The Mpox outbreak, caused by Monkeypox virus (MPXV), underscores the need for a serological assay to assess Mpox immunity. Modified Vaccinia Ankara (MVA) vaccine, an attenuated vaccinia virus (VACV), is authorised for Mpox prevention. We aimed to develop a quantitative immunoassay to differentiate infection- and vaccination-induced immunity and explore serological responses to Mpox infection and vaccination. We evaluated an electrochemiluminescence assay targeting IgG to 10 MPXV and 3 VACV antigens in plasma from adults in a cohort study with previous Mpox, MVA-vaccination, or historical controls. Sensitivity and specificity to distinguish i) seropositive versus naive and ii) infection- versus vaccination-induced seropositivity were determined using ROC curves. Antibody kinetics were analysed with generalised additive models. Eight of the thirteen IgG antibodies showed significant titre differences across groups identifying three key antigens: MPXVB6R, MPXVA27L, and VACVB5. A VACVB5 IgG titre of 0.082 IgG normalised units (nu) offered 74% (95% CI: 59-82%) sensitivity and 81% (73-96%) specificity for previous antigen exposure (infection or vaccine). For infection alone, an MPXVB6R IgG titre of 0.075 IgGnu provided 89% (82-98%) sensitivity and 94% (86-100%) specificity. To differentiate infection from vaccination-induced seropositivity, the sum of MPXVA27L IgG and the B6R/VACVB5 ratio provided 89% (80-96%) sensitivity and 80% (74-84%) specificity. VACVB5 IgG titres declined over time, with higher titres post-Mpox than post-vaccination (p < 0.0001). This assay demonstrates high sensitivity and specificity in quantifying and differentiating between antibody responses to Mpox infection and vaccination. Post-Mpox antibody responses were higher than post-vaccination, though both waned over time. Health Research Board (MONKEYVAX-2022-1), University College Dublin School of Medicine.

Background: Clade IIb mpox cases have declined globally, likely due to behavioral changes alongside vaccine- and infection-induced immunity. However, infections in vaccinated individuals raise concerns about immunity durability. We compared the longevity of antibody responses following mpox infection and modified vaccinia Ankara–Bavarian Nordic (MVA–BN) vaccination. Methods: In a multicenter, prospective cohort, we measured plasma IgG titers to vaccinia virus (VACV) B5 antigen in adults with prior mpox, MVA–BN vaccination, and historical controls, sampled up to 2 years postexposure. Receiver operating characteristic analysis determined the seropositivity threshold. Generalized additive mixed models compared antibody kinetics, and logistic regression identified factors associated with seropositivity. The results are median (interquartile range) unless specified. Results: A total of 122 vaccinated participants (100% male, aged 36 [32.5–43.5], 25% people with HIV [PWH]) were sampled at 22.0 (20.0–23.5) months post-MVA–BN vaccination, 72 of whom had a paired sample 12.5 (8.0–15.5) months prior, alongside 13 participants post-mpox (100% male, aged 32.5 [30.5–40], 23% PWH) sampled 25.0 (22.5–29.0) months postinfection, 12 with a paired sample 12.5 (8.5–15.5) months prior. At follow-up, 85% (11/13) of the post-mpox group remained seropositive, versus 32% (39/122) of the vaccinated group. Predicted geometric-mean anti-VACV-B5 titers fell below the seropositivity threshold at 15.5 (95% confidence interval [CI]: 13.0–19.5) months postvaccine. PWH had significantly lower odds of retaining seropositivity (odds ratio: 0.18; 95% CI: .04–.60; P = .01). Conclusions: Antibody titers declined more rapidly postvaccination than post-mpox, with most vaccinated recipients, particularly PWH, losing seropositivity at 2 years. How these data relate to reinfection risk or the need for boosters remains to be determined.

Introduction: A clear immune correlate of protection from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has not been defined. We explored antibody, B-cell, and T-cell responses to the third-dose vaccine and relationship to incident SARS-CoV-2 infection.Methods: Adults in a prospective cohort provided blood samples at day 0, day 14, and 10 months after the third-dose SARS-CoV-2 vaccine. Participants self-reported incident SARS-CoV-2 infection. Plasma anti–SARS-CoV-2 receptor-binding domain (RBD) and spike-subunit-1 and spike-subunit-2 antibodies were measured. A sub-study assessed SARS-CoV-2–specific plasma and memory B-cell and memory T-cell responses in peripheral blood mononuclear cells by enzyme-linked immunospot. Comparative analysis between participants who developed incident infection and uninfected participants utilised non-parametric t-tests, Kaplan–Meier survival analysis, and Cox proportional hazard ratios.Results: Of the 132 participants, 47 (36%) reported incident SARS-CoV-2 infection at a median 16.5 (16.25–21) weeks after the third-dose vaccination. RBD titres and B-cell responses, but not T-cell responses, increased after the third-dose vaccine. Whereas no significant difference in day 14 antibody titres or T-cell responses was observed between participants with and without incident SARS-CoV-2 infection, RBD memory B-cell frequencies were significantly higher in those who did not develop infection [10.0% (4.5%–16.0%) versus 4.9% (1.6%–9.3%), p = 0.01]. RBD titres and memory B-cell frequencies remained significantly higher at 10 months than day 0 levels (p < 0.01).Discussion: Robust antibody and B-cell responses persisted at 10 months following the third-dose vaccination. Higher memory B-cell frequencies, rather than antibody titres or T-cell responses, predicted protection from subsequent infection, identifying memory B cells as a correlate of protection.

The 2022 global mpox outbreak, caused by clade IIb of the monkeypox virus (MPXV), prompted emergency use authorisation of the Modified Vaccinia Ankara–Bavarian Nordic (MVA-BN) vaccine, previously approved for smallpox prevention. Understanding immune responses to the MVA-BN vaccine is critical to inform both current and future mpox vaccine policy, particularly amid reports of breakthrough infections in vaccinated persons, uncertainty about the durability of vaccine-induced protection, and the emergence of further outbreaks of mpox from different viral clades, including the clade I-driven public health emergency of international concern. MVA-BN elicits binding and neutralising antibody, memory B cells, and T cell responses. Immune responses vary by host factors, prior orthopoxvirus exposure, and dosing regimens. While seroconversion is generally robust, circulating antibody titres often wane rapidly, particularly in vaccinia-naïve and/or immunocompromised individuals, including people with HIV. Vaccine-induced neutralising antibody responses to MPXV are frequently lower than to vaccinia virus, and their role in protection remains ill-defined. In contrast, T cell responses appear more sustained and may support long-term immunity in the absence of persistent antibody titres. This narrative review synthesises current evidence on the immunogenicity and durability of MVA-BN vaccination, highlights challenges in assay interpretation, and outlines key research priorities, including the need to explore correlates of protection, booster strategies, and next-generation vaccine design.

Correlates of Protection (CoP) are biomarkers above a defined threshold that can replace clinical outcomes as primary endpoints, predicting vaccine effectiveness to support the approval of new vaccines or follow up studies. In the context of COVID-19 vaccination, CoPs can help address challenges such as demonstrating vaccine effectiveness in special populations, against emerging SARS-CoV-2 variants or determining the durability of vaccine-elicited immunity. While anti-spike IgG titres and viral neutralising capacity have been characterised as CoPs for COVID-19 vaccination, the contribution of other components of the humoral immune response to immediate and long-term protective immunity is less well characterised. This review examines the evidence supporting the use of CoPs in COVID-19 clinical vaccine trials, and how they can be used to define a protective threshold of immunity. It also highlights alternative humoral immune biomarkers, including Fc effector function, mucosal immunity, and the generation of long-lived plasma and memory B cells and discuss how these can be applied to clinical studies and the tools available to study them.

Higher circulating SARS-CoV-2 IgG titers correlate with SARS-CoV-2 ex vivo viral neutralization, but how well this translates to clinical protection in the real-world setting is unclear. In a prospective cohort study, we enrolled 44 SARS-CoV-2 negative, confirmed SARS-CoV-2 close contacts. Receptor-binding domain (RBD) and full-spike IgG and SARS-CoV-2 memory B-cell frequencies were measured at exposure, and participants were serially tested for incident infection over 14 days. Those who developed SARS-CoV-2 infection had significantly lower RBD titers, but not memory B-cell frequencies. An RBD IgG titer >6321 BAU/ml was associated with a reduced SARS-CoV-2 acquisition risk (HR 0.32, 95% CI 0.13–0.81), while an RBD IgG titer >456 BAU/ml was associated with a reduced moderate or severe COVID-19 risk (HR 0.15, 95% CI 0.03–0.81), identifying this threshold as a correlate of protection.