mpox vaccine appears safe and effective in adolescents

Interim analysis of a US National Institutes of Health (NIH)-funded mpox vaccine trial has found the vaccine is safe in adolescents and generates an antibody response equivalent to that seen in adults. The interim results were presented last month at the IDWeek 2024 conference in Los Angeles.

Two types of the virus that causes mpox have been identified: clade I is endemic in Central Africa and can cause severe illness, while clade II, endemic in West Africa, caused the global mpox outbreak that began in 2022 and tends to result in milder illness. People with compromised immune systems, children and pregnant people are especially vulnerable to severe mpox, regardless of the virus clade, with a large proportion of people affected in the current clade I outbreak in the Democratic Republic of the Congo and other African countries being adolescents and children.

The modified vaccinia Ankara-Bavarian Nordic (MVA-BN) vaccine is approved in several countries for the prevention of mpox and smallpox in adults, but insufficient data are available to support licensure for people younger than 18 years. With this in mind, NIH’s National Institute of Allergy and Infectious Diseases (NIAID) is sponsoring a mid-stage clinical trial in the United States to evaluate the safety and immune response generated by two doses of MVA-BN in adolescents aged 12–17 years, comparing outcomes to those in adults aged 18–50 years.

In a planned interim analysis, study investigators measured antibody levels two weeks after the second dose (study day 43) and monitored safety through 180 days after the second dose (study day 210). The analysis showed that the MVA-BN vaccine generated antibody levels in adolescents equivalent to those observed in adults at day 43 and found that the vaccine was well tolerated through study day 210. The overall frequency of adverse events was comparable between the study groups. Reports of dizziness were more common in adolescents than adults, but similar to the frequency of dizziness reported when other vaccines are administered in adolescents.

According to the study team, the interim data support the safety and quality of the immune response generated by the MVA-BN vaccine in adolescents — findings relevant to the United States and other areas where mpox cases have occurred. The authors underscored the need to evaluate the MVA-BN vaccine in younger children to extend the evidence base to all people affected by mpox.

Image caption: Colourised transmission electron micrograph of mpox virus particles (light green) found within VERO E6 cells (teal). Image credit: NIAID.