Immunogenicity and Reactogenicity of Modified Vaccinia Ankara Pre-Exposure Vaccination Against Mpox According to Previous Smallpox Vaccine Exposure

Background: Pre-exposure vaccination with MVA-BN has been widely used against mpox to face the 2022 outbreak. Many countries have prioritised administering a single dose, especially to those historically vaccinated for smallpox, to achieve quickly adequate coverage despite low supplies. Real-life effectiveness was estimated at around 80% through epidemiological models, and the few data available on immunogenicity showed that MVA-BN elicited a moderate or low humoral response and a strong cellular one. No clinical trials are available on the vaccine efficacy, and no correlates of protection are estimated. Methods: Blood samples were collected from participants eligible for pre-exposure MVA-BN vaccination before (T1) receiving a full course of vaccine (single-shot for vaccine-experienced and two-shot for smallpox vaccine-naïve) and one month after the last dose (T2 and T3, respectively). MPXV-specific IgGs were measured by in-house immunofluorescence assay, using 1:20 as screening dilution; MPXV-specific nAbs by 50% plaque reduction neutralization test (PRNT50, starting dilution 1:10); IFN-γ-producing specific T cells to MVA-BN vaccine, by ELISpot assay. Univariate analysis was fitted by paired or unpaired t-test for IgG and nAbs, and Wilcoxon or Mann-Whitney test for T-cell response. The probability of IgG and nAb response in vaccine-experienced versus vaccine-naïve was estimated in participants not reactive at T1. McNemar test was used to evaluate the overall response, while the proportion becoming responders one month after full-cycle completion by exposure groups was compared by logistic regression and then analyzed by HIV status strata (interaction test). The Average Treatment Effect (ATE) of the difference from baseline to schedule completion according to previous smallpox vaccination was estimated after weighting for HIV using a linear predictor. Self-reports of adverse effects following immunisation (AEFIs) were prospectively collected after the first MVA-BN dose (T1). Systemic (S-AEFIs: fatigue, myalgia, headache, GI effects, chills) and local (L-AEFIs: redness, swelling, pain) AEFIs were graded as absent (grade 0), mild (1), moderate (2), or severe (3). The maximum level of severity for S-AEFIs and L-AEFIs ever experienced over the 30 days post-dose by vaccination exposure groups were analysed using a univariable multinomial logistic regression model and after adjusting for HIV status; unadjusted comparisons of the mean days by exposure group stratified by individual symptoms (both systemic and local reactions) was also performed. Findings: Among the 164 participants included, 90 (54.8%) were smallpox vaccine-experienced. Median age was 49 yrs (IQR 41-55). Among the 76 (46%) PLWH, 76% had a CD4 count>500 cells/uL. Univariate analysis showed a significant increase in IgG and nAbs titers after the administration of each dose in each group. No difference was detected in ATE for variation of IgG and nAbs from baseline to the completion of a full cycle. In the subset of participants not reactive for nAbs at T1, the seroconversion rate after full cycle vaccination was not influenced by previous smallpox vaccination (p=1.00 by Fisher’s exact test). In the PLWH stratum, a reduced proportion of participants becoming reactive for nAb was observed in primed vs. not primed (48.4% vs. 61.3%), with a lower estimated probability of seroconversion (OR 0.59; 95% CI 0.22-1.62). A significant increase in T-cell response to MVA-peptides was observed over all the time points in both primed and non-primed. Univariate analysis showed a significantly higher response in recipients of two-shot than in single-shot ones (p=0.028), confirmed by ATE with a mean difference of -2.01 log2 (p=<0.001). No significant differences in the risk of developing any AEFIs of any grade were observed between vaccine-experienced and vaccine-naïve, except for the lower risk of grade 2 (moderate) fatigue, induration and local pain in vaccine experienced [OR 0.26 (0.08-0.92), p=0.037; OR 0.30 (0.10-0.88), p=0.029 and OR 0.19 (0.05-0.73), p=0.015, respectively]. No significant differences in symptom duration were also detected among the subgroups. Interpretation: The evaluation of the humoral and cellular response one month after the completion of the vaccination cycle suggested that MVA-BN is immunogenic and that the administration of a two-dose schedule is preferable regarding the previous smallpox vaccination status, especially in PLWH. MVA-BN was safe as well tolerated, with reactogenicity higher after the first administration in vaccine-naïve individuals than in vaccine-experienced ones, but with no difference in adverse effects duration. Further studies are needed to establish better the long-term duration of immunity and correlates of protection. Funding: The study was supported by the National Institute for Infectious Disease Lazzaro Spallanzani IRCCS “Advanced grant 5x1000, 2021” and by the Italian Ministry of Health “Ricerca Corrente Linea 2”. Declaration of Interest: The authors declare that no conflicting financial interests or other competing relationships exist. Ethical Approval: Mpox-Vac protocol (“Studio prospettico osservazionale per monitorare aspetti relativi alla sicurezza, all’efficacia, all’immunogenicità e all’accettabilità della vaccinazione anti Monkeypox con vaccino MVA-BN (JYNNEOS) in persone ad alto rischio”) was approved by the INMI Lazzaro Spallanzani Ethical Committee (approval number 41z, Register of Non-Covid Trials 2022). All subjects eligible for mpox vaccination according to the ministerial guidelines and who signed a written informed consent were enrolled in the study.