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The CDC encouraged populations at risk of contracting mpox, formerly known as monkeypox, to get fully vaccinated against the disease as part of preparations for summer during a briefing Thursday morning.

Demetre Daskalakis, MD, the White House National Mpox Response Deputy Coordinator, noted that with summer gatherings on the horizon, mpox vaccination should be considered, as should overall sexual health and wellness.

"Getting 'summer ready' means mpox vaccination, but that's not all it means -- it also means to be up to date on all of your sexual health, and that includes HIV and STIs [sexually transmitted infections] like syphilis, gonorrhea, and chlamydia," he said.

"Simply put, higher vaccination rates equal lower risk for an outbreak. So even though no vaccine is perfect, even in perfect immunity in many people, there still needs [to be] a lot of immunity in the population," he continued. "Now is the time to get vaccinated. If you didn't get your first dose, get it, and if you didn't get your second dose, get back, seek healthcare, and get tested if you have a rash, even if you have been previously vaccinated or had mpox in the past."

The briefing came after a recent outbreak of mpox in Chicago, in which many of the patients had received either partial or full vaccination with the modified vaccinia Ankara-Bavarian Nordic vaccine (Jynneos). As of Monday, no patients had been hospitalized.

Daskalakis said that these cases could be due to a number of factors, including vaccine effectiveness degradation, changes within the current virus, or even errors that occurred prior to or during administration.

Studies Show Vaccine Protection

Despite these cases, the vaccine has proven to be effective at preventing mpox, particularly when provided in two doses, according to a nationwide case-control study published in the .

The estimated adjusted vaccine effectiveness was 66.0% (95% CI 47.4-78.1) in those who received two doses, and 35.8% (95% CI 22.1-47.1) in those who only received one dose, reported Leora R. Feldstein, PhD, of the Mpox Emergency Response Team at the CDC, and co-authors.

"We found that two doses of vaccine, including those administered subcutaneously, intradermally, or heterologously, provided the highest protection against mpox disease," the group wrote. "Furthermore, our finding that vaccine effectiveness was higher among persons who received two doses, rather than one dose, highlights the importance of following the approved dosing schedule."

Patients who were not immunocompromised saw an adjusted vaccine effectiveness of 40.8% (95% CI 24.8-53.4) with a single dose and 76.3% (95% CI 57.7-86.8) with two doses.

Effectiveness rates with the vaccine decreased among male patients ages 18 to 49 who did not receive the smallpox vaccine ACAM2000. Among that population, effectiveness was 35.5% (95% CI 19.1-48.6) for those who received one dose and 58.7% (95% CI 33.9-74.3) for those who received both doses.

The CDC also released two reports on the effectiveness of the mpox vaccine in the Morbidity and Mortality Weekly Report. One study from the showed an adjusted vaccine effectiveness rate of 88.5% (95% CI 44.1-97.6) when both doses were received. The second study, using data from , showed an effectiveness rate of 85.9% (95% CI 73.8-92.4) with full vaccination.

Feldstein and colleagues noted that the vaccine should still be considered for any patients at risk of contracting mpox, which is not limited by sexual orientation or gender identity.

For this study, Feldstein's group used data from the Epic Cosmos electronic health records database from August to November 2022. They included 2,193 case patients and 8,319 control patients, 25 and 335 of whom received two doses of the mpox vaccine and 146 and 1,000 of whom received one dose.

Of the two groups, 89.2% identified as men, 9.1% identified as women, 1.0% identified as transgender women or men, and 0.7% as another gender identity. Among the case patients, 36% were Black, while 55.5% of the control group was white. The majority of both groups were immunocompromised.

Feldstein and colleagues noted that the data used in the study only reflects members of the population who have utilized healthcare services over the last 3 years, meaning these findings may not be fully applicable to a wider population. Furthermore, some data, including information regarding sexual orientation, may be inconsistent in the database used.

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