Detection of neutralizing antibodies of vaccinated sera against MERS-CoV infection. Mice were vaccinated with a recombinant protein expressing RBD of MERS-CoV S protein, and representative sera (Sera 1–3) collected from 10 days post-2nd vaccine were used for the test. Control sera were collected from mice injected with PBS. (A) MERS-CoV pseudovirus-based inhibition assay in DPP4-expreesing Huh-7 cells. The data are presented as mean percentages of inhibition ± SD of duplicate wells. (B) MERS-CoV live virus-based inhibition assay in Vero E6 cells. The titers were determined as the highest serum dilutions that completely prevent CPE in at least 50% of the wells (NT50) and are expressed as mean ± SD. The experiment was repeated three times, and similar results were obtained.