From the article:

“The vaccine is an innocuous parainfluenza virus (PIV5) carrying the "spike” protein that MERS uses to infect cells. All the vaccinated mice survived a lethal dose of the MERS coronavirus. […] The researchers note several factors that make PIV5 expressing a coronavirus spike protein an appealing platform for vaccine development against emerging coronaviruses. First, PIV5 can infect many different mammals, including humans, without causing disease. PIV5 is also being investigated as a vaccine for other respiratory diseases including respiratory syncytial virus (RSV) and influenza. Second, the fact that a low dose of the vaccine was sufficient to protect the mice might be beneficial for creating enough vaccine for mass immunization. And finally, the vaccine in the current study was the most effective MERS vaccine to date in animal models of the disease."