COVID-19 vaccines contain a surprisingly short list of ingredients: an active component that teaches your immune system to recognize the virus, a handful of fats or proteins that deliver or boost that active ingredient, and a few basic stabilizers like sugar and salt. None of the COVID-19 vaccines currently available in the U.S. contain preservatives, antibiotics, mercury, or formaldehyde.
Three vaccines are available in the United States as of 2025: two mRNA vaccines (Pfizer’s Comirnaty and Moderna’s Spikevax) and one protein-based vaccine (Novavax’s Nuvaxovid). Each uses a different approach to trigger immunity, but all target the spike protein found on the surface of the SARS-CoV-2 virus. Here’s what’s actually in each one.
The Active Ingredient: Teaching Your Immune System
The mRNA vaccines from Pfizer and Moderna contain a small strand of messenger RNA, a molecule that carries genetic instructions. This mRNA tells your cells to temporarily produce copies of the virus’s spike protein, the part of the virus that latches onto human cells. Your immune system spots these spike proteins, recognizes them as foreign, and builds defenses against them. The mRNA itself breaks down within days and never enters the nucleus of your cells, where your DNA is stored.
A single dose of Pfizer’s vaccine contains 30 micrograms of mRNA, while Moderna’s contains 100 micrograms. Both are currently formulated to match newer Omicron subvariants. The 2024-2025 formulas targeted the KP.2 strain (Pfizer and Moderna) and JN.1 strain (Novavax), and the formulas are updated periodically as the virus evolves, similar to how flu vaccines are updated each year.
Novavax takes a different approach. Instead of mRNA instructions, it contains 5 micrograms of the spike protein itself, grown in a lab using insect cells. Your immune system responds to this protein directly, without your own cells needing to produce anything. This is a more traditional vaccine technology, similar in concept to hepatitis B and shingles vaccines.
Lipid Nanoparticles: The Delivery System
Raw mRNA is fragile. Left unprotected, your body’s enzymes would shred it before it could reach a cell. So both Pfizer and Moderna wrap their mRNA in tiny fat bubbles called lipid nanoparticles. Think of them as microscopic envelopes that protect the mRNA during injection and help it slip inside your cells.
Each lipid nanoparticle is made of four types of fat:
- An ionizable lipid (called ALC-0315 in Pfizer’s vaccine and SM-102 in Moderna’s) that helps the particle fuse with your cell membranes and release the mRNA inside.
- A PEG-lipid (a fat molecule attached to polyethylene glycol) that acts as a coating, preventing the nanoparticles from clumping together and helping them stay stable in the liquid.
- A structural phospholipid (DSPC) that forms the shell of the nanoparticle, much like the fats that make up your own cell membranes.
- Cholesterol, the same molecule found naturally in every cell of your body, which fills gaps in the nanoparticle shell and keeps it sturdy.
The total amount of fat in a single dose is tiny. Pfizer’s vaccine contains less than 1 milligram of lipids combined. For comparison, a single almond contains roughly 600 milligrams of fat.
The Novavax Adjuvant: Matrix-M
Because Novavax delivers a protein rather than mRNA, it needs something extra to get a strong immune response. That something is an adjuvant called Matrix-M, which is made from saponin extracts of the soapbark tree (Quillaja saponaria). Each dose contains two fractions of this extract: 42.5 micrograms of Fraction-A and 7.5 micrograms of Fraction-C. Saponins stimulate your immune cells to pay closer attention to the spike protein in the vaccine, producing a stronger and longer-lasting response than the protein alone would generate.
Stabilizers, Buffers, and Salts
The remaining ingredients exist to keep the vaccine stable and effective from the moment it’s manufactured to the moment it’s injected. These are ordinary substances found in many foods and medicines.
Sucrose, plain table sugar, is the main stabilizer in both mRNA vaccines. It protects the fragile lipid nanoparticles during freezing, prevents them from sticking together, and keeps the vaccine intact during storage. Pfizer’s vaccine contains 31 milligrams of sucrose per dose, roughly one-eighth of a teaspoon.
Both vaccines also contain buffers, compounds that keep the liquid at the right pH so the mRNA and lipids don’t degrade. Pfizer’s original formula used common salts like sodium chloride and potassium chloride for this purpose. The current formula uses tromethamine and tromethamine hydrochloride, which are standard pharmaceutical buffering agents also used in many eye drops and injectable medications. Moderna’s vaccine uses tromethamine, tromethamine hydrochloride, and sodium acetate as its buffering system.
What’s Not in the Vaccines
None of the COVID-19 vaccines currently available in the U.S. contain preservatives of any kind. There is no thimerosal (a mercury-based preservative used in some multi-dose flu vaccines), no formaldehyde, no antibiotics, and no aluminum. The ingredient lists are publicly available in the FDA-approved package inserts for each vaccine.
A common question involves fetal cell lines. The Johnson & Johnson vaccine, which is no longer available in the U.S., used a laboratory-grown fetal retinal cell line (originally derived decades ago) during its manufacturing process. However, even that vaccine did not contain fetal cells in the final product. The manufacturing process purifies the vaccine components away from the cells used to grow them. The mRNA vaccines from Pfizer and Moderna do not use fetal cell lines in their manufacturing at all, since their mRNA is produced through a synthetic chemical process.
Potential Allergens
The ingredient most associated with rare allergic reactions in the mRNA vaccines is polyethylene glycol (PEG), the coating on the lipid nanoparticles. PEG is widely used in everyday products including laxatives, skin creams, and toothpaste, and true PEG allergy is uncommon. The Novavax vaccine does not contain PEG but does contain polysorbate 80, a chemically related compound also used in many foods and medications. People with a known severe allergy to PEG are generally advised to choose the Novavax option, and vice versa.
Severe allergic reactions to COVID-19 vaccines occur but are rare, estimated at roughly 2 to 5 cases per million doses administered. This is why vaccination sites ask you to wait 15 minutes after your shot before leaving.