Inactive ingredients in vaccines that serve various functions including stabilization, preservation, and manufacturing.
Excipients are inactive substances used as carriers for the active ingredients in vaccines. They serve important functions like stabilizing the vaccine, ensuring pH balance, and preserving the product. Excipients make up the majority of most vaccine formulations by volume.
Polyethylene glycol (PEG) is a compound used as part of the lipid nanoparticle delivery system in mRNA vaccines such as the Pfizer-BioNTech and Moderna COVID-19 vaccines. It helps stabilize the nanoparticle that carries the mRNA into cells, allowing the genetic instructions to reach the immune system effectively.
PEG received increased attention during COVID-19 vaccination because of a small number of reported anaphylactic reactions. These reactions are rare but prompted updated screening and monitoring protocols at vaccination sites. Public health agencies continue to monitor and study these events to refine guidance.
PEG is also used in some medications, laxatives, and personal care products. The form and concentration in vaccines is specific to its delivery function.
Individuals with a known allergy to PEG should discuss vaccination options with their healthcare provider. Post-vaccination observation periods, typically 15-30 minutes, exist in part to monitor for rare allergic reactions, including to PEG. For detailed information on vaccine-related anaphylaxis, see our Safety page.
Polysorbate 80 is an emulsifier that helps keep vaccine ingredients evenly mixed and prevents separation. It is used in some COVID-19, influenza, and HPV vaccines. Its primary function is to help maintain the stability and consistency of the vaccine formulation during storage and administration.
Polysorbate 80 and PEG share some structural similarities. Individuals who have experienced allergic reactions to one may be advised to discuss the other with their healthcare provider. This cross-reactivity is documented in medical literature and is considered when evaluating vaccination options for individuals with known sensitivities.
Gelatin is used as a stabilizer to protect vaccine potency during storage and transport. It is present in some vaccines including MMR, varicella, and some influenza vaccines. Gelatin helps maintain the integrity of the vaccine's active components across varying temperature conditions.
Gelatin is one of the more common vaccine ingredients associated with allergic reactions, though such reactions remain rare. Individuals with known gelatin allergies should inform their healthcare provider before vaccination. Vaccine providers are trained to recognize and respond to allergic reactions should they occur.