Figure 1. Analysis of IgG (A) and IgM (B) titres by IFA. Sera from MPXV-infected (M1-M5) or CPXV-infected (C1-C6) patients or individuals vaccinated against poxviruses (V1-V5) were analysed by using IFAs with different viruses (MPXV, VACV and CPXV). # = the titre is higher than the highest serum dilution analysed (≥). Different serum dilutions were analysed for some IFAs; therefore the highest measurable titre can vary by sample or test.

In addition to IFA, NTs are used in the lab at the Robert Koch Institute to detect neutralising antibodies against poxviruses. This assay – like IFA – is suited only for smaller numbers of samples. NTs were performed using different poxviruses (MPXV, CPXV and VACV) and the same set of serum samples were analysed that were used in the IFA cross-reactivity evaluation. For samples from MPXV-infected or IMVANEX-vaccinated individuals, neutralising antibodies could be detected when using all OPXVs (MPXV, VACV and CPXV; Figure 2) in the NT. Larger differences in titre against the different OPXVs were observed in the NTs as compared to the IFAs. These differences were most pronounced when using samples from MPXV-infected persons, where the lowest titres were observed when using CPXV and the highest titres when using VACV in the NTs (Figure 2). Neutralising antibodies in five out of six sera from CPXV-infected individuals were not detected by all OPXV NTs; however, if detected, their titre was low and close to the limit of detection of the test. In a single CPXV sample with a higher titre, neutralising antibodies were also detected with all three OPXV NTs.