Also known as: S. cerevisiae protein, yeast residual
Yeast Protein (Saccharomyces cerevisiae)
Sourced from FDA-approved package inserts (DailyMed). Last verified: 2026-07-07.
What It Is & Why It's in Vaccines
Residual yeast protein from the recombinant Saccharomyces cerevisiae (baker's yeast) expression system used to manufacture recombinant protein vaccines — specifically hepatitis B vaccines (Engerix-B, Recombivax HB, Heplisav-B, PreHevbrio) and the HPV vaccine (Gardasil 9). The antigen proteins are produced in genetically modified yeast cells, then purified; trace yeast protein may remain.
Not deliberately added — residual from the recombinant protein manufacturing process. Yeast is the preferred expression system for these vaccines because it produces correctly folded, immunogenic hepatitis B surface antigen (HBsAg) and HPV L1 capsid proteins at commercial scale.
Dose-in-Context
Per-Dose Quantity
≤5% yeast protein by total protein weight (varies by product and manufacturing lot). FDA/USP standards specify a maximum residual yeast protein content.
Reference Comparison
Baker's yeast (S. cerevisiae) is a ubiquitous environmental and dietary exposure. No established reference dose exists for injected trace yeast protein, but the quantities are immunologically relevant primarily for individuals with pre-existing yeast allergy.
Source: DailyMed package inserts: Engerix-B, Recombivax HB, Gardasil 9.
Route-of-Administration Note
Injected route. The principal clinical relevance is the contraindication for individuals with yeast allergy — this is a labeled contraindication for all recombinant yeast-derived vaccines.
Documented Adverse Effects
Labeled contraindication: individuals with known hypersensitivity to yeast should not receive yeast-derived recombinant vaccines (hepatitis B, HPV). Anaphylaxis in individuals with true IgE-mediated yeast allergy is plausible but extremely rare given the low prevalence of yeast allergy in the general population.
Vaccines Containing Yeast Protein (Saccharomyces cerevisiae)
Hepatitis B — Engerix-B (GlaxoSmithKline Biologicals)
Hepatitis B — Recombivax HB (Merck Sharp & Dohme LLC)
Aluminum salt is AAHS (chemically distinct from aluminum hydroxide used in Engerix-B).
Hepatitis B — Heplisav-B (Dynavax Technologies)
HPV — Gardasil 9 (Merck Sharp & Dohme LLC)
PCV (Pneumococcal) — Prevnar 20 (Pfizer Inc.)
ⓘ Methodology Disclaimer: Ingredient lists are sourced from official FDA-approved package inserts for the specific branded products named on each vaccine page. Formulations can change between manufacturing lots and over time — always verify against the current package insert before making a clinical decision. The presence of a substance in a vaccine does not by itself indicate harm; toxicology is dose-, route-, and context-dependent. Comparisons to reference exposure levels are provided with their sources where available, and are approximate — see the cited source for full methodology.