Luckily severe allergic reactions (anaphylaxis) to COVID-19 vaccines have been rare, averaging about 8 events per million vaccines.  However, since the Corona virus continues to circulate there will be an ongoing need for vaccination.  Based on current research it seems that most of these adverse events are reactions to polyethylene glycol (PEG).  However, attempts using traditional allergy testing techniques which look for IgE mediated sensitivity have led to poor discriminatory value in people who have had allergic reactions and poor predictive value when used to try and prophylactically identify individuals at high risk for allergic reactions.  This frustration has led to research to identify a more reliable test.  Georgetown University has used a research test called leukocyte activation with surprisingly good results.  The test is done by adding PEG to a sample of the patients’ blood and then analyzing it to see if the leukocytes (white blood cells) become activated.   

So far this is just a research tool but it could lead to a readily available testing mechanism, much as the home Covid test swab has become.