Do You Need Another Mpox Vaccine This Summer?

Fact checked by Nick Blackmer

Key Takeaways

• Summer is approaching and mpox is threatening to re-emerge.

• A slate of new studies show that the Jynneos mpox vaccine is effective, and two doses of vaccine are more protective than one.

• Health officials said that even when there are breakthrough cases, vaccinated individuals tend to have milder illness. They are not making a booster dose available this summer.

Warm weather is ushering in social gatherings and celebrations as well as concerns about an outbreak of mpox, the infectious disease formerly called “monkeypox” that circulated among men who have sex with men and gender diverse people last summer.

Fortunately, the Jynneos mpox vaccine appears to be 66% to 86% effective against symptomatic illness in people who are fully vaccinated, according to three large studies published last month.

In the year since the Centers or Disease Control and Prevention (CDC) shared its first mpox report, there have been more than 30,000 cases of the disease in the U.S., with the biggest outbreaks occurring during the summer. Public health officials now warn this season could see similar spikes in regions where vaccination coverage is spotty. Between March and May, for example, there was an outbreak of at least 24 cases in Chicago and its suburbs, including among some vaccinated people.

Despite the outbreak, Demetre Daskalakis, MD, MPH, deputy coordinator of the White House National Mpox Response, said there is “currently no change in vaccine strategy,” which consists of two doses spaced 28 days apart. While scientists are investigating the Chicago cases, he said there is no evidence that a third dose would improve or extend immunity for fully vaccinated people. But getting more people vaccinated for the first time would move the needle.

“It is important that to say that without renewed prevention efforts, especially vaccination, we are definitely at risk of a resurgence—in fact, a substantial risk of resurgence of mpox,” Daskalakis said in a press call last month.

New Research Confirms Mpox Vaccine Works

A collection of three large studies published last month indicate that two doses of the Jynneos vaccine is far more effective at preventing mpox symptoms than one shot. A person is considered fully vaccinated if they have two doses.

One study published by the CDC in the Morbidity and Mortality Weekly Report reported on cases in New York state while another from the agency assessed cases in 12 U.S. jurisdictions. A third, published in the New England Journal of Medicine (NEJM), included nationwide data from electronic health records.

These studies found that vaccine efficacy ranged from 36% to 86% for people vaccinated with one dose and 66% to 86% for two doses. When immunocompromised individuals were excluded from the NEJM study, the efficacy improved from 66% to 76%, on par with the other studies.

These estimates account for people who had symptomatic mpox and were diagnosed or treated by a medical professional.

Related: What Are the Symptoms of Mpox?

When analyzed together, Daskalakis said the data indicates that people vaccinated against mpox are 80% to 86% less likely to develop symptomatic illness from the virus than unvaccinated high-risk people.

“One dose is good and two doses is better,” Daskalakis said. “This definitely tells us that these vaccines work, and that our strategy of vaccinating people and getting both doses really remains core to our efforts to prevent mpox.”

No Change to Vaccination Strategy

Last year, a prolonged shortage of the Jynneos vaccine—the only FDA-approved mpox prevention option—meant many of the estimated 1.7 million gay and bisexual men most at risk for mpox couldn’t get the shots they’d need for full protection. To stretch the supply, health officials said providers could give the shot under the skin, rather than into the muscle, allowing them to vaccinate five people with a single-dose vial.

Related: Mpox Vaccines Will Now Be Administered Intradermally. Here’s What That Means

The research shared last month shows that intradermal and subcutaneous injections were both protective. Now, there is no shortage of the vaccine, but the CDC said providers can choose to give the vaccine via either route.

The data shows the administration routes are “interchangeable,” Amesh Adalja, MD, senior scholar at the Center for Health Security at the Johns Hopkins Mailman School of Public Health in Baltimore, told Verywell. The choice depends on the administrator’s level of comfort and experience as well as the patient’s preference.

Because the mpox vaccine program is less than a year old, there isn’t much data yet on how well the shot holds up over time. Still, two doses of the vaccine appears sufficient to offer long-term protection from symptomatic illness and decrease the risk of severe outcomes.

Most of the 24 patients reported sick with mpox in Chicago were at least partially vaccinated, according to Daskalakis. These patients had relatively mild disease, with smaller lesions and very few lesions in the mucosal lining of the rectum.

Related: How to Treat an Mpox Rash

“No one has ever claimed that the mpox vaccine is 100% efficacious,” Adalja said. “People are in alarm about the Chicago cases, but all those cases were very mild, which tells you that the if the vaccine wasn’t able to stop infection in all those Chicago cases in vaccinated individuals, it definitely attenuated the illness and made it milder.”

There are still many unknowns about those cases. For instance, the patients may have had prolonged or particularly close contact with an infected individual, or they may have received a compromised vaccine lot, or they could have become infected with a mutated version of the mpox virus which can better overcome immunity, Braden said.

The CDC is currently conducting three studies to better understand the duration of immunity from vaccination, according to Braden. If it turns out that there are more breakthrough infections among vaccinated people than expected based on the vaccine efficacy, officials may revisit the dosing strategy. But for now, there is no indication that more doses will give better protection.

“There’s a tendency after COVID to just reflexively say, ‘let’s just do another booster.’ But that’s not really how booster decision policy is made,” Adalja said. “Before you even start thinking about what to do about the fully vaccinated and if they’re susceptible, there’s lower hanging fruit that needs to be addressed first—that’s the unvaccinated high-risk population and single vaccinated ones who need to get their second dose.”

Who Is Eligible for Mpox Vaccination?

The CDC said men who have sex with men and gender diverse people should be vaccinated for mpox if they:

• Have confirmed or suspected exposure to mpox within in the last two weeks

• Have a new diagnosis of one or more sexually transmitted diseases

• Have more than one sex partner

• Had sex at a commercial sex venue or in exchange for money

• Have HIV or other causes of immune suppression
  

Now, the task is to increase the number of vaccinated individuals, said Christopher Braden, MD, CDC Mpox Response Incident Manager. While 1.2 million vaccine doses have been administered so far, he said fewer than a quarter of the people at highest risk are fully vaccinated.

Increasing vaccine coverage will likely be the most important factor for preventing mpox, Daskalakis said. Even an imperfect vaccine, when given to those at highest risk, will keep most people from getting seriously sick and will greatly reduce the incidence of illness.

“Jurisdictions with less than 35% coverage of at least one vaccine dose are more likely to have outbreaks which could be as large, or even larger than, those in 2022,” Braden said.

Mpox cases were higher and vaccination rates lower among Black and Hispanic men than White men throughout 2022.

Prior Infection Matters, Too

Immunity from prior infection may also shape the location and intensity of outbreaks, said Jeffrey Klausner, MD, MPH, a clinical professor of population and public health science at the University of Southern California.

“While it is encouraging to know the vaccine offers strong protection, what we also need to know is what percent of the population, particularly the core population of men who have sex with men, who attend circuit parties, sex clubs, or engage in group sex have immunity from prior infection,” Klausner told Verywell in an email.

More than 70% of the confirmed and probable cases in 2022 were among people living in urban areas. Relatively high levels of natural immunity and vaccination in urban areas mean the outbreaks there may be better contained this year than last, Klausner said.

“I have also heard that among the core population of men in urban areas, men well-connected within the gay community, who engage in sex with multiple partners, attend group sex events, etc., mostly all are vaccinated,” Klausner said. “Immunity in that core population is key because that’s the population that contributes disproportionately to the spread of sexually transmitted infections like mpox through tight, interconnected sexual networks. A highly immune core population serves a damper or shield to wider spread within the larger community of men who have sex with men.”

People living in rural areas, who are not “closely linked to the gay community,” who remain unvaccinated, or who were not infected last year may be the most susceptible to the disease this year, he added.

Immunity is just one of several tools high-risk people can use to protect themselves, Daskalakis emphasized. Individuals should avoid contact with people who have mpox, and should seek medical attention and get tested if they have a rash, even if they have been vaccinated or previously infected.

Read Next: How Does Mpox Spread?