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.