BACKGROUND AND AIMS: Infliximab attenuates serological responses to SARS-CoV-2 infection. Whether this is a class effect, or if anti-TNF level influences serological responses, remains unknown. METHODS: Seroprevalence and the magnitude of SARS-CoV-2 nucleocapsid antibody responses were measured in surplus serum from 11422 (53.3% (6084) male; median age 36.8 years) patients with immune-mediated inflammatory diseases, stored at six therapeutic drug monitoring laboratories between 29 th January and 30 th September 2020. Data were linked to nationally-held SARS-CoV-2 PCR results to 4 th May 2021. RESULTS: Rates of PCR confirmed SARS-CoV-2 infection were similar across treatment groups. Seroprevalence rates were lower in infliximab- and adalimumab- than vedolizumab-treated patients (infliximab: 3.0% (178/5893), adalimumab: 3.0% (152/5074), vedolizumab: 6.7% (25/375), p = 0.003). The magnitude of SARS-CoV-2 reactivity was similar in infliximab- vs adalimumab-treated patients (median 4.30 cut-off index (COI) (1.94 - 9.96) vs 5.02 (2.18 - 18.70), p = 0.164), but higher in vedolizumab-treated patients (median 21.60 COI (4.39 - 68.10, p< 0.004). Compared to patients with detectable infliximab and adalimumab drug levels, patients with undetectable drug levels (<0.8 mg/L) were more likely to be seropositive for SARS-CoV-2 antibodies. One-third of patients who had PCR testing prior to antibody testing failed to seroconvert, all were anti-TNF treated. Subsequent positive PCR-confirmed SARS-CoV-2 was seen in 7.9% (12/152) patients after a median time of 183.5 days (129.8 - 235.3), without differences between drugs. CONCLUSION: Anti-TNF treatment is associated with lower SARS-CoV-2 nucleocapsid seroprevalence and antibody reactivity when compared to vedolizumab-treated patients. Higher seropositivity rates in patients with undetectable anti-TNF levels supports a causal relationship, although confounding factors, such as combination therapy with immunomodulator, may have influenced the results.