COVID-19 vaccines contain a small number of ingredients, most of them familiar substances like sugar, salts, and fats. The exact formula depends on which vaccine you receive, but none of the currently available COVID vaccines in the U.S. contain preservatives like thimerosal (mercury), formaldehyde, aluminum, egg proteins, gelatin, or latex.
Three main vaccine types have been used worldwide: mRNA vaccines (Pfizer and Moderna), a protein-based vaccine (Novavax), and viral vector vaccines (Johnson & Johnson, now discontinued in the U.S.). Here’s what’s actually inside each one.
The Active Ingredient: Instructions for Your Immune System
Every COVID vaccine delivers the same basic message to your body: here’s what the spike protein on the surface of the SARS-CoV-2 virus looks like. Your immune system then learns to recognize and attack that protein if the real virus shows up. The vaccines differ in how they deliver that message.
In the Pfizer and Moderna vaccines, the active ingredient is a strip of modified mRNA, a molecule that tells your cells to temporarily produce a harmless copy of the spike protein. Pfizer’s adult dose contains 30 micrograms of mRNA, while Moderna’s contains 50 micrograms. For context, a microgram is one-millionth of a gram. The mRNA breaks down in your body within days and never enters the nucleus of your cells where your DNA is stored.
Novavax takes a different approach. Instead of instructions, it delivers a lab-made copy of the spike protein directly, produced using insect cells. Your immune system reacts to this protein and builds defenses against it. The now-discontinued Johnson & Johnson vaccine used a modified, harmless cold virus (adenovirus type 26) that carried DNA instructions for the spike protein into your cells.
Lipid Nanoparticles: The Protective Coating
mRNA is fragile. Without protection, your body’s enzymes would shred it before it reached a single cell. Both Pfizer and Moderna solve this problem by wrapping the mRNA in tiny fat bubbles called lipid nanoparticles, made from four types of lipids that each serve a specific purpose.
The first is an ionizable lipid, the key player. This fat carries a charge that shifts depending on acidity, which allows it to bind tightly to the mRNA and then release it once inside a cell. Pfizer uses a lipid called ALC-0315, while Moderna uses one called SM-102. The second is a helper lipid (DSPC in both vaccines), which stabilizes the bubble’s structure. The third is cholesterol, the same molecule found naturally in every cell membrane in your body, which keeps the nanoparticle from falling apart. The fourth is a PEG-lipid, a fat molecule attached to polyethylene glycol, which acts like a shield to prevent the immune system from destroying the nanoparticle before it can do its job.
The total lipid content per dose is small. Pfizer’s adult dose contains about 0.76 mg of lipids total. Moderna’s contains about 1.01 mg.
Sugars, Salts, and Buffers
The remaining ingredients are stabilizers and buffers, substances that keep the vaccine effective during storage and ensure it stays at the right pH when injected.
Sucrose, ordinary table sugar, is the most abundant inactive ingredient in both mRNA vaccines. Pfizer’s dose contains 31 mg of sucrose, and Moderna’s contains 43.5 mg. The sugar acts as a cryoprotectant, preventing the lipid nanoparticles from clumping together or breaking apart during freezing and thawing. Without it, the carefully constructed fat bubbles would be destroyed by ice crystals.
Both vaccines also contain tromethamine (often called Tris), a buffering agent that keeps the solution at a stable, neutral pH around 7.4, matching your blood’s natural acidity. Moderna additionally includes trace amounts of acetic acid and sodium acetate for the same purpose. These are the same kinds of salts and acids found in many injectable medications and IV fluids.
Novavax: Adjuvant and Protein
Because Novavax delivers a pre-made protein rather than mRNA, it doesn’t need lipid nanoparticles. Instead, it includes an adjuvant called Matrix-M, a compound derived from the bark of the soapbark tree (Quillaja saponaria). The adjuvant’s job is to amplify your immune response. Without it, the spike protein alone might not trigger strong enough immunity.
This approach is closer to how vaccines have been made for decades. Protein-based vaccines with plant-derived adjuvants have a long track record in other diseases, which is one reason some people who were hesitant about mRNA technology felt more comfortable with Novavax.
Johnson & Johnson’s Ingredients
Though no longer available in the U.S., the J&J vaccine is worth understanding since millions of people received it. Its active ingredient was a modified adenovirus (Ad26) engineered to carry spike protein instructions but unable to replicate in your body. The inactive ingredients were straightforward: polysorbate 80 (an emulsifier common in foods and medicines), citric acid monohydrate (closely related to lemon juice), sodium chloride (table salt), ethanol (a small amount of alcohol), and a sugar compound called 2-hydroxypropyl-β-cyclodextrin used to stabilize the virus.
Potential Allergens
The ingredient most likely to cause an allergic reaction in mRNA vaccines is polyethylene glycol (PEG), the compound attached to one of the lipids in the nanoparticle shell. PEG is widely used in laxatives, skin creams, and other medications, so most people have already been exposed to it without issue. Severe allergic reactions to COVID mRNA vaccines are rare, occurring in roughly 2 to 5 cases per million doses.
The viral vector vaccines (J&J and AstraZeneca) contained polysorbate 80 instead of PEG. The two compounds are chemically related, which is why people with a known severe allergy to PEG were sometimes directed to avoid both types. Novavax also contains polysorbate 80.
What COVID Vaccines Don’t Contain
None of the COVID vaccines authorized in the U.S. contain thimerosal (a mercury-based preservative), aluminum adjuvants, formaldehyde, egg proteins, gelatin, or neomycin. These ingredients appear in some other vaccines, but they are not part of any COVID vaccine formula. The vials also do not use latex rubber stoppers, which matters for people with latex allergies.
As with all biologics manufactured using living cells, trace amounts of residual DNA from the production process can be present. The FDA sets strict limits: no more than 10 nanograms per dose, with fragments no larger than 200 base pairs. At those quantities, independent risk assessments have found the safety margin to be substantial. For perspective, 10 nanograms is 10 billionths of a gram, far less DNA than you’d encounter eating a piece of fruit.