Single Vaccination with BNT162b2 or ChAdOx1 in Older People Induces Equivalent Antibody Generation but Enhanced Cellular Responses after ChAdOx1

Background: Extended-interval Covid vaccination regimens are now used widely in order to accelerate population coverage but the relative immunogenicity of different vaccines in older people remains uncertain. Methods: We recruited 165 participants aged 80+ years who had received a single dose of either BNT162b2 mRNA or ChAdOx1 adenovirus vaccine and studied adaptive immune responses after 5 weeks. Findings: Antibody responses against spike protein were detectable in 93% and 87% of mRNA or ChAdOx1 recipients respectively with median antibody titres of 19.3 and 19.6 U/ml (p=0.41). Spike-specific T cell responses were observed in 12% and 31% of mRNA and ChAdOx1 recipients respectively and median responses were 3-fold higher in ChAdOx1 vaccinees at 2 vs 6 spots/million respectively (p=<0.0001). Humoral and cellular immune responses against spike were correlated in both cohorts. Evidence of previous natural infection was seen in 8 donors and associated with 691-fold and 4-fold increase in humoral and cellular immune responses across the whole cohort. Interpretation: Single doses of either the BNT162b2 or ChAdOx1vaccine in older people thus induce humoral immunity in most donors and are markedly enhanced by previous infection. Cellular responses are weaker but show relative enhancement after the ChAdOx1 platform. Funding Statement: This work was partially supported by the UK Coronavirus Immunology Consortium (UK-CIC) funded by DHSC/UKRI and the National Core Studies Immunity programme. Declaration of Interests: The authors declare no conflicts of interest. Ethics Approval Statement: The work was performed under the CIA UPH IRAS approval (REC 20\NW\0240) and conducted according to the Declaration of Helsinki and good clinical practice.