The article doesn't go into it, but the actual paper[1] states that this vaccine has "has previously been reported to be immunogenic and protective against pneumonia in a rhesus macaque challenge model", and cites to [2].

[1] https://www.thelancet.com/journals/lancet/article/PIIS0140-6...

[2] https://www.biorxiv.org/content/10.1101/2020.05.13.093195v1