The Middle East respiratory syndrome coronavirus (MERS-CoV) is a highly pathogenic and zoonotic virus with a fatality rate in humans of over 35%. Although several vaccine candidates have been developed, there is still no clinically available vaccine for MERS-CoV. In this study, we developed two types of MERS-CoV vaccines: a recombinant adenovirus serotype 5 encoding the MERS-CoV spike gene (Ad5/MERS) and spike protein nanoparticles formulated with aluminum (alum) adjuvant. Next, we tested a heterologous prime-boost vaccine strategy, which compared priming with Ad5/MERS and boosting with spike protein nanoparticles and vice versa, with homologous prime-boost vaccination comprising priming and boosting with either spike protein nanoparticles or Ad5/MERS. Although both types of vaccine could induce specific immunoglobulin G against MERS-CoV, neutralizing antibodies against MERS-CoV were induced only by heterologous prime-boost immunization and homologous immunization with spike protein nanoparticles. Interestingly, Th1 cell activation was induced by immunization schedules including Ad5/MERS, but not by those including only spike protein nanoparticles. Heterologous prime-boost vaccination regimens including Ad5/MERS elicited simultaneous Th1 and Th2 responses, but homologous prime-boost regimens did not. Thus, heterologous prime-boost may induce longer-lasting immune responses against MERS-CoV because of an appropriate balance of Th1/Th2 responses. However, both heterologous prime-boost and homologous spike protein nanoparticles vaccinations could provide protection from MERS-CoV challenge in mice. Our results demonstrate that heterologous immunization by priming with Ad5/MERS and boosting with spike protein nanoparticles could be an efficient prophylactic strategy against MERS-CoV infection.

An outbreak of nosocomial infections with Middle East respiratory syndrome coronavirus occurred in South Korea in May 2015. Spike glycoprotein genes of virus strains from South Korea were closely related to those of strains from Riyadh, Saudi Arabia. However, virus strains from South Korea showed strain-specific variations.

The full genome sequence of a Middle East respiratory syndrome coronavirus (MERS-CoV) was identified from cultured and isolated in Vero cells. The viral genome sequence has high similarity to 53 human MERS-CoVs, ranging from 99.5% to 99.8% at the nucleotide level.