PEGylation has proven to be a valuable tool to prolong the half-life of proteins in drug delivery. Covalent binding of one or more polyethylene glycol (PEG) molecules-either with the attachment of small (5-10 kDa) PEG groups, or site-directed attachment of large (≤60 kDa) PEG molecules via linkers-increases the hydrodynamic radius of a protein, improving drug stability and reducing clearance receptor interaction. 1,2 At least 12 PEGylated biopharmaceuticals have been approved in Europe and the United States, across multiple indications. PEGylated products have a clinical track record of >20 years, and no long-term PEG-related safety signals have been identified in humans. Most of the approved products are used to treat chronic diseases, including hepatitis, immunodeficiency disorders, renal failure and autoimmune diseases. 1 Short-term effects of PEG immunogenicity on safety, by detection of either pre-existing or PEGylated biologic-induced anti-PEG IgM and IgG antibodies, have been reported, but will not be the focus of this letter.