**7. Ad26.COV2.S**

The one-shot Ad26.COV2.S vaccine is a recombinant; replication-deficient viral vector denoted human adenovirus type 26 (Ad26) vector expressing the entire sequence of SARS-CoV-2 spike protein in a prefusion-stabilized shape [45, 46]. Other Ad26-based vaccines, such as an authorized Ebola vaccine, have been found to be safe and generate long-lasting immune responses. In preclinical SARS-CoV-2 challenge studies [47, 48], Ad26.COV2.S caused persistent protection at low doses [49], and preliminary clinical data showed that a single dosage of virus particles was safe and elicited sufficient humoral and cellular immune responses [46]. ENSEMBLE trial showed that the vaccine achieved 52.0 and 64.0% efficacy against moderate to severe–critical COVID-19 with onset at least 2 weeks and at least 3 weeks after vaccination, respectively, and achieved 73.1 and 81.7% efficacy against severe–critical COVID-19, respectively [50]. Ad26.COV2.S could be preserved for up to 2 years in a conventional freezer and 3 months in a fridge, making travel, storage, and use in a pandemic much easier.
