The COVID vaccine trains your immune system to recognize and fight the coronavirus before you ever get sick. It does this by teaching your body to identify a specific part of the virus called the spike protein, the structure the virus uses to enter your cells. Once your immune system learns to spot this protein, it can respond faster and more effectively if you encounter the real virus, dramatically reducing your chances of severe illness, hospitalization, and death.
How mRNA Vaccines Work
Most COVID vaccines available today use mRNA technology. These vaccines contain a small piece of genetic instructions that tell your cells how to build a harmless copy of the spike protein found on the surface of the coronavirus. Your cells read these instructions, produce the spike protein, and then display it on their surface. The mRNA itself breaks down within days and never enters the nucleus of your cells, where your DNA is stored.
Your immune system spots these spike proteins, recognizes them as foreign, and mounts a defense. It produces antibodies, specialized proteins that latch onto the spike protein and mark it for destruction. At the same time, your body activates two other key players: B cells, which are responsible for producing those antibodies, and T cells, which can directly kill infected cells. This coordinated response is what gives you protection.
The critical part happens after the initial response fades. Your body retains memory B cells and memory T cells that persist long after vaccination. These memory cells live in your lymph nodes and tissues, ready to reactivate quickly if the real virus shows up. Think of it like a fire drill: your immune system has already rehearsed its response, so when the actual threat arrives, it reacts faster and more powerfully than it would encountering the virus for the first time.
Protein-Based Vaccines Take a Different Route
Not all COVID vaccines use mRNA. The Novavax vaccine uses an older, more established technology. Instead of giving your cells genetic instructions to build the spike protein, Novavax delivers the spike protein itself, pre-made in a lab and assembled into tiny clusters called nanoparticles. The vaccine also includes an adjuvant, a substance that amplifies your immune response to the protein.
The end result is the same: your immune system sees the spike protein, learns to recognize it, and builds memory cells for future protection. The difference is simply how the protein gets introduced to your body. Some people who prefer a non-mRNA option choose Novavax for this reason.
What the Vaccine Protects Against
The primary goal of COVID vaccination has always been preventing severe disease rather than blocking every infection. All forms of COVID immunity, whether from vaccination, prior infection, or both, provide strong protection against severe, critical, or fatal illness regardless of the variant involved. CDC data from the 2024-2025 season showed that the updated vaccine reduced hospitalizations by 45-46% among adults 65 and older compared to those who hadn’t received the latest dose. For immunocompromised adults in the same age group, that figure was about 40%.
These numbers may seem modest compared to the 90%+ efficacy figures from the original vaccine trials in 2020. That’s partly because the virus has evolved significantly, and partly because most people now have some level of existing immunity from prior infections or earlier vaccinations. The vaccine is doing its heaviest lifting in a population that already has baseline protection, so the additional benefit appears smaller in percentage terms even though it remains clinically meaningful.
One thing the vaccine does less effectively is prevent transmission. Research published in The Lancet found that vaccinated and unvaccinated people carry similar levels of virus in their upper airways during infection. So while vaccination substantially lowers your risk of getting seriously ill, it doesn’t reliably stop you from passing the virus to others if you do get infected.
Reducing the Risk of Long COVID
Beyond preventing hospitalization, vaccination appears to cut the risk of developing long COVID, the persistent symptoms like fatigue, brain fog, and shortness of breath that can linger for months after infection. A large study across the UK, Spain, and Estonia found that vaccinated people were roughly 30-50% less likely to develop long COVID symptoms compared to unvaccinated individuals. The CDC now lists lowering your risk of long COVID as one of the specific reasons to stay current with vaccination.
Hybrid Immunity: Infection Plus Vaccination
If you’ve already had COVID and also been vaccinated, you have what researchers call hybrid immunity, and it’s the strongest form of protection observed so far. Studies from 2021 found that people with hybrid immunity from a prior infection plus two vaccine doses had 94-97% effectiveness against infection with the Beta and Delta variants. Three doses pushed protection against Delta to 98%.
This doesn’t mean you should seek out infection on purpose. But if you’ve already had COVID, getting vaccinated afterward adds a significant layer of protection. The immune effects of infection and vaccination appear to work independently, building on each other without redundancy. Your body essentially gets trained by two slightly different versions of the same threat, broadening its ability to respond.
Common Side Effects
The most frequent reactions to COVID vaccination are short-lived and predictable. At the injection site, you can expect soreness or tenderness, and occasionally some swelling or redness. Systemically, people commonly report fatigue, headache, muscle aches, chills, joint pain, and fever. These symptoms typically peak within a day or two and resolve on their own. They’re signs that your immune system is responding to the vaccine, not signs that something has gone wrong.
Younger children, particularly those under 3, tend to show irritability, crying, sleepiness, and reduced appetite rather than the headache-and-fatigue pattern seen in adults.
Rare but serious side effects do occur. Myocarditis and pericarditis (inflammation of the heart muscle or its surrounding lining) have been observed in a small number of cases, most frequently in adolescent and young adult males within a week of receiving their second mRNA dose. Cases have also occurred in females and after other doses, and data suggest some risk following Novavax vaccination as well. Most cases of vaccine-associated myocarditis have been mild and resolved with standard care.
Current Vaccine Options and Recommendations
The CDC recommends a 2025-2026 COVID vaccine for everyone 6 months and older, with the decision framed as an individual one. Four options are currently available: Moderna’s Spikevax (ages 6 months and up), Moderna’s mNexspike (ages 12 and up), Pfizer-BioNTech’s Comirnaty (ages 5 and up), and Novavax’s Nuvaxovid (ages 12 and up).
Vaccination is considered especially important if you are 65 or older, have never received a COVID vaccine, are at high risk for severe illness, live in a long-term care facility, or are pregnant or planning to become pregnant. If you recently had COVID, you can wait about three months after your symptoms started before getting vaccinated, since your body already has fresh immunity from the infection.