Does One Dose of the Mpox Vaccine Offer Enough Protection?

New York City has been particularly hard-hit by the mpox (formerly known as monkeypox) outbreak. As of Friday, the city had recorded 839 cases, nearly a third of the cases in the United States.

With the mpox vaccine in short supply, NYC health officials said they’re working to get as many high-risk residents vaccinated as possible. This might mean that some people will have to wait longer than the recommended 28 days for the second dose.

“Given the rapid increase in cases, the Health Department has decided that providing first doses to offer protection to more at-risk New Yorkers is the best strategy until we receive adequate vaccine supply,” the NYC health department said in a press release last week. “Until there is sufficient supply in the city, all vaccine doses will be treated as first doses, and we will only begin scheduling second dose appointments once we have enough vaccine to do so.”

The U.S. has a supply of two vaccines to prevent mpox, but Jynneos is the only one specifically licensed for the disease. Although the city received an additional 26,000 doses on Wednesday, it is still far from being enough to fully vaccinate the entire at-risk population.

During a press conference last week, Peter Marks, MD, director of the FDA’s Center for Biologics Evaluation and Research, emphasized that two doses of the Jynneos are required to offer sufficient protection against mpox.

“A single dose of this vaccine will not provide the kind of protection over time that is necessary if people continue this risky behavior,” Marks said. “So the two-dose regimen is the best that we can do to make sure that we actually have people get the protection that the vaccine is intended to provide.”

But some research indicates that a single dose of Jynneos is effective at protecting against symptomatic disease, and that protection doesn’t wane very quickly.

One Dose or Two?

In 2019, Bavarian Nordic, the manufacturer of Jynneos, submitted data to the FDA that showed the vaccine induced modest antibody levels after the first dose. The second dose increased the antibody response more than six-fold, and the immune response peaked two weeks after.

In a study from 2008, monkeys who were vaccinated with a single dose of Modified Vaccinia Ankara (MVA)—the virus used in Jynneos—were protected against mpox as soon as four days after the first shot.

It’s possible that the immunity from one dose of vaccine could last for years, said Grant McFadden, PhD, director for the Biodesign Center for Immunotherapy, Vaccines and Virotherapy at Arizona State University.

“It’s a perfectly justifiable decision and it allows you to protect twice as many people with a limited amount of vaccine,” McFadden told Verywell. “Scientifically there’s nothing magical about [28] days between the first and the second dose. Immunologically, even if the next dose is given weeks or months later, there still will be a massive boosting effect.”

McFadden added that when Bavarian Nordic applied for licensure, the researchers didn’t do a robust review of how the immune response differs when someone receives a booster shot at different intervals. It’s possible, he said, that antibody levels could be as strong, if not stronger, when boosted six months after the primary dose as three weeks later.

The FDA makes recommendations based on the information available to the agency from clinical research. Because health officials have only reviewed data about the vaccine’s efficacy when the doses were given on schedule, they can’t say for sure how the time lag will affect the outcome.

Still, McFadden said that most poxvirus experts would agree that the antibody levels from the first dose won’t wane quickly and a second dose further supports the immune system, whether given one month or six months after the first.

“At almost any point during that time, if you give a boost, it will again cause a rise in the protective titers,” he said.

Other countries, including the United Kingdom and Canada, have also adopted the single-dose vaccine strategy. Some of these countries used a similar strategy in the COVID-19 vaccine rollout, though the U.S. didn’t.

Vaccinating High-Risk Communities

Mpox can infect anyone. In the current outbreak, however, it is spreading most among men who have sex with men.

Those who have multiple sexual partners or participate in anonymous sex are more likely to come in contact with the virus. For these groups, two doses are especially important to ensure ongoing protection, CDC Director Rochelle Walensky said in the press call last week.

Mpox isn’t a very lethal infection. The disease usually resolves itself within a matter of weeks and has a mortality rate of about 3%–6%, according to the World Health Organization.

Still, the disease can be quite painful and disruptive of day-to-day activities. The vaccines protect against the symptoms of the disease, including the development of lesions. The virus is mostly transmitted through contact with the lesions, so this could reduce the spread of the disease.

There’s some benefit to being vaccinated, even if an individual has already been exposed or is in the very early stages of the disease. That’s because the mpox virus has a long latent period in the body—it can take weeks for the virus to replicate enough to cause symptomatic disease.

“It’s a little different than many other diseases where if you’re not vaccinated ahead of time, the game’s over,” McFadden said. “Even after exposure, a vaccination can reduce the severity of the disease if it’s caught early.”

Status of Vaccine Deployment

Demand for mpox vaccines has been high in New York City. The few times the city has opened vaccine appointments, the slots were filled in mere minutes.

Federal health officials are holding weekly calls with leading health organizations that provide care to gay communities, according to Walensky. McFadden said these groups are robust and he is hopeful “they will do a very good job” of doling out vaccinations and care to those who need it.

“If we can get the vaccine to everyone, chances are that this is a disease that can be stamped out,” McFadden said.

The Department of Health and Human Services has made some 374,000 doses of the vaccine available to states and has so far distributed nearly 200,000 doses. Walensky said 780,000 more doses will soon be approved for release. About 7 million doses of the vaccine will be available in the U.S. by 2023.

“We’re not asking New York—or anybody—to hold back doses right now because… we’re pretty confident that what’s going to come in is going to be able to cover those second doses,” Walensky said.