COVID-19 vaccines contain a small number of ingredients, each with a specific job. The core component is either a strand of genetic instructions (mRNA) or a lab-made protein that teaches your immune system to recognize the virus. Everything else in the vial exists to protect that active ingredient, keep it stable, and deliver it into your cells. Here’s what’s actually in each type and why.
The Active Ingredient: Spike Protein Instructions
The Pfizer (Comirnaty) and Moderna (Spikevax) vaccines use messenger RNA, a molecule that carries instructions for building a single protein. In this case, the mRNA tells your cells to produce copies of the spike protein found on the surface of SARS-CoV-2. Your immune system sees these spike proteins, recognizes them as foreign, and builds antibodies and other defenses against them. If you later encounter the actual virus, your immune system is already trained to fight it.
The mRNA itself is chemically modified to make it more stable inside the body, but it’s still short-lived. It doesn’t enter the nucleus of your cells where DNA is stored, and it breaks down naturally within days. A single adult dose of Comirnaty contains 30 micrograms of mRNA. Spikevax contains a somewhat higher amount. Children’s doses contain less.
The Novavax vaccine (Nuvaxovid) takes a different approach. Instead of sending instructions, it delivers a pre-made version of the spike protein directly. This recombinant protein is grown in insect cells in a lab, purified, and then packaged into nanoparticles. Your immune system responds to it the same way: by learning to recognize and attack the spike protein.
Lipid Nanoparticles: The Delivery System
Raw mRNA would fall apart almost instantly in the bloodstream. To get it safely into your cells, both Pfizer and Moderna wrap the mRNA in tiny fat bubbles called lipid nanoparticles. Think of them as microscopic envelopes that protect the mRNA during transit and help it slip through cell membranes.
Each lipid nanoparticle is built from four types of fat:
- An ionizable lipid (ALC-0315 in Pfizer, SM-102 in Moderna) that carries a charge at low pH, helping the particle merge with and enter cells.
- A PEG-lipid (a fat attached to polyethylene glycol) that acts as a coating, preventing the nanoparticles from clumping together and helping them last long enough to reach cells.
- A phospholipid (DSPC) that provides structural support to the particle, similar to the fats in natural cell membranes.
- Cholesterol, which fills gaps between the other lipids and makes the particle more stable.
The amounts are tiny. A single dose of Comirnaty contains less than 1 milligram of lipids total. These fats are metabolized by the body the same way dietary fats are.
Stabilizers, Buffers, and Salts
The remaining ingredients keep the vaccine in working condition from the moment it’s manufactured through freezing, shipping, storage, and injection. In both mRNA vaccines, sucrose (ordinary table sugar) serves as a stabilizer, protecting the lipid nanoparticles from damage during freezing. Comirnaty contains 31 milligrams of sucrose per dose, roughly one-sixth of the sugar in a single Tic Tac.
Tromethamine and tromethamine hydrochloride act as pH buffers, keeping the solution at the right acidity level so the mRNA and lipids don’t degrade. Moderna’s formula also includes acetic acid and sodium acetate for the same purpose. These are common buffering agents used across many injectable medications.
The Novavax Adjuvant
Because Novavax delivers a protein rather than mRNA, it needs something extra to get a strong immune response. That something is Matrix-M, an adjuvant made from saponins purified from the bark of the Quillaja saponaria tree, combined with cholesterol and phospholipids. These components form tiny cage-like nanoparticles about 40 nanometers across. Matrix-M is a blend of two particle types called Matrix-A (85%) and Matrix-C (15%), each containing different saponin fractions that work together to amplify the immune response.
What’s Not in COVID Vaccines
COVID-19 vaccines do not contain thimerosal (the mercury-based preservative used in some older multi-dose flu vaccines). They contain no preservatives at all. They do not contain microchips, tracking devices, or any form of the live virus. You cannot get COVID-19 from any of these vaccines.
The mRNA vaccines do not contain eggs, latex, or metals. They do contain polyethylene glycol (PEG), which is worth knowing about because a small fraction of people have PEG sensitivity. PEG is widely used in everyday products like toothpaste, laxatives, and skin creams, so most people with a PEG allergy already know about it. Severe allergic reactions to the mRNA vaccines are rare but have been linked to PEG as the likely trigger.
Updated Formulas Target Current Variants
The ingredients listed above stay the same from year to year. What changes is the mRNA sequence itself. Just as flu vaccines are updated each season to match circulating strains, COVID vaccine formulas are revised to target the dominant variants. The 2024-2025 formula was updated to target the KP.2 strain, a descendant of the Omicron JN.1 lineage. The 2025-2026 Comirnaty formula targets the LP.8.1 sublineage. The lipids, buffers, and stabilizers remain identical. Only the genetic instructions are swapped out to match the current virus.
This is one of the practical advantages of mRNA technology: updating the vaccine doesn’t require redesigning the delivery system. Manufacturers change the mRNA sequence while keeping every other component the same, which speeds up the process of producing reformulated doses.