Crohn's disease can be thought of a primary immunodeficiency of macrophages, where an adaptive immune response tries to compensate for a lack of bacterial clearance in the intestine.
TNF-alpha blockers block TNF-alpha signaling molecules, cytokine released by macrophages. They dampen the adaptive response (triggerd by antigen presentation) which dampens secondary tissue damage in the intestine due to an inflammatory cascade.
But this leaves the patient with a weakened adaptive immune response, which leaves patients very vulnerable to infections, especially infections from intracellular bacteria. Which is why the mantoux test (which tests for the presence of latent Tuberculosis) is so important before people are allowed to take a TNF-alpha blocker. People with crohn's disease are vulnerable to mycobacteria, both from a genetic standpoint and from a treatment standpoint, especially TB and Leprae, thankfully both are rather rare in the Western world.
Coronaviruses are viruses, not intracellular bacteria like Tuberculosis. It is possible that people on TNF-alpha blockers are not that much more vulnerable than the general population because a humoral response might be more important than a cell mediated response used to clear intracellular bacteria. But I would think the response would still be noticeably weaker in people on TNF-alpha blockers.
I would argue people on TNF-alpha blockers are high risk groups compared to the general population, and hospital staff should be wearing masks when they come into contact with crohn's disease patients on TNF-alpha blockers.
