| Summary: | We report that in a cohort of 45,965 adults, who were receiving either the ChAdOx1 or the BNT162b2 SARS-CoV-2 vaccines, in
those who had no prior infection with SARS-CoV-2, seroconversion rates and quantitative antibody levels after a single dose
were lower in older individuals, especially in those aged >60 years. Two vaccine doses achieved high responses across all
ages. Antibody levels increased more slowly and to lower levels with a single dose of ChAdOx1 compared with a single dose of
BNT162b2, but waned following a single dose of BNT162b2 in older individuals. In descriptive latent class models, we identified
four responder subgroups, including a ‘low responder’ group that more commonly consisted of people aged >75 years, males
and individuals with long-term health conditions. Given our findings, we propose that available vaccines should be prioritized
for those not previously infected and that second doses should be prioritized for individuals aged >60 years. Further data
are needed to better understand the extent to which quantitative antibody responses are associated with vaccine-mediated
protection.
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