SARS-CoV-2 delta variant neutralisation after heterologous ChAdOx1-S/BNT162b2 vaccination

Safety considerations associated with the Oxford–AstraZeneca COVID-19 ChAdOx1-S vaccine (AZD1222) have led many public health agencies to recommend a heterologous boost with an mRNA vaccine after prime vaccination with ChAdOx1-S instead of a homologous boost. Given the strong immune response after heterologous prime-boost vaccination, mixing of vaccines has been suggested as a suitable strategy to contain emerging SARS-CoV-2 variants.5 Heterologous boosting with BNT162b2 has been shown to induce higher counts of spike-specific CD4+ and CD8+ T cells and, in particular, high titres of neutralising antibodies in a surrogate test against the SARS-CoV-2 variants of concern (VOCs) alpha, beta, and gamma.3 However, the rapid spread of the delta variant is a concern for both ChAdOx1-S-primed vaccinees who are expecting a boost vaccination and for individuals who have been fully vaccinated with ChAdOx1-S. If confirmed in a large study, our data also support a heterologous boost vaccination of individuals with completed homologous ChAdOx1-S vaccination, once humoral immunity is declining and patients become susceptible to infection.