Results
For the detection of antibodies against OPXV in low sample numbers, IFA is the assay of choice and was the first assay adapted to detect antibodies against MPXV in serum samples. To verify the antibody cross-reactivity, cells infected with different OPXV (MPXV, CPXV and VACV) were used to detect IgG and IgM antibodies by IFA. Moreover, in addition to sera from MPXV-infected individuals, sera were analysed from individuals with CPXV infection or from individuals that were recently vaccinated against poxviruses by using the IMVANEX vaccine. Using the different OPXV IFAs, IgG antibodies binding to MPXV, CPXV and VACV could be detected with comparable titres in all samples (Figure 1A). Although some differences could be observed for some sera when tested on MPXV-, CPXV- or VACV-infected cells, no clear trend could be seen for homologue serum/antigen sets when IgG and IgM were considered, as most sera gave similar results irrespective of the virus strain used as antigen. The only exception was for IgG where slightly higher titres could be seen after MPXV infection on MPXV IFA slides. However, differences were minor (usually a single 1:4 dilution step of the serum in the IFA). The same pattern was observed for IgM antibodies (Figure 1B).