Comirnaty is the brand name for the Pfizer-BioNTech COVID-19 vaccine, an mRNA vaccine that received full FDA approval on August 23, 2021. It was the first COVID-19 vaccine to move from emergency use authorization to full approval in the United States. The vaccine is manufactured by BioNTech Manufacturing GmbH in partnership with Pfizer and is approved for people ages 5 and older, with different doses for children and adults.
How Comirnaty Works
Comirnaty uses messenger RNA (mRNA) technology, which works differently from traditional vaccines. Instead of injecting a weakened or inactivated virus, the vaccine delivers a small piece of synthetic mRNA wrapped in tiny fat bubbles called lipid nanoparticles. These fat bubbles protect the mRNA and help it get inside your cells.
Once inside a cell, the mRNA provides instructions for building a copy of the spike protein found on the surface of the SARS-CoV-2 virus. Your cells read these instructions, produce the spike protein, and display it on their surface. Your immune system recognizes the spike protein as foreign and mounts a response, creating antibodies and training immune cells to fight the real virus if you encounter it later. The mRNA itself is short-lived and breaks down quickly. It never enters the cell’s nucleus where your DNA is stored.
What’s in the Vaccine
The active ingredient is a modified mRNA molecule that encodes the spike protein of a specific SARS-CoV-2 variant. For people 12 and older, each dose contains 30 micrograms of this mRNA. The children’s dose (ages 5 through 11) contains 10 micrograms.
The inactive ingredients are relatively simple. Four types of lipids (fats) form the protective nanoparticle shell around the mRNA, including cholesterol. The remaining ingredients are tromethamine (a buffering agent that keeps the solution at the right pH), tromethamine hydrochloride, and sucrose (table sugar), which acts as a stabilizer. There are no preservatives, antibiotics, or egg proteins in the formulation.
Updated Formulas and Variant Targets
Like the flu vaccine, Comirnaty’s formula is updated periodically to match circulating virus strains. The 2024-2025 formula targets the Omicron KP.2 variant lineage. A newer 2025-2026 formula has been developed targeting the Omicron LP.8.1 sublineage. Each update swaps in a new mRNA sequence coding for the spike protein of the targeted variant while keeping the same lipid nanoparticle delivery system and inactive ingredients.
Clinical Efficacy
Comirnaty’s efficacy has been measured across multiple clinical trials and formula updates. In the original pivotal trial for children ages 5 through 11, the two-dose series showed 88.2% efficacy against symptomatic COVID-19 infection, measured in children with no prior evidence of infection. Among children in that age group who had at least one underlying health condition, efficacy was even higher at 92.3%.
Efficacy numbers shift with each new variant and updated formula. For the most recent formulations, the FDA has required ongoing post-marketing studies, including randomized, placebo-controlled trials in adults ages 50 through 64, to continue tracking how well updated vaccines prevent confirmed COVID-19 illness.
Common and Rare Side Effects
The most frequently reported side effects are what you would expect from any vaccine that activates the immune system: pain or swelling at the injection site, fatigue, headache, muscle aches, chills, and sometimes fever. These typically appear within a day or two of vaccination and resolve on their own within a few days.
The most closely watched rare side effect is myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the tissue surrounding the heart). Based on insurance claims data from the 2023-2024 formula, the estimated rate was roughly 8 cases per million doses among people ages 6 months through 64. The risk is highest in males ages 12 through 24, where the rate was approximately 27 cases per million doses. Most cases occurred within seven days of vaccination. The FDA now requires updated warning labeling on all mRNA COVID-19 vaccines regarding this risk.
Dosing by Age Group
Comirnaty is given as an intramuscular injection in the upper arm (or thigh for young children). The dose volume is 0.3 mL for both age groups, but the concentration of mRNA differs. Adults and adolescents 12 and older receive 30 micrograms of mRNA per dose, while children ages 5 through 11 receive 10 micrograms. The number of doses and timing depend on vaccination history and age, so the schedule varies from person to person. The adult dose comes in a prefilled syringe, while the children’s dose comes in a vial.
Storage Requirements
One of the early logistical challenges with mRNA vaccines was their cold storage needs. Comirnaty can be shipped and stored frozen at ultra-cold temperatures between negative 90°C and negative 60°C (roughly negative 130°F to negative 76°F). However, it can also be transferred to a standard refrigerator (2°C to 8°C) upon receipt and stored there for up to 10 weeks, as long as it doesn’t exceed the printed expiration date. This flexibility has made distribution to pharmacies and clinics much more practical compared to the earliest days of the vaccine rollout, when ultra-cold freezers were a significant barrier.