Safety and immunogenicity of an rAd26 and rAd5 vector-based heterologous prime-boost COVID-19 vaccine in two formulations: two open, non-randomised phase 1/2 studies from Russia Denis Y. Logunov , Inna V. Dolzhikova , Olga V. Zubkova +29 more 2020 The Lancet 1109 citations