Immunogenicity, safety and reactogenicity of heterologous (third dose) booster vaccination with a full or fractional dose of two different COVID-19 vaccines: A phase 4, single-blin... by Costa Clemens, SA, Marchevsky, N, Kelly, S, Felle, S, Eldawi, A, Rajasingam, R, Mahmud, R, Lambe, T, Voysey, M, Gonzalez, I, Milan, EP, Justino, MC, Bibi, S, Aley, P, Clemens, R, Pollard, AJ