U.S. health authorities at the CDC indicate that additional mpox vaccine boosters aren't required at the moment, as rumors swirl concerning diminishing immunity levels.
Last month, the World Health Organization marked an mpox outbreak in the Democratic Republic of Congo as a significant global health issue, and the illness has been spreading to an increasing number of countries across Africa.
Mpox, previously known as monkeypox, is renowned for two genetic clades, I and II. This recent outbreak involves the Ib subtype, a relatively latest strain that is associated with more severe symptoms.
A world-wide spread of mpox in 2022 was triggered by the IIb subtype, primarily transmitted through sexual contact. Over 30,000 people in the United States fell ill as a result, leading to a global health alert. However, vaccination with the Jynneos mpox vaccine helped curtail the illness in the U.S. to the point where it was almost eradicated.
Jynneos, designed to protect against mpox and smallpox, calls for two doses administered four weeks apart.
As the new outbreak spreads, experts in public health are discussing the need for booster shots for those most susceptible to the disease. Data indicates that immunity may weaken over time, necessitating additional vaccinations for boosters.
"I've had individuals contact me, asking if they need a booster, given it's been two years, and my advice at this time is, I don't see a compelling 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 advised on other public health concerns, including the mpox outbreak in 2022. "However, we should reconsider booster shots if Clade Ib arrives in the U.S. and starts spreading rapidly, resulting in significant infections among vaccinated individuals."
United States on alert, but risk remains low
In the U.S., health departments have not seen high mpox case numbers this year, said Lori Tremmel Freeman, CEO of the National Association of County and City Health Officials. "However, we're being vigilant in the face of global outbreaks and are constantly in a preparedness stance," she said.
According to the US Centers for Disease Control and Prevention (CDC), the overall risk of mpox in the U.S. continues to remain low. "Intermediate contact during travel, such as as a traveler on 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 stated that the agency will monitor vaccination data. "Two doses of JYNNEOS vaccines can prevent mpox and can reduce the severity of symptoms in infected individuals," Daigle said. Data gathered during the previous outbreak "indicated that breakthrough infections after the two-dose JYNNEOS vaccine series were uncommon."
Experts in public health advocate that the primary focus should be on ensuring that those at the highest risk complete their initial vaccination series.
The CDC recommends receiving the mpox vaccine if you have been exposed to the virus, if you are a man who has sex with men, or if you have had sex in a commercial setting or event, among other at-risk factors.
Additional 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 Clade I cases emerge in the U.S., current CDC projections don't predict these will result in significant spread in U.S. communities that have more than 50% immunity amongst at-risk groups either through vaccination or previous infection," he said. "Therefore, people who are eligible for mpox vaccination ought to be fully vaccinated now."
Data indicates that a two-dose regimen of Jynneos has an effectiveness that ranges from 66% to 89%, according to the CDC.
"Still, there are ongoing research questions, including potential future needs for boosters. However, that's not the main focus now; the main focus now is on vaccinating people and providing them with 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 U.S. population most at risk has been fully vaccinated, based on CDC data.
"Vaccination rates in the U.S. remain unacceptably low, and it's even lower for those who return for the second dose of the vaccine," Harvey said. "The immediate priority now should be for gay men and bisexual men and other at-risk individuals to obtain the vaccine now."
Research analysts from the medical center and the National Institutes of Health examined blood specimens from each adult at the point of vaccination or infection and subsequently additional samples taken three months later, six months later, nine months later, and a year later.
They ascertained that, among individuals who received two doses, vaccine-triggered antibodies against the virus peaked around three weeks and then declined substantially over a year later.
"Our data indicates that the antibody responses elicited by the mpox vaccine are of moderate to high titer following the two-dose vaccine," stated Dr. Dan Barouch, the main 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, these antibody titers diminish rapidly and return nearly to their initial levels at 12 months."
The same trend of waning immunity was observed among people who received only one dose of the vaccine.
"After a single dose of the vaccine, the antibody titers are significantly lower and essentially revert to baseline levels after six to 12 months," Barouch stated.
Three persons had natural immunity from a prior mpox infection, and they exhibited high titers of mpox antibodies at three months that persisted through nine months post-infection, according to the researchers' findings.
The study's results align with those of two papers published this year that suggest immune responses to the mpox virus can ** decline within one year** following vaccination.
Barouch also mentioned a cluster of mpox cases in Chicago last year during which "some of the people had been fully vaccinated" in 2022, further supporting the theory of waning immunity.
Preventing serious illness is the primary concern
The findings from the newer study are "extremely compelling," Jha said, implying that protection against infection may have decreased significantly. The question is what it means for people, particularly in terms of severe illness.
"A decrease in antibody levels suggests that there may be individuals who are susceptible to infection again," Jha said. "However, if they contract an infection due to vaccination, their immune response to that virus – the immune response – will be much more robust, and they will likely clear the infection more quickly."
"Empirically, I would think that people would have less severe illness," he said. "While we strive to prevent infections, what we truly aim to achieve is to prevent serious illness."
It's unclear what these markers of immunity imply in real-world scenarios, stated 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, they may suggest the need for a booster dose," he said. "However, I don't think we're there yet. I don't think there's any reason to believe that, due to this outbreak in Africa, we need to obtain a booster dose right now. I would still encourage people who haven't received their second dose to do so now."
Bavarian Nordic, the manufacturer of Jynneos, noted that the last mpox outbreak that affected the United States was two years ago, and many people were vaccinated months after it began. The company stated that it is communicating with research groups to learn more about how long immunity lasts.
As for the current mpox outbreak, the first doses of Jynneos arrived in the Democratic Republic of Congo last week, Bavarian Nordic stated in a press release on Thursday. More than 250,000 doses have been distributed, and further donations of more than 500,000 doses have been pledged by other nations.
The press release added that Bavarian Nordic has increased production of the vaccine for the remainder of the year and could provide up to 13 million doses by the end of 2025, including 2 million in 2024.
‘We’re not isolated’
It's uncertain how the outbreak in Africa may develop elsewhere, and Barouch expressed hope that the emerging data on the potential for waning immunity could help inform any response efforts in the United States.
"Obviously, we're not isolated from the rest of the world," he said. "So, it's only a matter of time before it reaches here. There has been at least one case in Sweden," the first case identified outside Africa in the current outbreak.
There are no reported cases involving the clade Ib strain in the United States, but Jha anticipates that it will arrive eventually.
"We're a rather globalized nation. We have numerous individuals who travel," he said. "I'd be surprised if we don't encounter a case at some point."
A fact sheet on the CDC's website states that the agency has been "closely monitoring" the spread of mpox, specifically clade I, using wastewater testing and various other methods.
And more than 1 million vials of the Jynneos vaccine have been disseminated across the country since August 2022.
However, the mpox vaccines have transitioned to the commercial market, Harvey noted, and they can be expensive for individuals not accessing them 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 obtain a free vaccine through a Ryan White HIV/AIDS service organization or community health center," Harvey said.
"Yo, let's keep it real, alright?" Harper shared. "The likelihood of clade Ib monkeypox popping up in the States is quite high, mate. But just how much or when, we're unsure."
He sounded the alarm, stating that the U.S. is "flat-out unprepared" to tackle another monkeypox outbreak due to a scarcity of disease control specialists. These are the folks who dig into infectious disease situations and educate the public about them.
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In a press release back in November, Harper advocated for Congress to pump funds into public health initiatives and experts. He recommended $322.5 million for sexually transmitted infection (STI) prevention at the CDC and $200 million for STI clinical services at HRSA.
According to Crowley, to curb a clade Ib monkeypox outbreak in the States, resources should also be channeled towards Africa. If we have the means to contain the outbreak there, he said, it lowers the chance of monkeypox spreading globally further.
"As citizens of America," Crowley urged, "we need to focus on sourcing vaccines, treatments, and resources to regions in East and Central Africa right now. That's our safest bet and helps keep this from reaching our shores."
The current discussion among public health experts involves the possibility of booster shots for those most susceptible to mpox, as immunity may weaken over time.
According to recent studies, the antibody responses to the mpox vaccine decline substantially over a year after the two-dose regimen, indicating the potential need for boosters in the future.
