Background: Anti-tumour necrosis factor (TNF) drugs such as infliximab are associated with attenuated antibody responses after COVID-19 vaccination. It is unknown how infliximab impacts vaccine-induced serological responses against highly transmissible Omicron variants, which possess the ability to evade host immunity and are now the dominating variants causing current waves of infection. Methods: CLARITY IBD is a prospective, multicentre, observational cohort study investigating the impact of infliximab and vedolizumab on SARS-CoV-2 infection and vaccination in patients with inflammatory bowel disease (IBD). In the current work, the primary outcome was neutralising antibody responses against SARS-CoV-2 wild-type and Omicron BA.1 and BA.4/5 variants after three doses of COVID-19 vaccination in 1288 patients with IBD without prior COVID-19 infection, who were established on either infliximab (n=871) or vedolizumab (n=417). Cox proportional hazards models were constructed to investigate the risk of breakthrough infection in relation to neutralising antibody titres. Findings: Following three doses of COVID-19 vaccine, neutralising titre NT50 (half-inhibitory neutralising titre) was significantly diminished in patients treated with infliximab as compared to patients treated with vedolizumab, against wild-type (geometric mean [95% CI], 1990 [1781, 2223] vs 3212 [2780, 3712], p