Background Patients with inflammatory rheumatic and musculoskeletal diseases (iRMD) receiving mycophenolic acid (MPA) may have a less favourable outcome from COVID-19 infection. Our aim was to investigate whether MPA treatment is associated with severe infection and/or death.

Methods IRMD patients with and without MPA treatment with highly suspected/confirmed COVID-19 were included in this observational multicentre study. The primary outcome was death rate from COVID-19 with secondary objectives to determine the severity of infection and length of hospital stay. Outcome comparisons were made using regression models with and without adjustment on prespecified confounding factors. ORs, sub-HR (sHR) and 95% CIs were calculated using patients not treated with MPA as a reference group.

Results Of the 1977 patients, 1928 were not treated with MPA (393 were MPA eligible), and 49 patients were treated with MPA. MPA-treated patients had more severe disease, longer hospital stays and higher death rate from COVID-19 than non-MPA patients (OR 8.02 (95% CI 3.35 to 19.20), p<0.001; sHR 0.57 (95% CI 0.33 to 0.98), p=0.040; OR 11.58 (95% CI 4.10 to 32.69), p<0.001). In adjusted analyses, however, no outcome was independently associated with MPA treatment. Death rate, severity and length of hospital stay of MPA-treated patients were not significantly different from those of not treated but MPA-eligible patients.

Conclusion MPA therapy is not associated with a more severe COVID-19 infection. However, due to increased vulnerability of developing a severe form of COVID-19, careful consideration should be taken with iRMD patients likely to be treated with MPA.

Trial registration number NCT04353609.