Vaccine excipients & allergy relevance
For a suspected component allergy: which US vaccines carry each excipient, and whether it is actually a contraindication. Most true IgE-mediated vaccine reactions are to an excipient (gelatin, PEG, polysorbate), not the antigen, but most cited "allergies" (egg, neomycin contact, aluminum nodules) are not contraindications.
Evidence tier: Guideline-derived.
| Excipient | Representative US vaccines containing it | Allergy relevance & approach |
|---|---|---|
| Polyethylene glycol (PEG) | mRNA COVID-19 (Comirnaty/Pfizer, Spikevax/Moderna), mRNA RSV (mResvia) | The excipient implicated in rare mRNA-vaccine anaphylaxis. PEG is in many drugs (bowel preps, laxatives, depot steroids, some injectables), ask about those exposures. Suspected PEG allergy → allergist evaluation; the protein-subunit COVID vaccine (Novavax) is PEG-free. Cross-reacts with polysorbate. |
| Polysorbate 80 | Many, Shingrix, Gardasil 9, Prevnar 13/20, Boostrix, several influenza (Fluarix, Flulaval, Flucelvax, Fluad), Novavax, RotaTeq, RSV (Abrysvo, Arexvy) | Structurally related to PEG, so it can cross-react, a patient labeled "PEG-allergic" may also react to polysorbate, and vice versa. A rare cause of reactions in its own right; evaluate with an allergist. |
| Gelatin (porcine) | MMR-II, ProQuad (MMRV), Varivax (varicella), FluMist (LAIV); yellow fever (YF-VAX) | A leading cause of vaccine anaphylaxis. Ask about gelatin-containing food allergy and mammalian-meat / α-gal allergy; gelatin skin testing is available. Note: Shingrix (the current zoster vaccine) is gelatin-free, only the discontinued live Zostavax contained gelatin. |
| Egg protein (ovalbumin) | Egg-based influenza (Afluria, Fluarix, Fluvirin, Fluad); yellow fever; MMR-II / ProQuad are grown in chick fibroblasts (negligible egg) | NOT a contraindication to influenza or MMR. Give any age-appropriate flu vaccine with no egg-specific measures (ACIP 2023); MMR is safe (negligible ovalbumin). Egg-free flu options exist (Flucelvax cell-based, Flublok recombinant). Yellow fever has higher egg content → evaluate severe egg allergy before giving. |
| Neomycin / other antibiotics (polymyxin B, gentamicin, streptomycin) | MMR-II, Varivax, ProQuad, IPV-containing combos (Kinrix, Pediarix, Pentacel, Vaxelis), Havrix, some influenza | Most neomycin allergy is a delayed contact dermatitis, NOT a contraindication. Only a history of anaphylaxis to the antibiotic is a precaution warranting allergist input. |
| Yeast (Saccharomyces cerevisiae) | Recombinant hepatitis B (Engerix-B, Recombivax HB, Heplisav-B), HPV (Gardasil 9), combos (Pediarix, Twinrix, Vaxelis, Vaxneuvance) | Yeast allergy is rare; only a documented history of anaphylaxis to baker’s/brewer’s yeast is a precaution. Routine yeast intolerance is not a contraindication. |
| Aluminum salts (adjuvant) | Many non-live vaccines, hepatitis A/B, HPV, Tdap/DTaP, pneumococcal conjugate, meningococcal B | Causes delayed, itchy injection-site nodules and contact hypersensitivity, NOT anaphylaxis and NOT a contraindication. Reassure; the nodule may persist for months. |
| Natural rubber latex (vial stopper / syringe) | Product- and lot-dependent, check the specific package insert | Only a history of severe (anaphylactic) latex allergy warrants choosing a latex-free presentation. Routine/contact latex sensitivity does not contraindicate vaccination. |
Notes
- The CDC Pink Book Appendix B / Institute for Vaccine Safety excipient summary is the authoritative per-product reference, confirm against the specific product’s current package insert, as brands and formulations change.
- Most true IgE-mediated vaccine reactions are to an excipient, not the antigen, phenotype the reaction and identify the component (see the vaccine allergy pathway).
- Excipient/vaccine skin testing has limited predictive value: a negative test does not fully exclude allergy, and a positive (possibly irritant) result can mislead, interpret alongside the clinical history.
Evidence & citations
- Kelso JM, Greenhawt MJ, Li JT, et al. Adverse reactions to vaccines practice parameter 2012 update. J Allergy Clin Immunol. 2012;130(1):25-43. PMID 22608573
- Greenhawt M, Shaker M, Golden DBK, et al. Diagnostic accuracy of vaccine and vaccine excipient testing in the setting of allergic reactions to COVID-19 vaccines: a systematic review and meta-analysis. Allergy. 2023;78(1):71-83. PMID 36321821
- Grohskopf LA, Blanton LH, Ferdinands JM, et al. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the ACIP, United States, 2023-24 Influenza Season. MMWR Recomm Rep. 2023;72(2):1-25. ACIP / MMWR Recommendations and Reports (egg-allergy influenza guidance)
- Vaccine Excipient Summary, Excipients Included in US Vaccines, by Vaccine. Institute for Vaccine Safety, Johns Hopkins Bloomberg School of Public Health; 2024 (mirror of CDC Pink Book Appendix B). https://www.vaccinesafety.edu/components-excipients/
Clinician decision support. Verify against the cited source. Not a substitute for clinical judgment. 100% on-device; no patient data is stored or transmitted.