Homologous and heterologous boosting of the Chadox1-S1-S COVID-19 vaccine with the SCB-2019 vaccine candidate: a randomized, controlled, phase 2 study by Costa Clemens, SA, Milan, EP, Sprinz, E, Neto, JC, Pacciarini, F, Li, P, Chen, HL, Smolenov, I, Pollard, A, Clemens, R