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The modified vaccinia Ankara-Bavarian Nordic mpox vaccine (MVA-BN; Jynneos) generated robust immune responses in adolescents against mpox and was safe, according to an interim analysis of a phase II clinical trial.

Two doses of the vaccine generated geometric mean titers (GMT) in adolescents (470.3, 95% CI 422.3-523.8) that were higher than titers in adults (293.2, 95% CI 249.8-344.2), with a geometric mean titer ratio (GMTR) of 1.60 (95% CI 1.32-1.95), reported C. Mary Healy, MD of Baylor College of Medicine in Houston, during the IDWeek annual meeting in Los Angeles.

Also, on any study day, the peak antibody response among adolescents was non-inferior to that of adults. For example, on day 29 after the first dose, the GMT was 51.1 (95% CI 45.6-57.4) in adolescents and 44.4 in adults (95% CI 37.3-53.0), for a GMTR of 1.15 (95% CI 0.93-1.42).

"These findings are particularly relevant to adolescents both in the U.S., but probably particularly in areas where mpox is endemic, such as the DRC [Democratic Republic of Congo], which are affected by the ongoing outbreak," Healy said. "However, evaluations in younger children are really urgently needed to extend protection to those who are most vulnerable, particularly in the DRC."

The interim analysis is "the first data that for older children this vaccine is safe and effective," she said. "Hopefully that will open the door to other studies being done in younger age groups, so we can get licensure across the lifespan. That would be really, really important."

The NIH-funded trial was conducted at sites throughout the U.S.

There are now two FDA-approved vaccines against mpox, but only for use in adults -- and ACAM2000. An FDA-approved mpox vaccine for adolescents is an unmet public health need, Healy said. Currently the MVA-BN vaccine is only available in the U.S. under emergency use authorization (EUA) as younger than 18 years of age considered at high-risk for mpox.

"Unfortunately, children are always lagging behind in having vaccine studies performed, even though they are very often the most vulnerable," Healy commented earlier in a press briefing, hosted by IDWeek organizers.

Solicited systemic reactions were similar between adolescents and adults for each vaccine dose, and occurred at similar frequencies, Healy told attendees. Fatigue, headache, and myalgia were the most common solicited systemic reactions. Local reactions -- more common in both groups after the second dose -- included pain, erythema, induration, and pruritus and affected about 80% to 90% of participants.

Vaccine-related dizziness occurred in 3% of adolescents versus none of the adults, but did not result in syncope or require medical attention or discontinuation of the vaccine.

The MVA-BN vaccine was generally well-tolerated in both groups, Healy said. "We have a lot of safety data even before this study," she commented. "We knew that there were no safety signals down to a few months of age."

For the interim analysis of the open-label trial, adolescents ages 12 to 17 years and adults ages 18 to 50 years received two doses of MVA-BN vaccine administered subcutaneously 28 days apart. The modified intention-to-treat population included 304 adolescents and 208 adults.

Researchers evaluated safety 180 days after the second dose (day 210) and immunogenicity through 14 days after the second dose (day 43).

The mean age of adolescents enrolled in the study was 14 and about half of the enrolled adolescents were ages 12 to 14. Most adolescents and adults were non-Hispanic white and 10% to 15% were Black or African American. No adolescents were HIV positive, versus 2% of the adult study population.

The trial excluded adolescents or adults who had previously received a smallpox or mpox vaccine, had COVID-19 infection or received a COVID-19 vaccine within the previous month, or recent or current use of immunosuppressive medication. The study also excluded people with an immunocompromising condition, a history of myocarditis, or other significant medical comorbidity.

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