Safety and immunogenicity of heterologous COVID-19 vaccine regimens to deal with product shortage: A randomised clinical trial in an elderly population. - Kundro MA; Losso MH; Macchia A; Pastor I; Alonso Serena M; Gestoso C; Moreno Macías L; Crupi F; Acosta MC; Ivalo S; Ghioldi M; Bouzas MB; et al. (Public health in practice (Oxford, England), 2022)