IGHE and anaphylaxis: However, a recent case series of patients who experienced anaphylaxis at their first mRNA vaccination found that 19 of 19 patients who still chose to receive the second dose did not experience anaphylaxis on the second dose, possibly suggesting a role for an allergic, but non-IgE-mediated mechanism, such as mast-cell activation or complement activation to a lipid or PEG component of the vaccine, as has been occasionally observed with other PEGylated lipid nanoparticles (34, 35).