Ongoing surveillance will be important for monitoring the duration of protection offered by booster doses and any need for additional doses in the future. Acknowledgements We would like to thank Dorothy Blundell, Dr Caroline Sayer and the team at Haverstock Healthcare GP Federation and the whole CONSESUS team at General public Health England (PHE) including those within the Computer virus Reference Department at Colindale who assisted with the laboratory testing. Funding: This surveillance was funded by PHE now UK Health Security Agency (UKHSA). Applications for relevant anonymised data should be submitted to the UKHSA Office for data Oxypurinol release. Notes Note From 1 October 2021, the United Kingdom Health Security Agency (UKHSA) became fully operational. against death, for the Vaxzevria vaccine (ChAdOx1-S, AstraZeneca, Cambridge, United Kingdom (UK)) and the Comirnaty vaccine (BNT162b2 mRNA, BioNTech-Pfizer, Mainz, Germany/New York, United States (US)), respectively. Greater waning of immunity was observed among older adults and those with underlying comorbidities [1] who were the first groups to be offered COVID-19 vaccination. In view of this and issues about waning immunity, the emergence of the highly-transmissible Delta variant (Phylogenetic Assignment of Named Global Outbreak (Pango) lineage designation B.1.617.2) which causes more severe disease and can infect vaccinated individuals [2,3], high and sustained community contamination rates in the UK and winter pressures around the national healthcare system, in September 2021 the UK Joint Committee on Vaccination and Immunisation recommended a third dose (booster) of a vaccine against Oxypurinol COVID-19. The recommendation comprises either a single dose of Comirnaty vaccine or a half dose (50?g) of Spikevax vaccine (mRNA-1273, Moderna, Cambridge, US) for selected population groups. Adults aged?50 years and older, individuals aged 16C49 years in clinical risk groups, adult carers and household contacts of immunosuppressed individuals and frontline health and social care workers are offered a booster dose at least 6 months after their second vaccine dose [4-6]. COVID-19 vaccine responses after extended immunisation schedules In England, the UK Health Security Agency initiated an evaluation of vaccine responses in adults aged?50 years and older who received the Comirnaty or Vaxzevria vaccines as part of the national immunisation programme to compare short vs longer interval vaccine schedules and monitor antibody waning over time [7]. The COVID-19 vaccine responses after extended immunisation schedules (CONSENUS) cohort has been explained previously [7,8]. Immunocompetent adults aged 50 years and older in London were recruited in January 2021 to provide serial blood Oxypurinol samples at 0, 3, 6, 9, 12, 15 and 20 weeks after their first dose of COVID-19 vaccine. As part of the national COVID-19 vaccine roll-out, participants received either (i) two Comirnaty doses? ?30 days apart (Comirnaty-control); (ii) two Comirnaty doses??30 days apart (Comirnaty-extended); or (iii) two Vaxzevria doses??30 days apart (Vaxzevria-extended). Additional blood samples were taken before and after a Comirnaty booster dose. We describe the antibody kinetics after main immunisation and booster vaccination in adults aged?50 years and older who were vaccinated as part of the national COVID-19 immunisation programme. Serum samples were tested for nucleoprotein (N) antibodies as a marker of previous severe acute respiratory syndrome coronavirus 2 FLI1 (SARS-CoV-2) contamination (Elecsys Anti-SARS-CoV-2 total antibody assay, Roche Diagnostics, Basel, Switzerland). Results were expressed as a cut-off index (positive??1). Serum samples were also tested for spike (S) protein antibodies which could be contamination- or vaccine-derived (Elecsys Anti-SARS-CoV-2 S total antibody assay, Roche Diagnostics). Results were expressed as arbitrary models (au)/mL (positive??0.8) to assess vaccine response. Individuals with??0.4 (au)/mL around the Roche N assay were considered to have had prior infection with SARS-CoV-2. This was assessed at enrolment. If participants tested N antibody positive after vaccination, indicating vaccine breakthrough, this and subsequent samples were removed from the analysis. The S antibody geometric mean titres (GMTs) were calculated with 95% confidence intervals (CI). Geometric imply ratios (GMR) of responses between time points were estimated using a mixed regression model on log responses including a random effect for each participant. Separate models were fitted for each vaccine group. The GMR of responses by vaccine type at each post-vaccination time point was estimated via regression on log Roche S responses and included age group and sex. Statistical analyses were performed using STATA version 14.2 (StataCorp, College Station, Texas, US). Antibody kinetics following main COVID-19 vaccination Of the 750 recruited participants, 626 provided serum samples for up to 38 weeks after their second vaccine dose (Table 1). Table 1 Characteristics of participants with samples after second dose of the primary COVID-19 vaccination and after booster dose, London, United Kingdom, SeptemberCDecember 2021 (n?=?626)a thead th rowspan=”3″ valign=”middle” align=”left” scope=”col” style=”border-left: solid 0.50pt; border-top: solid 0.50pt; border-right: solid 0.50pt; border-bottom: solid 0.50pt” colspan=”1″ Vaccine routine /th th rowspan=”3″ valign=”middle” align=”center” scope=”col” style=”border-left: solid 0.50pt; border-top: solid 0.50pt; border-right: solid 0.50pt; border-bottom: solid 0.50pt” colspan=”1″ n /th th rowspan=”3″ valign=”middle” align=”center” scope=”col” style=”border-left: solid 0.50pt; border-top: solid 0.50pt; border-right: solid 0.50pt; border-bottom: solid 0.50pt” colspan=”1″ First and second dose routine median in days (IQR) /th th rowspan=”3″ valign=”middle” align=”center” scope=”col” style=”border-left: solid 0.50pt; border-top: solid.