Serological responses to COVID-19 Comirnaty booster vaccine, London, United Kingdom, September to December 2021.

Serum samples were collected pre- and post-booster vaccination with Comirnaty in 626 participants (aged ≥ 50 years) who had received two Comirnaty doses < 30 days apart, two Comirnaty doses ≥ 30 days apart or two Vaxzevria doses ≥ 30 days apart. Irrespective of primary vaccine type or schedule, spike antibody GMTs peaked 2-4 weeks after second dose, fell significantly ≤ 38 weeks later and rose above primary immunisation GMTs 2-4 weeks post-booster. Higher post-booster responses were observed with a longer interval between primary immunisation and boosting.