MERS-coronavirus: From discovery to intervention

Widagdo, W.; Okba, Nisreen M.A.; Stalin Raj, V.; Haagmans, Bart L.

Despite being highly specific and sensitive, RT-PCR-based assays still have limitations as MERS-CoV can only be detected when it is actively shed by the host. Serology-based assays were subsequently developed to distinguish those individuals that had been exposed to MERS-CoV in the past. Indirect immunofluorescence assays (IFA) and neutralization tests (plaque reduction neutralization test and microneutralization test) were set up using susceptible cell lines and whole virus particles [5, 12] . These assays require biosafety level 3 facilities to work with the infectious MERS-CoV in-vitro, limiting their usage. In addition, whole virus IFA showed limited specificity to MERS-CoV due to cross-reactivity with other human CoV [5, 12] . Alternative assays using a pseudoparticle virus and specific MERS-CoV antigens were then developed to solve this issue. Two CoV structural proteins known to be highly antigenic are the N and S proteins. Both proteins have been used to develop serology-based assays in various platforms, i.e. recombinant IFA, western blot, enzyme-linked immunosorbent assay (ELISA), luciferase-based antibody detection assay and protein microarray [5, [13] [14] [15] [16] . The N protein is relatively conserved among CoVs, whereas the S1 domain, located in S, is more divergent among CoVs, making it an ideal candidate for CoV specific diagnostic serological assays. However, it is important to note that none of the serological assays available to date has been fully validated for specificity and sensitivity, therefore due care must be taken in interpreting the results of large serosurveillance studies. Possible cross reactivity and/or low sensitivity of these assays can lead to failure in determining the prevalence of true MERS-CoV positive cases in a given population. In turn, this has an impact on the calculated fatality rate of the viral infection. Further studies using a set of well-characterized sera are required for the determination of cut-off values and assessing cross-reactivity between MERS-CoV and other human CoVs. It is crucial to properly determine the MERS-CoV prevalence at a population level to develop adequate control programs.