COVID-19 vaccines teach your immune system to recognize and fight the virus that causes COVID-19, specifically by targeting a structure on the virus called the spike protein. Once your immune system learns to identify this protein, it can respond quickly if you’re ever exposed to the real virus, reducing your chances of getting seriously ill. Several types of COVID-19 vaccines exist, each using a different method to introduce your body to that spike protein.
How mRNA Vaccines Work
The most widely used COVID-19 vaccines, made by Pfizer-BioNTech and Moderna, use a technology called mRNA. These vaccines deliver a small piece of genetic instructions into your muscle cells. Your cells read those instructions, build a harmless copy of the spike protein found on the surface of the coronavirus, and display it. Your immune system spots the protein, recognizes it as foreign, and produces antibodies designed to latch onto it and mark it for destruction.
The mRNA itself breaks down quickly and never enters the nucleus of your cells, meaning it doesn’t interact with your DNA. What remains are the antibodies and immune memory cells your body created in response. If you encounter the actual virus later, those antibodies recognize the spike protein immediately and mount a defense before the infection can take hold and cause serious illness.
How Protein Subunit Vaccines Work
The Novavax vaccine takes a more traditional approach. Instead of giving your cells instructions to build the spike protein, it delivers pre-made spike proteins directly. These lab-grown protein pieces are combined with an adjuvant, an ingredient that acts like a signal flare to get your immune system’s attention. Your immune cells detect the foreign proteins, and with the adjuvant amplifying the alarm, your body produces antibodies and activates other immune defenses just as it would with an mRNA vaccine. The end result is the same: trained immunity against the coronavirus.
What’s Actually in the Vaccines
Beyond the active ingredient (either mRNA or spike proteins), COVID-19 vaccines contain a short list of inactive ingredients that keep the vaccine stable and functional. The mRNA vaccines use tiny fat particles called lipids to wrap the fragile mRNA and help it enter your cells, along with cholesterol, salts, buffers to maintain the right pH, and sucrose (table sugar) as a stabilizer. There are no preservatives, no fetal cells, and no microchips. The protein subunit vaccine similarly contains salts, buffers, and its adjuvant. The full ingredient lists are published by the FDA.
Two Layers of Immune Protection
Vaccination triggers two distinct arms of your immune system. The first is antibody production. Antibodies circulate in your blood and can neutralize the virus on contact, which is your front line of defense against infection. These antibody levels do decline over time. Research has shown that neutralizing antibody levels drop significantly within about six months of vaccination.
The second arm involves T-cells, a type of white blood cell that hunts down and destroys cells already infected by the virus. T-cell responses tend to be more durable than antibody levels, and studies have found that virus-specific T-cells remain detectable in most people at least 14 months after exposure. T-cells are also better at recognizing newer variants. Even when antibody levels against variants like Omicron dropped substantially, T-cell responses remained largely intact. This is one reason why vaccinated people who do get infected typically have milder illness: even if antibodies couldn’t block infection entirely, T-cells limit the damage.
How Effective the Vaccines Are
COVID-19 vaccines are most effective at preventing severe illness, hospitalization, and death. Their ability to prevent mild infection is more modest and fades faster. CDC data from the 2024-2025 season found that the updated vaccines reduced emergency department and urgent care visits by about 33% in the first four months after vaccination. Among adults 65 and older, effectiveness against hospitalization was roughly 45% to 46% during the same window. For older adults with weakened immune systems, that figure was around 40%.
These numbers may seem moderate compared to the 90%-plus efficacy reported in the original clinical trials, but context matters. The virus has mutated significantly since 2020, and most people now have some baseline immunity from prior infections or vaccinations. Protection against the most severe outcomes, including ICU admission and death, has historically been considerably higher and longer-lasting than protection against milder illness, though hospitalization rates during this period were too low to generate precise estimates for critical outcomes.
Current Recommendations
The CDC’s Advisory Committee on Immunization Practices recommends an updated 2024-2025 COVID-19 vaccine for everyone aged 6 months and older. The updated formulas target currently circulating strains of the virus, similar to how the flu shot is reformulated each year.
Most people need one dose of the updated vaccine. Adults 65 and older should receive two doses, spaced at least six months apart (with a minimum interval of two months), because older immune systems produce a less robust response. People with moderate or severe immune compromise should also receive at least two doses, and their doctors may recommend three or more based on individual circumstances.
Common Side Effects
The most frequent side effects are mild and short-lived. Soreness, redness, or swelling at the injection site is the most common reaction across all vaccine types. Systemic effects like fatigue, headache, muscle pain, chills, and low-grade fever are also common, particularly after mRNA vaccines. Some people experience joint pain or nausea. In young children, irritability, decreased appetite, and sleepiness are the typical reactions.
These side effects generally appear within a day or two of vaccination and resolve on their own within one to three days. They’re signs that your immune system is responding to the vaccine and building protection. Even if you experience side effects like vomiting, shortness of breath, or a rash at the injection site within four hours of your shot, you can typically still receive the same type of vaccine for future doses.
Why Development Was Fast but Not Rushed
COVID-19 vaccines went from concept to emergency authorization in under a year, which understandably raised questions about whether safety steps were skipped. They weren’t. The speed was possible because of decades of groundwork. Researchers at the National Institutes of Health had been studying mRNA technology and coronavirus biology for years before the pandemic. The clinical trial infrastructure built during the HIV/AIDS epidemic and refined during the Ebola outbreak in 2014-2018 provided a ready-made framework for enrolling tens of thousands of volunteers quickly.
The federal initiative known as Operation Warp Speed, launched in May 2020, also made a difference by allowing vaccine manufacturing to begin at the same time as clinical trials, rather than waiting for results first. This was a financial gamble, not a scientific shortcut. If the trials had failed, the pre-manufactured doses would have been discarded. The clinical trials themselves followed the standard three-phase process: small safety studies first, then expanded trials for dosing, and finally large-scale Phase 3 trials with tens of thousands of participants to confirm both safety and efficacy. Moderna’s Phase 3 trial reported promising interim results in November 2020, just eight months after clinical testing began in March of that year.