Vaccines work by showing the immune system a harmless piece of a pathogen — called an antigen — so the body builds antibodies and memory cells without suffering the actual disease. Getting a vaccine to that point takes years of preclinical testing and three phases of human trials before regulators such as the FDA license it. Full protection after vaccination typically takes a few weeks, as the immune system needs time to produce lasting defenses.
Every vaccine, regardless of technology, relies on the same basic trick: it teaches the immune system to recognize a specific pathogen before a person ever encounters the real thing. According to the World Health Organization, vaccines contain weakened or inactivated parts of an organism — an antigen — that trigger the body's defenses without causing the disease itself.
What happens inside the body after a vaccine
Once injected, the antigen (or the genetic instructions to make one) is picked up by immune cells. The U.S. Centers for Disease Control and Prevention (CDC) explains that this process activates B-lymphocytes, which produce antibodies that bind to and neutralize the pathogen, and T-lymphocytes, which help coordinate the response and destroy infected cells.
The CDC's Pink Book, a reference manual for health professionals, describes this as triggering humoral immunity — antibody production by B-cells — and cell-mediated immunity, driven by T-cells. After the initial response, some B-cells become long-lived memory cells that remain in the blood and bone marrow, ready to multiply quickly if the real pathogen appears later.
This is why active immunity built by a vaccine or an infection can last for years or a lifetime, according to the CDC, in contrast to passive immunity — such as antibodies passed from a mother to a newborn — which fades within weeks to months because the body did not make the antibodies itself.
The WHO notes that full protection is not immediate: it takes the immune system time to generate antibodies and memory cells, and some vaccines require multiple doses spaced weeks or months apart to build durable defense. The CDC adds that for COVID-19 vaccines specifically, it typically takes a few weeks after vaccination for the body to produce the relevant T- and B-lymphocytes.
The different technologies behind vaccines
Not every vaccine delivers the antigen the same way. The CDC's Pink Book and the WHO describe several established categories still in use.
Main vaccine types and how they deliver the antigen
Type
How it works
Example diseases
Live-attenuated
A weakened but living version of the virus replicates briefly in the body, closely mimicking a natural infection
Measles, mumps, rubella, varicella
Inactivated (whole-cell)
A killed version of the virus or bacterium that cannot replicate; often needs multiple doses
Polio, hepatitis A, rabies
Subunit / recombinant
Only a specific protein piece of the pathogen is used to trigger a response
Influenza, pneumococcal, hepatitis B, HPV
Toxoid
Uses an inactivated toxin produced by the bacterium rather than the bacterium itself
Diphtheria, tetanus
Viral vector
A harmless, unrelated virus is engineered to carry and deliver a piece of the target pathogen
Ebola vaccines
mRNA / nucleic acid
Genetic instructions tell the body's own cells to briefly produce the antigen, which the immune system then targets
COVID-19 vaccines (Pfizer-BioNTech, Moderna)
mRNA vaccines work differently from older types because they do not contain any part of the virus itself. The CDC explains that laboratory-made mRNA instructs the body's own cells to produce a harmless piece of the target protein — for COVID-19 vaccines, a fragment of the spike protein. The immune system reacts to this protein as foreign, generates antibodies, and activates T-cells, after which the cells break down the mRNA and remove it as waste.
Protein subunit vaccines such as Novavax's COVID-19 vaccine instead inject the actual spike protein fragment directly, paired with an adjuvant — an ingredient added specifically to strengthen the immune reaction, according to the CDC.
How a vaccine moves from the lab to the clinic
Before any vaccine reaches a clinical trial, it goes through an exploratory and preclinical stage, in which researchers test it in laboratory and animal studies to judge whether it is likely to be safe and to provoke a useful immune response, according to the U.S. Food and Drug Administration (FDA). Developers then file an Investigational New Drug application with the FDA before human testing can begin.
Human testing proceeds through three main clinical phases, as described by the FDA:
Phase 1 — a small trial, generally 20 to 100 healthy volunteers, focused mainly on safety and early signs of immune response.
Phase 2 — expands to hundreds of participants with varying health profiles, comparing different doses and monitoring common side effects.
Phase 3 — tests thousands of participants, often against a placebo group, to measure real-world effectiveness and catch rarer side effects.
If the data support it, the manufacturer submits a Biologics License Application for the FDA to review before granting approval. Even after a vaccine is licensed, the FDA keeps monitoring its safety through post-marketing surveillance systems such as the Vaccine Adverse Event Reporting System (VAERS). The FDA states there is no fixed timeline for the overall process, since it depends on the vaccine's complexity and the data generated at each stage.
The WHO notes that historically only about one in five vaccine candidates that reach clinical trials is eventually approved.
Before the COVID-19 pandemic, none had yet been through the full approvals process for use in humans, though some DNA vaccines, including for particular cancers, were undergoing human trials.
— World Health Organization
Questions
How long does it take for a vaccine to start working?
According to the CDC, it typically takes a few weeks after vaccination for the immune system to produce enough antibody- and T-cell-based protection, which is why people can still get infected shortly after being vaccinated.
Can a vaccine give you the disease it protects against?
Inactivated, subunit, toxoid, viral-vector and mRNA vaccines cannot cause the disease because they do not contain a live, disease-causing pathogen, per the CDC. Live-attenuated vaccines contain a weakened virus that is designed not to cause illness in healthy people, though they are generally avoided in people with weakened immune systems.
What is the difference between an mRNA vaccine and a traditional vaccine?
Traditional vaccines usually deliver a piece of the pathogen itself (or a weakened/killed version of it), while mRNA vaccines deliver genetic instructions that tell a person's own cells to temporarily produce a piece of the antigen, according to the CDC.
Why does vaccine development usually take so long?
The FDA outlines a multi-stage process — preclinical testing, three phases of human trials, and regulatory review — each designed to confirm safety and effectiveness before a vaccine can be licensed; the agency says there is no fixed timeline because it depends on the data generated at each stage.
Does a vaccine provide lifelong immunity?
It depends on the vaccine and disease. The CDC's Pink Book notes that active immunity generated by vaccination can last for years or a lifetime, though some vaccines require booster doses to maintain protective antibody and memory-cell levels.
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