Protects vaccinated Ad5/hDPP4-transduced BALB/c mice and majority (4/6) of vaccinated hDPP4-Tg mice from MERS-CoV (EMC2012 strain, 105 PFU for BALB/c; 103–4 TCID50 for Tg) challenge, without immunological toxicity or eosinophilic immune enhancement