "These 1000s of vaccines, given by the NHS to those at highest risk of exposure, should have a significant impact on the transmission of the virus."

Although these outbreaks are "far from extinguished," there are signs that "they're a bit more under control than they had been" in some areas, said Donald Vinh, an infectious disease specialist at McGill University.

However, Vinh noted that it's too soon to determine whether the vaccine is the primary factor contributing to the decline in monkeypox cases in these countries. "No single measure is going to really be the solution here."

In addition to vaccination, people at high risk are recommended to take precautions to protect themselves from infection. For example, WHO earlier this month 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."

Health experts also say that providers need to learn how to identify monkeypox cases, since many cases are being misdiagnosed as sexually transmitted diseases, such as syphilis. "I can tell you, from the lens of a clinician, that monkeypox is very, very easy to mistake for another infectious disease," Titanji said.

"You really have to maintain a very high index of suspicion because some of the lesions are so subtle and the clinical presentation is so variable," she added. "At this phase of the outbreak, we should be over testing rather than under testing. If a doctor even remotely suspects monkeypox, they should be sending a test for it." (Bean, Becker's Hospital Review, 8/19; Mitropoulos, ABC News, 8/17; Doucleff, "Goats and Soda," NPR, 8/18; Furlong, Politico, 8/17)