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CDC: Vaccination Key to Shielding U.S. from Potent Mpox Virus

A significant outbreak of a more transmissible and severe mpox strain in Africa has led to urgent calls for at-risk gay and bisexual men to receive both doses of the Jynneos vaccine.

Amid rising concerns about a severe mpox outbreak in central Africa and an increase in U.S. cases since early last year, the CDC reported Thursday that the mpox vaccine provides long-term protection.

A separate CDC report noted that while new mpox infections in the U.S. have remained low—about 60 cases per week compared to 3,000 weekly cases at the peak in summer 2022—cases have increased nationally compared to the same period in 2023, with a sharp rise in New York City.

The CDC confirmed that individuals who received two doses of the Jynneos mpox vaccine are protected and currently do not require a booster.

Public health experts express concern that the summer travel season and upcoming LGBTQ Pride festivals could increase sexual connectivity among gay and bisexual men, potentially accelerating mpox transmission.

Christina Hutson, chief of the CDC’s Poxvirus and Rabies Branch, emphasized to NBC News that this is not a time for complacency. The factors that have kept the U.S. outbreak in check since late 2022—vaccination, infection-driven immunity, and changes in sexual behavior—may be fragile.

"The mpox outbreak is 'poised to return under the right conditions,' stated Dr. Jeffrey Klausner, an infectious disease expert at the University of Southern California. Infections spread rapidly across the globe starting in May 2022, causing significant distress among gay and bisexual men before subsiding worldwide.

However, the vaccination rate among at-risk American gay and bi men is currently inadequate to ensure long-term protection for this vulnerable group.

'Vaccination is a critical way to help protect yourself and others,' emphasized Christina Hutson. 'It’s important that people at risk for mpox exposure who have not previously recovered from mpox — including certain gay, bisexual, and other men who have sex with men — complete the two-dose Jynneos vaccination series.'

On May 16, the CDC published a concerning report about nearly 20,000 recent cases of clade 1 mpox documented in the Democratic Republic of Congo (DRC) since January 2023. This viral clade appears to be more transmissible and causes higher rates of severe disease and death compared to clade 2 mpox, which drove the recent global outbreak. Five percent of those diagnosed with clade 1 in the DRC have died, compared to just 0.2% of the 96,000 people in the global clade 2 outbreak."

To date, there have been no reports of clade 1 mpox cases outside the African nations where the virus has been endemic for decades. In December, the CDC first warned healthcare providers about the possibility of this more dangerous viral clade reaching the United States.

"We're facing a potentially dangerous situation," said Ira Longini, a biostatistician at the University of Florida, regarding the potential global spread of clade 1. "But we really don’t know."

Dr. Boghuma Titanji, an assistant professor at Emory University School of Medicine, suggested that the Jynneos vaccine is likely to provide protection against clade 1. The Jynneos vaccine is approved in the U.S. for mpox regardless of the clade. "It’s reasonable to anticipate some immune cross-protection," she said, referring to immunity from both the vaccine and previous infection with clade 2.

A clinical trial of the Jynneos vaccine is ongoing among healthcare workers in the DRC and is not expected to be completed until the end of next year. However, the vaccine has not been widely deployed in the DRC, which Dr. Titanji views as a missed opportunity to control the outbreak and prevent the global spread of clade 1.

Dr. Placide Mbala, head of epidemiology and global health at the University of Kinshasa in the DRC, noted that there has been "a lot of interest" from global public health leaders in providing assistance, "but we are still waiting for concrete action," he said.

During the clade 2 outbreak, mpox has predominantly spread through sexual contact, both oral and anal, between men, and has also disproportionately affected transgender people. The virus has not spread easily through air, surfaces, healthcare settings, or casual, nonsexual contact.

In an interview earlier this year, CDC epidemiologist Dr. Jennifer McQuiston said mpox "needs to be thought about as an STD. The good news is it’s a preventable STD."

Multiple studies on the real-world use of the Jynneos vaccine suggest that receiving both doses is associated with a 66% to 89% lower infection rate, while one dose is 36% to 75% effective.

In one of the new CDC reports, investigators analyzed demographic data and clinical characteristics of 271 mpox cases among fully vaccinated individuals in 27 U.S. jurisdictions from the outbreak's start in May 2022 to May 2024.

A CDC official reported that the agency estimates only 0.1% of the fully vaccinated population developed a breakthrough infection, with such cases accounting for 0.8% of all 32,819 mpox diagnoses nationally. Additionally, 13% of mpox cases occurred in partially vaccinated individuals.

The CDC estimates that 2 million men in the U.S. are at significant risk of mpox due to having multiple male sexual partners or being HIV positive, a group with a high mpox diagnosis rate. Among at-risk gay and bisexual men, only about 25% are fully vaccinated, and an additional 14% have received one Jynneos shot.

Consistent with previous studies, the CDC’s new report found that full vaccination is associated with less severe disease and a lower hospitalization rate. Notably, all 56 mpox deaths occurred among unvaccinated people.

Recent research has raised concerns that antibodies from the Jynneos vaccine wane over time. However, CDC investigators found that the vaccine’s protection against infection hasn't diminished, possibly due to innate or cell-based immunity.

To address the temporary vaccine shortage in mid-2022, U.S. health authorities shifted from traditional subcutaneous (under the skin) shots to intradermal (within the skin) shots, allowing for a lower dose. The new CDC report found no evidence that this change compromised vaccine effectiveness.

A separate CDC analysis of recent U.S. mpox diagnoses shows that the virus remains predominantly seen among gay and bisexual men, with just 0.4% of recent cases in people under 18.

Between October and April, 1,802 probable and confirmed mpox cases were reported to the CDC from 42 jurisdictions, averaging 59 cases per week. In contrast, the outbreak's peak in mid-July to late August 2022 saw 2,000 to 3,300 weekly cases. However, nearly 750 mpox cases have been reported through mid-April this year, more than double the number for the same period in 2023. New York City has experienced a nearly five-fold increase in cases, with 191 reported thus far in 2024.

Infectious disease experts can only speculate about how a U.S. outbreak of clade 1 might unfold. Improved healthcare quality might result in a lower death rate than in the DRC, where most global clade 2 deaths occurred among those with compromised immune systems, primarily due to untreated HIV.

Other uncertainties include the potential transmission patterns of clade 1 in Western nations. The DRC outbreak has involved men with multiple male sex partners, female sex workers and their contacts, and has seen significant transmission in children, unlike the global clade 2 outbreak. Epidemiologists have noted sustained person-to-person transmission in one DRC province, primarily among adults.

While clade 1 has largely spread through sexual contact, it shows signs of transmitting more readily without sexual contact compared to clade 2, though still requiring close personal contact, often in household settings. The CDC report suggests substantial transmission in children may be driven by encounters with wild animals in rural areas.

CDC officials believe widespread clade 1 transmission among U.S. children is unlikely due to the absence of an animal reservoir, less crowded living conditions, and better hygiene.

Dr. Boghuma Titanji from Emory University expressed concern that interest in mpox vaccination among gay and bisexual men waned after the outbreak was no longer a primary concern. In fall 2022, the nation’s weekly distribution rate of Jynneos shots dropped in line with the mpox case rate. Since then, there has been no significant progress in increasing the proportion of at-risk gay and bisexual men receiving both doses of the Jynneos vaccine.


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