Anti-PEG Immunogenicity of mRNA-LNP Vaccines in Humans: Evidence for Population-Level Changes in the Anti-PEG Antibody Repertoire

Background/Objectives: Polyethylene glycol (PEG) is widely used to enhance the stability and pharmacokinetics of nanomedicines, including lipid nanoparticle (LNP)-based mRNA vaccines. However, both pre-existing and vaccine-induced anti-PEG antibodies may compromise the efficacy and safety of PEGylated therapeutics. Methods: In this study, we analyzed the specificity and avidity of anti-PEG antibodies in human blood donor samples from Unvaccinated individuals and recipients of PEGylated mRNA-LNP vaccines (Comirnaty and Spikevax), polysorbate-containing vaccines, or vaccines lacking both PEG and polysorbates. Quantitative ELISA was used to characterize anti-PEG and anti-polysorbate IgM and IgG responses in 325 plasma samples, while an equilibrium titration method was applied to assess IgG binding to PEG molecules, micelles, and PEGylated liposomes with defined structural features in 36 plasma samples. Results: Vaccination with PEGylated mRNA-LNPs was associated with increased anti-PEG antibody levels and qualitative changes in antibody binding behavior. Anti-PEG IgG antibodies displayed progressively higher avidity toward larger and structurally more complex PEG-containing antigens, with the strongest binding observed for PEGylated liposomes. Notably, vaccinated individuals, particularly those who received Spikevax, showed increased end-group and backbone-specific avidity, as well as an enhanced ability of antibody paratopes to engage shorter PEG chains. In contrast, polysorbate-containing or PEG-free vaccines did not elicit comparable effects. Conclusions: These findings suggest that vaccination with PEGylated mRNA-LNPs is associated with the emergence of altered antibody populations with increased end-group reactivity and higher avidity toward PEG-directed immune responses, despite PEG being a synthetic, nonprotein polymer. Antibody binding to LNPs, accompanied by the emergence of high-avidity anti-PEG IgG seems to be consistent with an increased risk of adverse events, particularly following repeated vaccinations, including complement activation-related pseudoallergy (CARPA) and anaphylaxis. It may also contribute to altered immune protection against the vaccine target, underscoring the need for avidity-aware risk-benefit assessment of PEGylated therapeutics.