Early evaluation of the safety, reactogenicity, and immune response after a single dose of modified vaccinia Ankara–Bavaria Nordic vaccine against mpox in children: a national outbreak response

Background: In response to a national mpox (formerly known as monkeypox) outbreak in England, children exposed to a confirmed mpox case were offered modified vaccinia Ankara–Bavaria Nordic (MVA-BN), a third-generation smallpox vaccine, for post-exposure prophylaxis. We aimed to assess the safety and reactogenicity and humoral and cellular immune response, following the first reported use of MVA–BN in children. Methods: This is an assessment of children receiving MVA–BN for post-exposure prophylaxis in response to a national mpox outbreak in England. All children receiving MVA–BN were asked to complete a post-vaccination questionnaire online and provide a blood sample 1 month and 3 months after vaccination. Outcome measures for the questionnaire included reactogenicity and adverse events after vaccination. Blood samples were tested for humoural, cellular, and cytokine responses and compared with unvaccinated paediatric controls who had never been exposed to mpox. Findings: Between June 1 and Nov 30, 2022, 87 children had one MVA–BN dose and none developed any serious adverse events or developed mpox disease after vaccination. Post-vaccination reactogenicity questionnaires were completed by 45 (52%) of 87 children. Their median age was 5 years (IQR 5–9), 25 (56%) of 45 were male, and 22 (49%) of 45 were White. 16 (36%) reported no symptoms, 18 (40%) reported local reaction only, and 11 (24%) reported systemic symptoms with or without local reactions. Seven (8%) of 87 children provided a first blood sample a median of 6 weeks (IQR 6·0–6·5) after vaccination and five (6%) provided a second blood sample at a median of 15 weeks (14–15). All children had poxvirus IgG antibodies with titres well above the assay cutoff of OD_450nm 0·1926 with mean absorbances of 1·380 at six weeks and 0·9826 at 15 weeks post-vaccination. Assessment of reactivity to 27 recombinant vaccina virus and monkeypox virus proteins showed humoral antigen recognition, primarily to monkeypox virus antigens B6, B2, and vaccina virus antigen B5, with waning of humoral responses observed between the two timepoints. All children had a robust T-cell response to whole modified vaccinia Ankara virus and a select pool of conserved pan-Poxviridae peptides. A balanced CD4⁺ and CD8⁺ T-cell response was evident at 6 weeks, which was retained at 15 weeks after vaccination. Interpretation: A single dose of MVA–BN for post-exposure prophylaxis was well-tolerated in children and induced robust antibody and cellular immune responses up to 15 weeks after vaccination. Larger studies are needed to fully assess the safety, immunogenicity, and effectiveness of MVA–BN in children. Our findings, however, support its on-going use to prevent mpox in children as part of an emergency public health response. Funding: UK Health Security Agency.