Differential immunogenicity of BNT162b2 or ChAdOx1 vaccines after extended-interval homologous dual vaccination in older people

Several SARS-CoV-2 vaccines have shown clinical efficacy against Covid-19 infection but there remains uncertainty about the immune responses elicited by different regimens. This is a particularly important question for older people who are at increased clinical risk following infection and in whom immune senescence may limit vaccine responses. The BNT162b2 mRNA and ChAdOx1 adenovirus vaccines were the first two vaccines deployed in the UK programme using an 8-12 week 'extended interval'. We undertook analysis of the spike-specific antibody and cellular immune response in 131 participants aged 80+ years after the second dose of 'extended interval' dual vaccination with either BNT162b2 mRNA (n = 54) or ChAdOx1 (n = 77) adenovirus vaccine. Blood samples were taken 2-3 weeks after second vaccine and were paired with samples taken at 5-weeks after first vaccine which have been reported previously. Antibody responses were measured using the Elecsys® electrochemiluminescence immunoassay assay and cellular responses were assessed by IFN-γ ELISpot. Antibody responses against spike protein became detectable in all donors following dual vaccination with either vaccine. 4 donors had evidence of previous natural infection which is known to boost vaccine responses. Within the 53 infection-naïve donors the median antibody titre was 4030 U/ml (IQR 1892-8530) following BNT162b2 dual vaccination and 1405 (IQR 469.5-2543) in the 74 patients after the ChAdOx1 vaccine (p =