As a deadly mpox outbreak continues to spread in Africa and other parts of the world, questions are emerging about whether some vulnerable people in the United States may need booster doses of the mpox vaccine.
Last month, the World Health Organization declared an mpox outbreak in the Democratic Republic of Congo to be a public health emergency of international concern, and the outbreak has spread to a growing number of countries in Africa.
Mpox, formerly known as monkeypox, is characterized by two genetic clades, I and II. This outbreak involves a subtype called clade Ib, a relatively new strain that causes more severe disease.
A global outbreak in 2022 was caused by clade IIb mpox, spread primarily through sexual contact. More than 30,000 people in the United States were infected in that outbreak, which was also declared a public health emergency of international concern. But vaccination with the Jynneos mpox vaccine helped slow transmission to the point where the virus was almost gone in the US.
Jynneos, developed to help protect against mpox and smallpox infections, is recommended in two doses given four weeks apart.
But amid the new outbreak, public health experts are debating whether people most at risk of illness may need booster vaccinations to maintain robust immunity, as data suggests that it may wane over time.
“I’ve literally had people call me and say, ‘Should I get a booster, because it’s been two years?’ and my suggestion right now is, I don’t see an obvious reason for a booster,” said Dr. Ashish Jha, dean of the Brown School of Public Health, who served as the White House’s Covid-19 response coordinator and assisted the administration on other public health concerns including the mpox outbreak in 2022.
“My view is, when we see clade Ib arrive in the US, if it starts spreading quickly, if you start seeing a lot of people who were previously vaccinated getting infected, then a booster campaign makes sense,” he said. “I just don’t think we have enough data right now to recommend boosters for everybody who got previously vaccinated.”
US in ‘preparedness’ mode, as risk still low
Local health departments in the US have not seen high numbers of mpox cases this year, said Lori Tremmel Freeman, chief executive officer for the National Association of County and City Health Officials.
“But we’re remaining diligent in the face of the global outbreaks and constantly in the preparedness stance,” she said.
The overall risk of mpox to the United States remains low, according to the US Centers for Disease Control and Prevention.
“Casual contact in travel settings, for example, as a passenger in an airplane, is unlikely to pose significant risks for transmission,” CDC spokesperson David Daigle said in an email.
Daigle added that the CDC does not recommend more than two doses of the Jynneos vaccine for the general public and said the agency will continue to monitor data on vaccination.
“Two doses of JYNNEOS vaccine may prevent mpox and can reduce the severity of symptoms in infected persons,” Daigle said. Data collected during the previous outbreak “show that breakthrough infections after the two dose JYNNEOS vaccine series were rare.”
Many public health experts emphasize that right now, their focus is on making sure those at highest risk complete the initial vaccination series.
The CDC recommends getting vaccinated against mpox if you have been exposed to the virus, if you are a man who has sex with men, or if you have had sex at a commercial venue or event, among other risk factors.
More than two vaccine doses are not recommended “unless you are a lab worker who works with live mpox and other related viruses in a research lab. Vaccine recommendations are based on the level of risk from potential exposures,” Dr. Justin Chan, director of infection prevention and control at NYC Health + Hospitals/Bellevue, said in an email.
“If we did see Clade I cases in the US, current projections by the CDC don’t expect these to result in significant spread in US communities that have more than 50% immunity amongst at-risk groups either from vaccination or previous infection,” he said. “Therefore, people who are eligible for mpox vaccination should get fully vaccinated now.”
Data suggests that a two-dose regimen of Jynneos has an effectiveness that ranges from 66% to 89%, according to the CDC.
“There still are some open research questions, including potentially in the future some need for boosters. But that’s not the focus now; the focus now is on getting people vaccinated and getting them access to the vaccine,” said David C. Harvey, executive director of the National Coalition of STD Directors.
It’s estimated that only about 25% of the US population most at risk has been fully vaccinated, according to CDC data.
“Vaccine rates in the US are unacceptably low, and it’s even lower for people coming back and completing the two doses of vaccine,” Harvey said. “The urgent priority now has to be for gay men and bisexual men and other people at risk to get the vaccine now.”
Questions around waning immunity
A new study suggests that immunity from Jynneos can wane within a year of vaccination – and due to this, there are questions around whether some people might need booster doses to maintain “robust levels” of protective immunity.
The study, which hasn’t been published yet in a peer-reviewed journal but was posted Wednesday to the server medrxiv.org, included data on 45 adults who either received a dose or two of the Jynneos vaccine or had a confirmed mpox infection at Beth Israel Deaconess Medical Center in Boston during the 2022 outbreak. None had a weakened immune system or a history of diabetes, and four were living with HIV.
Researchers from the medical center and the National Institutes of Health analyzed blood samples from each adult at the time of their vaccination or infection and then additional samples taken three months later, six months later, nine months later and one year later.
The researchers found that, among people who received two doses, vaccine-induced antibodies against the virus peaked around three weeks and then declined around a year later.
“Our data shows that the antibody responses induced by the mpox vaccine are moderate to high titer after the two-dose vaccine,” said Dr. Dan Barouch, lead author of the study and director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center. “After six to 12 months, those antibody titers wane quickly and go back almost to baseline levels at 12 months.”
The same waning immunity was seen among people who got only one dose of the vaccine.
“After one dose of the vaccine, the antibody titers are much lower and really are back at the baseline after six to 12 months,” Barouch said.
Three participants had natural immunity from a previous mpox infection, and they had high titers of mpox antibodies at three months that persisted through nine months post-infection, the researchers found.
The study’s findings parallel those of two papers released this year that suggest immune responses to the mpox virus can wane within one year after vaccination.
Barouch added there was a cluster of mpox cases in Chicago last year in which some of the people had been fully vaccinated in 2022, also suggesting waning immunity.
Preventing serious illness is top priority
The findings in the new study are “very compelling,” Jha said, and they suggest that protection against infection may have waned a lot. The question is what it means for people – especially when it comes to severe illness.
“A drop in antibody levels makes me believe that you probably do have people who may be susceptible to infection again. But if they get an infection, because they have been vaccinated, their response to that virus – the immune response – is going to be much more robust, and they will likely clear that infection more quickly,” Jha said.
“Empirically, I would think that people are going to have less severe illness,” he said. “While we would love to prevent infections, what we really care about is preventing serious illness.”
It’s unclear what these markers of immunity mean in the real world, said Jeffrey Crowley, director of the Center for HIV and Infectious Disease Policy at the O’Neill Institute for National and Global Health Law at Georgetown University Law Center.
“In the future, maybe they’ll say we need a booster dose, but I don’t think we’re there yet,” he said. “I don’t think we can dismiss that possibility. But there’s no reason to think now, because of this outbreak in Africa, that we need to get a booster dose. I would still just reinforce the message for people that didn’t get the second dose before to get their second one now.”
Bavarian Nordic, which manufactures Jynneos, noted that the last mpox outbreak that hit the United States was two years ago, and many people got vaccinated months after it began. The company said it is in contact with research groups about learning more about how long immunity lasts.
As for the current mpox outbreak, the first doses of Jynneos arrived last week in the Democratic Republic of Congo, Bavarian Nordic said in a statement Thursday. More than 250,000 doses have been shipped, and further donations of more than 500,000 doses have been pledged by other countries.
The statement added that Bavarian Nordic has ramped up production of the vaccine for the rest of the year and could supply up to 13 million doses by the end of 2025, including 2 million in 2024.
‘We’re not isolated’
It’s not clear how the outbreak in Africa may play out elsewhere, and Barouch said he hopes the emerging data on the potential of waning immunity can help inform any response efforts in the United States.
“Obviously, we’re not isolated from the rest of the world, so it’s only a matter of time before it comes here,” he said. “There has been at least one case in Sweden,” the first case identified outside Africa in the current outbreak.
There are no known cases involving the clade Ib strain in the United States, but Jha expects it to arrive eventually.
“We’re a pretty globalized country. We have lots of people who travel,” he said. “I’d be surprised if we don’t end up seeing a case at some point.”
A fact sheet on the CDC’s website notes that the agency has been “closely monitoring” the spread of mpox, specifically clade I, through wastewater testing and other tools.
And more than 1 million vials of the Jynneos vaccine have been distributed across the country since August 2022.
However, the mpox vaccines have transitioned to the commercial market, Harvey noted, and they can be expensive for those not accessing them for free through a government-supported program or through insurance coverage – costing around $300 out of pocket.
The US Health Resources and Services Administration, or HRSA, sent a letter to the Ryan White HIV/AIDS Program and HRSA-supported health center programs this month announcing renewed availability of the Jynneos mpox vaccine from the Strategic National Stockpile. The stockpile has a limited supply of doses that expire October 31, the letter said.
The letter means that “you can still get a free vaccine through a Ryan White HIV/AIDS service organization or community health center,” Harvey said.
What’s needed to prevent a US outbreak
The potential risk of clade Ib mpox still worries Harvey.
“Let’s not fool ourselves,” he said. “This is highly likely to emerge in the US. To what extent it will emerge, we do not know.”
He warned that the United States is “not ready” to adequately respond to another mpox outbreak because of a shortage of disease intervention specialists, the public health workers who investigate infectious disease outbreaks and educate the public about them.
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In a statement last month, Harvey called on Congress to invest in public health programs and specialists by allocating $322.5 million for STI prevention at the CDC and $200 million for STI clinical services at HRSA.
To prevent a clade Ib mpox outbreak in the United States, Crowley said, funding and resources should also be directed to Africa. If there are enough resources to mitigate the outbreak there, he said, it reduces the risk of mpox further spreading around the world.
“As Americans, what we should be doing now is really focusing on how we can get vaccines, therapies and resources to places in East and Central Africa,” he said. “That will make us safer and can keep this from getting to our country.”