Allergic reactions to Covid vaccines have been discussed on our blog posts previously when both the Pfizer and Moderna vaccines came out. Several months have passed now and we are here to provide an update.
Allergic reactions to vaccines in general are rare, occurring at a rate of 1.3 per 1 million doses given. Recent immediate allergic reactions clinically compatible with anaphylaxis have occurred in the Pfizier-BioNTech and Moderna mRNA vaccines against SARS-CoV-2. At present, the specific mechanisms have not been identified.
Back in December 2020, 21 adverse events identified as anaphylaxis after the Pfizer vaccine were reported. This came out to approximately 11 cases per million doses. 71% of these occurred within 15 minutes of vaccination.
The CDC has recommended that the mRNA SARS-CoV-2 vaccines should not be administered to individuals with a known history of a severe allergic reaction to any component of the vaccine. Although the specific vaccine component hasn’t been identified, polyethylene glycol (PEG) is one of the ingredients and it has been known to cause anaphylaxis in other clinical settings.
Ingredients of the Pfizer-BioNTech and Moderna Covid-19 vaccines.
Pfizer-Nucleoside-modified mRNA encoding the viral spike glycoprotein of SARS-CoV-2. (4-hydroxybutyl)azanediyl)bis(hexane-6,1diyl)bis(2-hexyldecanoate), 2[(PEG)-2000]-N,N-ditertradecylacetamide, 1,2-Distearoyl-sn-gycero-3-phosphocholine, Cholesterol, potassium chloride, monobasic potassium phosphate, sodium chloride, dibasic sodium phosphate dehydrate, sugar (sucrose), Diluent (sodium chloride).
Moderna-Nucleoside-modified mRNA encoding the viral spike glycoprotein of SARS-CoV-2. SM-102 (proprietary to Moderna), PEG 2000 dimyristoyl gycerol, 1,2-Distearoyl-sn-glycero-3-phosphocholine, Cholesterol, Tromethamin, Tromethamin hydrochloride, acetic acid, sodium acetate, Sugar (sucrose), Diluent (none).
At present, no other vaccine uses PEG 2000, the vaccine component for both of these vaccines. Polysorbate, found in medications and foods, can cross react with PEG. Patients with documented allergic reactions to polysorbate should not be vaccinated with the mRNA vaccines.
According to the CDC, if a person experiences a severe allergic reaction or an immediate allergic reaction of any severity within 4 hours after the first dose, the person shouldn’t receive the 2nd dose.
A recent review associated with the mRNA vaccines hypothesized a potential relationship between the lipid-PEG2000 component of the lipid nanoparticle mRNA carrier system and increased risk of anaphylaxis.
The ACAAI does not currently endorse any testing protocol for PEG, polysorbate or the mRNA COVID-19 vaccines. There are testing protocols that have been published. However, there are no established predictive values and safety data for the skin testing procedures.
For those who had suspected allergic reactions, a graded vaccine protocol could be an option for administration. With 15 minute intervals the dosing can be increased with the following dosages:
0 minutes- 0.05ml of 1:10 dilution
15 minutes- 10% of a full dose
30 minutes-20% of a full dose
45 minutes-30% of a full dose
60 minutes-40% of a full dose
Followed by a 30 minute observation period and alternating arms.
This is experimental, and there is no data yet to confirm the efficacy and safety of the above protocol. This is currently not endorsed by major medical allergy societies.
Patients at high risk should be screened before allergic reactions with the following questions.
- Do you have a history of a severe allergic reaction to an injectable medication?
- Do you have a history of a severe allergic reaction to a previous vaccine?
- Do you have a history of a severe allergic reaction to polyethylene glycol (PEG), a polysorbate, or polyoxyl 35 castor oil (eg. paclitaxel)?
Any affirmative response should prompt a referral to a board-certified allergist for further evaluation before Covid-19 vaccination. The guidance underscores that those who have a history of food, venom, latex, inhalant or medication allergy can still get the vaccine.