“Breakthrough” cases of monkeypox are being reported, World Health Organization officials said Wednesday—confirming that the vaccine used to fight it isn’t the “silver bullet” many had hoped for.

Such cases provide “important information that tells us the vaccine is not 100% effective in any given circumstance,” Dr. Rosamund Lewis, the WHO’s technical lead on monkeypox, said at a press conference.

Breakthrough infections have occurred when the vaccine was given before exposure to high-risk individuals, as well as when it was given after exposure in hopes of preventing infection, Lewis said.

While data on the vaccine most commonly used against monkeypox, smallpox vaccine Jynneos, is sparse, limited data from studies in the 1980s suggested that smallpox vaccines are only about 85% effective against monkeypox, Lewis said. There is no vaccine available specifically for monkeypox.

“We have known from the beginning this vaccine will not be a silver bullet and meet all the expectations put on it,” Lewis said.

Those who feel they’re at heightened risk and wish to lower that risk level can reduce their number of sex partners and avoid group and/or casual sex, she added.

More than 35,000 cases of monkeypox had been reported globally since January, WHO officials said Wednesday. There had been 12 deaths. Reported U.S. cases were at just over 13,500, according to the U.S. Centers for Disease Control and Prevention.

Globally, nearly 7,500 cases were reported last week—a 20% increase over the previous week, which was a 20% increase over the week before, according to WHO officials. Almost all cases are being reported in Europe and the Americas among men who have sex with men.

Earlier this month the U.S. Food and Drug Administration announced it would allow health care providers to administer a fraction of the normal dose of Jynneos to patients in a bid to stretch limited vaccine supply.

The vaccine is now to be administered between layers of skin versus under the skin, a technique that improves effectiveness, making a smaller dose acceptable, experts say.

“In recent weeks the monkeypox virus has continued to spread at a rate that has made it clear our current vaccine supply will not meet the current demand,” FDA Commissioner Dr. Robert Califf said in an Aug. 9 news release. "By increasing the number of available doses, more individuals who want to be vaccinated against monkeypox will now have the opportunity to do so.”

The authorization of a nontraditional route of administration was issued because the now globally circulating pathogen "has a significant potential to affect national security or the health and security of United States citizens living abroad," according to the release.

It was not immediately clear if the administration of smaller doses of vaccine between layers of skin would increase the risk of breakthrough cases, though experts say reduced doses delivered in such a manner could be more effective than whole doses delivered in the traditional manner.

This story was originally featured on Fortune.com