CDC: Mpox Vaccine Is Effective Regardless of Administration Route

Intradermal administration of the JYNNEOS mpox vaccine is as effective and safe as subcutaneous administration, according to new data released this week by the Centers for Disease Control and Prevention (CDC).

Unvaccinated people were nearly 10 times as likely to be infected with the mpox virus compared with those who had completed the two-dose series of the vaccine, and the reporting rates for adverse health events are similar for both routes of administration.

"There has been some hesitancy around intradermal vaccination, so I think these data are very reassuring," said Timothy Wilkin, MD, an infectious disease expert at Weill Cornell Medicine in New York City, in an interview with Medscape Medical News. He was not involved with the research.

JYNNEOS, a vaccine for protection against smallpox and monkeypox, was approved by the US Food and Drug Administration in 2019 and was traditionally given via subcutaneous administration. In August, the FDA issued emergency-use authorization for intradermal dosing of the vaccine to help extend the supply of the in-demand vaccine.

Since the beginning of the mpox outbreak in May, there have been 29,711 reported cases in the United States, and more than 1.1 million doses of the JYNNEOS mpox vaccine have been administered. Mpox incidence in the US has fallen by more than 95% from the peak of the outbreak in July, said Jonathan Mermin, MD, MPH, director of the National Center for HIV, Viral Hepatitis, STD, and TB Prevention at the CDC. Currently, there are fewer than 10 new cases every day in the US.

Mermin told Medscape that both behavioral changes to mitigate spread of the virus as well as vaccination led to this large reduction, but he also noted that improved access to mpox testing and vaccination for communities most affected by the outbreak — gay and bisexual men and transgender persons — is necessary to maintain progress. "If these efforts do not continue, declines could be reversed and we could find ourselves responding to a resurgence of infection," he said.

Vaccination Protects Against Mpox Infection

The CDC compiled data from 43 jurisdictions between July 31 and October 1 of this year to estimate overall vaccine efficacy. The analysis, released today in the CDC's Morbidity and Mortality Weekly Report (MMWR), included 9544 reported mpox cases in men aged 18-49 years old. Of these cases, 87.2% were in unvaccinated individuals. Of the mpox cases in vaccinated individuals, 61% were in people whose illness began 13 days or less after receiving one dose of the vaccine. In the 39% (392 cases) whose illness occurred at least 2 weeks after the initial dose, most (295) occurred before an individual received their second dose. Just 0.5% of all mpox cases (48) were in people who were fully vaccinated — where the illness began at least 2 weeks after the second dose.

Researchers calculated that the average mpox incidence was 7.4 times higher in unvaccinated persons than in those who had received one dose of the mpox vaccine at least two weeks earlier (incidence rate ratio [IRR], 7.4; 95% CI, 6.0 - 9.1). This is lower than estimates from an earlier analysis from the CDC, which suggested that unvaccinated individuals were on average 14 times as likely to get infected with the mpox virus compared with those who had received one dose of the vaccine.

But that estimate had a wide 95% confidence interval, ranging from 5.0 - 41.0, Mermin noted, and these new findings fall within this window. This most recent analysis used much more data, including the addition of the equivalent of over 1 million person-weeks of follow up. "Even though the point estimate is lower, it is consistent with the prior estimates because of the confidence intervals in the prior study, and because you have additional data that that makes the estimates more precise," he said.

The new analysis also found that the second dose of the vaccine conferred even more protection: unvaccinated persons were 9.6 times as likely to contract mpox compared with those who had received both doses of the vaccine at least 2 weeks earlier (IRR, 9.6; 95% CI, 6.9 - 13.2). The analysis also suggested there was no difference in efficacy between subcutaneous and intradermal administration of the vaccine.

Wilkin noted that while this data is reassuring, more research is necessary to understand vaccine efficacy between both intradermal and subcutaneous administration routes. There are ongoing studies looking into these questions, he noted. He also emphasized that further studies are necessary to understand vaccine efficacy in immunocompromised populations, like those with uncontrolled HIV. In these populations, mpox infection can be severe and even fatal.

Surveillance Supports Vaccine Safety

From May 22 through October 21 of this year, the Vaccine Adverse Event Reporting System (VAERS) received 1350 reports for the JYNNEOS vaccine, according to a second analysis published today in MMWR. The most commonly reported adverse health event for both subcutaneous and intradermal administration was injection site erythema. For doses given intradermally, other common side effects were dizziness, urticaria, injection site swelling, and syncope. For subcutaneous injections, common reported adverse health events were injection site swelling, injection site pain, pain, and erythema.

Whereas there was no difference in reporting rates for adverse events between intradermal and subcutaneous injection, vaccine administration errors were reported more often for intradermal injections. The most commonly reported error (54% of error reports) was the absence of a wheal after injection; however, Mermin noted that the CDC's clinical considerations for the use of the mpox vaccine state that a wheal is not necessary for valid administration.

In the past, many clinicians have given intradermal injections for a PPD skin test for tuberculosis, where a wheal was expected, he explained. "I think what's happened is for many clinicians, they've thought a wheal is necessary [for mpox vaccination] because it's important for the TB skin test, but it's not so," Mermin said.

Fourteen reports were classified as serious adverse health events. Nine individuals were hospitalized for myocarditis (two), pericarditis (two), appendicitis (one), aseptic meningitis (one), atrial fibrillation (one), idiopathic thrombocytopenic purpura (one), and methemoglobinemia (one).

There were two deaths reported within 2 days of vaccine administration, in males aged 37 and 58. In one case, the cause of death was listed as drowning, while the death certificate is pending on the other case. Three individuals reported injection site discoloration (one), injection site pain (one), and injection site scar (one) as representing disability or permanent damage. No serious adverse events were reported in persons younger than 18 years.

Although live, replicating smallpox vaccines like ACAM are associated with higher rates of myocarditis, safety data for JYNNEOS suggest no increased risk for myocarditis following vaccination. The VAERS myocarditis reporting rate was seven times lower than what would be expected based on the estimated background rate of condition. "Myocardititis does not seem to be a concern with this vaccine," Wilkin said.

Vaccination Efforts Remain Vital

With the decline in number of cases, thanks to both vaccination efforts and behavioral changes, federal officials recently announced they plan to let the mpox emergency declaration expire at the end of January. But both Mermin and Wilkin noted that mpox prevention efforts need to be continued to stay ahead of the virus. Communities of color tend to be disproportionally affected by mpox, so higher vaccine coverage is necessary, Wilkin said. "My hope is that this new data will be reassuring to these communities and improve vaccination rates," he added.

Mermin noted that mpox vaccination should be integrated into HIV and STI clinics as a part of routine care. "We'll also need to continue vaccine-equity activities by bringing mpox vaccination to communities and venues and events, rather than asking people to come to us to get vaccinated," he said; "We're in the fourth quarter and ahead, but the epidemic is not over."

Mermin and Wilkin report no relevant financial relationships.

MMWR. Published online December 8, 2022. Reduced Risk for Mpox, Safety Monitoring of JYNNEOS

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