The persistence of immunogenicity in patients with immune-mediated inflammatory diseases (IMID) on disease-modifying antirheumatic therapy (DMARD) has been less well studied. This extension study evaluates the SARS-CoV2 antibody decay kinetics 6 months following two doses of ChAdO1nCov-19 (AZ) and BNT162b (Pfizer) and subsequent response following an mRNA booster. RESULTS: 175 participants were included. Six months after initial AZ vaccination, 87.5%, 85.4% and 79.2% (p=0.756) in the withhold, continue and control groups remained seropositive compared with 91.4%, 100% and 100% (p=0.226), respectively, in the Pfizer group. Both vaccine groups developed robust humoral immune responses following a booster with seroconversion rates being 100% for all three intervention categories. The mean SARS-CoV-2 antibody levels were significantly lower in the targeted synthetic DMARD (tsDMARD) group that continued therapy compared with the control (2.2 vs 4.8 U/mL, p=0.010). The mean time interval until loss of protective antibodies in the IMID group was 61 days for the AZ and 137.5 days for the Pfizer vaccine. Within each DMARD class the interval until loss of protective antibody titres in the csDMARD, bDMARD and tsDMARD groups were 68.3, 71.8 and 64.0 days in the AZ group and 185.5, 137.5 and 116.0 days in the Pfizer group, respectively. CONCLUSION: Antibody persistence was longer in the Pfizer group due to a higher peak antibody level following second vaccination with levels of protection in IMID on DMARD therapy similar to controls except in those on tsDMARDs where it was lower. A third mRNA vaccine booster can restore immunity in all groups.