COVID-19 vaccines contain a small number of ingredients, and most of them are surprisingly ordinary. The active ingredient teaches your immune system to recognize the virus, and everything else in the vial exists to protect that active ingredient and keep it stable. There are no preservatives, no antibiotics, and no mercury-containing compounds in any of the COVID-19 vaccines available in the United States.
The Active Ingredient
The Pfizer and Moderna vaccines use messenger RNA (mRNA) as their active ingredient. Each dose of the Pfizer vaccine contains 30 micrograms of it, a nearly invisible amount. This mRNA carries instructions for your cells to build a harmless piece of the SARS-CoV-2 virus called the spike protein. Your immune system spots this protein, recognizes it 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.
One key modification makes this mRNA work well as a vaccine ingredient. A natural building block called uridine is swapped out for a modified version. This change prevents your immune system from destroying the mRNA before it can do its job, giving your cells enough time to read the instructions and produce the spike protein.
The Novavax vaccine takes a different approach. Instead of giving your body instructions to make the spike protein, it delivers the protein directly. Each dose contains 5 micrograms of a lab-made version of the spike protein, grown in insect cells derived from the fall armyworm (a moth relative). It also includes an adjuvant, a substance that strengthens your immune response, made from natural extracts of the soapbark tree.
The Lipid Shell
mRNA is fragile. Left on its own, it would fall apart before reaching your cells. Both the Pfizer and Moderna vaccines wrap their mRNA in a tiny bubble of fats called a lipid nanoparticle. This shell is made of four types of lipids, each with a specific job.
- Ionizable lipid (ALC-0315 in Pfizer, SM-102 in Moderna): This is the main structural fat. It carries a neutral charge in your bloodstream, which keeps it from being toxic to cells. Once it enters a cell and encounters a more acidic environment, it switches to a positive charge, which helps release the mRNA inside.
- PEG-lipid: A fat attached to polyethylene glycol, a compound widely used in laxatives, skin creams, and toothpaste. In the vaccine, it coats the outside of the nanoparticle, preventing the particles from clumping together and helping them circulate in your blood long enough to reach cells. Each Pfizer dose contains just 0.05 mg of this ingredient.
- Cholesterol: The same molecule found naturally in every cell membrane in your body. It fills gaps in the lipid shell and helps the nanoparticle hold its shape.
- DSPC (a phospholipid): A structural fat that stabilizes the outer layer of the nanoparticle, similar to how phospholipids stabilize your own cell membranes.
These lipid nanoparticles are tiny, roughly 80 to 100 nanometers across. For perspective, a human hair is about 80,000 nanometers wide. After delivering their mRNA cargo, the lipids are broken down and cleared by your body through normal metabolic processes.
Salts, Sugars, and Buffers
The remaining ingredients are simple compounds you’d find in many foods and medicines. Their job is to keep the vaccine stable and at the right pH level (not too acidic, not too basic) so it works properly when injected.
Sucrose, ordinary table sugar, is the most abundant inactive ingredient. The Pfizer vaccine contains 31 mg per dose. Sucrose acts as a cryoprotectant, shielding the delicate lipid nanoparticles from damage during freezing and thawing. Without it, the particles swell and lose their ability to deliver mRNA effectively. Both Pfizer and Moderna use tromethamine and tromethamine hydrochloride as pH buffers. Moderna also includes acetic acid (the same acid in vinegar) and sodium acetate for the same purpose.
The Novavax vaccine uses a slightly different set of stabilizers, including sodium chloride (table salt) and phosphate buffers, along with polysorbate 80, an emulsifier commonly found in ice cream and salad dressings.
What COVID-19 Vaccines Do Not Contain
COVID-19 vaccines do not contain preservatives of any kind. Thimerosal, a mercury-based preservative that was once common in flu vaccines and became the subject of safety debates, is not in any COVID-19 vaccine. They contain no antibiotics, no formaldehyde, no egg proteins, and no latex in the vial stoppers.
Fetal cell lines have come up in public discussion. The mRNA vaccines from Pfizer and Moderna are not manufactured using fetal cell lines, though cell lines originally derived from tissue in the 1970s and 1980s were used during early laboratory testing. These cell lines are lab-grown copies many generations removed from the original tissue, and no fetal cells are present in any finished vaccine.
Trace Residuals From Manufacturing
The manufacturing process for mRNA vaccines uses DNA templates (called plasmids) as a starting blueprint to create the mRNA. After the mRNA is produced, the DNA is removed through purification steps. Tiny fragments can remain, but in quantities measured in nanograms or picograms, billionths or trillionths of a gram. The FDA sets a maximum limit of 10 nanograms of residual DNA per dose, and extensive testing in animal studies has shown that even amounts far exceeding this limit pose negligible risk.
PEG and Allergic Reactions
Polyethylene glycol (PEG) is the one ingredient that has been linked to rare allergic reactions. PEG is found in both the Pfizer and Moderna vaccines as part of the lipid shell. The amount is very small, 0.05 mg in a Pfizer dose, which is thousands of times less than the amount of PEG 3350 in a single dose of common laxatives like MiraLAX. Most people who have experienced reactions to PEG-containing products reacted to doses of 28 mg or higher, with a molecular weight of 3350, a much larger molecule than the PEG 2000 used in vaccines. The Novavax vaccine does not contain PEG, making it an option for people with known PEG sensitivity. It uses polysorbate 80 instead, which is structurally related but distinct.
Comparing Ingredients Across Vaccines
All three available vaccines share a core simplicity: an active ingredient that teaches immune recognition, fats or surfactants that help with delivery or stability, salts that maintain pH, and sugar that protects during storage. The total amount of material in a single dose is remarkably small. A Pfizer dose is 0.3 mL, less than a tenth of a teaspoon, and the combined weight of all inactive ingredients is under 32 mg.
The Novavax vaccine stands apart because it uses a more traditional protein-based approach rather than mRNA. Its adjuvant, Matrix-M, contains 50 micrograms of saponin extracts split into two fractions. Saponins are naturally occurring compounds found in many plants, and they work by drawing immune cells to the injection site and making them respond more vigorously to the spike protein.