Rutherford AI, et al. - Experts compared the incidence of serious infection (SI) across biologic drugs used to treat rheumatoid arthritis (RA) using data from the British Society for Rheumatology Biologics Register for Rheumatoid Arthritis (BSRBR-RA). Data suggested that in the primary analysis use of certolizumab pegol led to a lower rate of SI than etanercept but in several sensitivity analyses performed, the result was no longer significant these analyses suggested that residual confounding could account for the observed difference. Based on the findings, deriving a conclusion that certolizumab pegol has a lower rate of SI than other biologics would be wrong. Nonetheless, the risk does not appear to be significantly higher as has previously been suggested.