COVID-19 vaccines contain a small number of ingredients, and most of them serve simple purposes: deliver the active ingredient, keep the solution stable, and maintain the right acidity. The specific contents depend on which vaccine you receive, but none of the vaccines currently used in the United States contain preservatives, antibiotics, or mercury-based compounds like thimerosal.
Three COVID-19 vaccines are available in the U.S.: the Pfizer-BioNTech vaccine (Comirnaty), the Moderna vaccine (Spikevax), and the Novavax vaccine (Nuvaxovid). The first two use mRNA technology. Novavax uses a more traditional protein-based approach. Here’s what’s in each one and why.
The Active Ingredient
In the Pfizer and Moderna vaccines, the active ingredient is a small piece of messenger RNA, or mRNA. This genetic instruction tells your cells to temporarily produce copies of the spike protein found on the surface of the SARS-CoV-2 virus. Your immune system recognizes that protein as foreign and builds defenses against it. The mRNA itself breaks down within days and never enters the nucleus of your cells, where your DNA is stored.
A standard adult dose of the Pfizer vaccine contains 30 micrograms of mRNA. The Moderna vaccine originally contained 100 micrograms per dose, with booster doses using 50 micrograms. For context, a microgram is one-millionth of a gram.
The Novavax vaccine skips the mRNA step entirely. Instead of giving your body instructions to make the spike protein, it delivers the protein directly. Each dose contains 5 micrograms of lab-grown spike protein, produced using insect cells. Your immune system responds to this protein much the same way it would respond to the mRNA-generated version.
All three vaccines are updated periodically to match circulating virus strains. The formulas available starting in fall 2025 target a sublineage called LP.8.1, part of the JN.1 family of Omicron variants.
Lipid Nanoparticles in mRNA Vaccines
Raw mRNA is fragile. If injected on its own, your body would destroy it before it could do anything useful. Both Pfizer and Moderna solve this problem by wrapping the mRNA in tiny fat bubbles called lipid nanoparticles. These particles protect the mRNA during storage and transport it into your cells after injection.
The Pfizer vaccine uses four lipids. One is a specialty fat designed to fuse with cell membranes. Another contains polyethylene glycol (PEG), which helps the nanoparticle stay stable in solution. The remaining two are cholesterol and a phospholipid called DSPC, both of which are naturally found in human cell membranes. Moderna’s vaccine uses a similar four-lipid combination, with its own proprietary ionizable lipid called SM-102 in place of Pfizer’s version.
These lipids also function as a mild adjuvant, meaning they help stimulate the immune system’s response. In the Pfizer vaccine, the total amount of all four lipids combined comes to about 0.76 milligrams per dose.
The Novavax Adjuvant
Because the Novavax vaccine delivers a protein rather than mRNA, it needs a separate adjuvant to strengthen the immune response. It uses Matrix-M, which contains 50 micrograms per dose of saponin extracts from the bark of the soapbark tree. These natural compounds, split into two fractions called Fraction-A and Fraction-C, help recruit immune cells to the injection site and improve antibody production.
Stabilizers, Buffers, and Salts
The remaining ingredients keep the vaccine stable during storage and maintain the right pH level so the injection isn’t irritating.
Both the Pfizer and Moderna vaccines contain sucrose, which is ordinary table sugar. It acts as a stabilizer, protecting the lipid nanoparticles from damage during freezing. A single Pfizer dose contains 31 milligrams of sucrose. For comparison, a single grain of table sugar weighs about 625 micrograms, so the amount in a vaccine dose is roughly equivalent to a tiny pinch.
Both mRNA vaccines also contain tromethamine and tromethamine hydrochloride, which work together as a buffer to keep the solution at the correct acidity. Moderna adds acetic acid and sodium acetate for the same purpose. Novavax uses a combination of phosphate salts and sodium chloride (table salt) to maintain stability and pH, adjusted with small amounts of sodium hydroxide or hydrochloric acid as needed.
What’s Not in the Vaccines
None of the COVID-19 vaccines currently available in the U.S. contain preservatives, thimerosal, mercury, formaldehyde, or antibiotics. While some other vaccines do use formaldehyde during manufacturing or contain trace antibiotics, the COVID-19 vaccines do not.
The Johnson & Johnson COVID-19 vaccine, which was produced using fetal retinal cell lines, is no longer available in the United States. The Pfizer, Moderna, and Novavax vaccines are not manufactured using fetal cell lines. The Novavax vaccine may contain residual trace amounts of insect cell proteins and DNA from the manufacturing process, listed at no more than 0.96 micrograms and 0.00016 micrograms respectively.
Potential Allergens
The ingredient most commonly flagged for allergic reactions is polyethylene glycol, present in both the Pfizer and Moderna vaccines as part of their lipid nanoparticles. PEG is widely used in laxatives, skin creams, and other medications, and true allergies to it are rare but do exist.
The Novavax vaccine does not contain PEG. It does contain polysorbate 80 at 0.05 milligrams per dose, a chemically related compound used as an emulsifier. Polysorbate 80 appears in many foods and other vaccines. People with a known allergy to PEG are sometimes advised to choose the Novavax option instead, since the two compounds can occasionally cause cross-reactive allergic responses.
Full Ingredient Lists at a Glance
Pfizer-BioNTech (Comirnaty)
- Active ingredient: 30 mcg nucleoside-modified mRNA (adult dose)
- Lipids: ALC-0315 (ionizable lipid), ALC-0159 (PEG-lipid), DSPC, cholesterol
- Buffers: tromethamine, tromethamine hydrochloride
- Stabilizer: sucrose
Moderna (Spikevax)
- Active ingredient: nucleoside-modified mRNA
- Lipids: SM-102 (ionizable lipid), PEG 2000-DMG, DSPC, cholesterol
- Buffers: tromethamine, tromethamine hydrochloride, acetic acid, sodium acetate trihydrate
- Stabilizer: sucrose
Novavax (Nuvaxovid)
- Active ingredient: 5 mcg recombinant spike protein
- Adjuvant: 50 mcg Matrix-M (soapbark tree saponin extracts)
- Stabilizers and buffers: cholesterol, phosphatidylcholine, sodium chloride, phosphate salts, polysorbate 80
- Residual traces: insect cell proteins, DNA, and processing agents in extremely small amounts