The impact of antigenic distance on Orthopoxvirus Vaccination and Mpox Infection for cross-protective immunity

Immunological memory mediates rapid protection following infection or vaccination including heterologous exposure. However, cross-reactive memory responses in humans remain poorly characterized. We explored the longevity and specificity of cross-protective responses to orthopoxviruses through smallpox vaccination and Mpox virus (MPXV) infection. Smallpox vaccination using Vaccinia virus (VACV)-based vaccines provides a unique opportunity to study long-term cross-protective immunity without antigen re-exposure. We assessed systemic and mucosal responses in four human cohorts, including first-(Dryvax) and/or third-generation (JYNNEOS) smallpox vaccine recipients (vaccinated 1 week-80 years ago), along with Mpox-infected individuals. First-and third-generation smallpox vaccines elicited strong VACV- and MPXV-specific antibodies. VACV-neutralizing antibodies persisted for decades in first-generation vaccine recipients and were further enhanced after JYNNEOS vaccination. However, despite the high levels of anti-MPXV-specific antibodies in the plasma, cross-neutralization activity was directly correlated with the antigenic distance. Higher neutralization was observed for the cowpox virus (CWPXV) than for MPXV, which showed lower antigenic conservation with VACV. Similarly, Mpox-infected patients had lower neutralization titers for VACV than for CWPXV. Individuals who received vaccination boosters showed more robust, diverse, and prolonged cross-neutralizing responses. Long-term memory analysis revealed an increase in neutralization capacity for VACV over decades, with 80-years-old displaying the most robust humoral response, although this trend was not observed for cross-reactive antigens. Finally, T-cell reactivity to VACV and MPXV epitopes was detected decades post-vaccination, suggesting a role of long-lasting cross-reactive T-cell memory responses in vaccine efficacy. Our findings underscore the pivotal influence of antigenic distance on vaccine effectiveness with implications for cross-protective vaccine design. ### Competing Interest Statement Alessandro Sette is a consultant for Gritstone Bio, Flow Pharma, Moderna, AstraZeneca, Qiagen, Fortress, Gilead, Sanofi, Merck, RiverVest, MedaCorp, Turnstone, NA Vaccine Institute, Emervax, Gerson Lehrman Group and Guggenheim. Alba Grifoni is a consultant for Sanofi and Pfizer. LJI has filed for patent protection for various aspects of T cell epitope and vaccine design work. All other authors declare no competing interests. ### Funding Statement SBO and CL secured funding for this study. VSM is supported by the CAPES-YALE Fellowship. Brazilian cohorts set up, sample collection and processing was supported in part by grants from Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro/FAPERJ and from UFRJ. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The Human Research Protection Program Institutional Review Board of Yale University gave ethical approval for this work (IRB protocol ID 2000033415). The Research Ethics Committee of the Hospital Universitario Clementino Fraga Filho/Universidade Federal Rio de Janeiro gave ethical approval for this work (protocol number CAAE 62281722.5.0000.5257). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.