Vaccines work by showing the immune system a harmless piece or weakened version of a pathogen, so the body builds antibodies and memory cells before it ever meets the real infection. When exposed later, a vaccinated immune system reacts faster and more effectively, which is why most vaccines require more than one dose. This same principle underlies all vaccine types, from decades-old live-attenuated shots to newer mRNA vaccines.
Vaccination trains the immune system against a specific pathogen without causing the disease itself. According to the World Health Organization (WHO), vaccines contain either weakened or inactivated parts of a pathogen — called antigens — or, in newer vaccines, the genetic instructions (DNA or RNA) that let the body's own cells produce those antigens temporarily.
What happens inside the body after a vaccine
Once an antigen enters the body, the immune system identifies it as foreign and produces antibodies, proteins that the WHO describes as acting like soldiers trained to recognize one specific antigen. The U.S. Centers for Disease Control and Prevention (CDC) describes this process as the vaccine imitating an infection closely enough to teach the immune system how to respond, without producing the symptoms of the actual disease.
The first encounter with an antigen — whether through a vaccine or a real infection — takes the immune system time to respond to. During this process, the body also creates memory cells that persist long after the antigen is cleared. The WHO notes that if the same pathogen appears again, the antibody response is much faster and more effective than the first time around, because memory cells are already primed to act.
This is also why many vaccines are given in a series of doses spaced weeks or months apart: according to the WHO, this spacing helps the immune system build longer-lasting antibodies and more robust memory-cell responses than a single dose would.
The different types of vaccines
Not all vaccines use the same method to present an antigen to the immune system. The main approaches used in licensed vaccines today are summarized below, based on WHO and CDC descriptions.
Main vaccine types and how they work
Type
How it works
Example use
Live-attenuated
Uses a weakened form of the live pathogen that can still replicate a little, triggering a strong, often long-lasting immune response
Measles, mumps, rubella vaccines
Inactivated
Uses a killed version of the pathogen; generally needs multiple doses over time for lasting protection
Some polio and influenza vaccines
Subunit/toxoid
Uses only a piece of the pathogen (a protein or sugar) or an inactivated toxin rather than the whole organism
Hepatitis B, tetanus vaccines
mRNA
Delivers genetic instructions so the body's own cells temporarily produce an antigen (e.g., a viral protein), without using a live virus
Certain COVID-19 vaccines
Viral vector
Uses a modified, harmless version of a different virus to carry genetic material from the target pathogen into cells
Certain COVID-19 and Ebola vaccines
What vaccines actually contain
Beyond the antigen itself, the CDC lists several categories of ingredients used in manufacturing: stabilizers such as sugars or gelatin that keep the vaccine effective; adjuvants such as aluminum salts that help boost the immune response; inactivating agents such as formaldehyde used during production; cell-culture materials such as egg protein; and small amounts of antibiotics to prevent contamination during manufacturing. The CDC states that modern vaccines contain only the ingredients needed for safety and effectiveness.
On a frequently raised concern, the CDC clarifies that most vaccines do not contain mercury at all; only some multi-dose flu vaccines and certain tetanus vaccines contain a mercury-based preservative called thimerosal, which the agency states is safe for use in vaccines.
Herd immunity and protecting the wider community
When a large share of a population is immune to a pathogen — through vaccination or, less safely, prior infection — the pathogen has fewer opportunities to spread. The WHO explains that this indirect protection, known as herd immunity, helps shield people who cannot be vaccinated themselves, such as those with certain health conditions or severe allergies.
The UK's National Health Service (NHS) gives a concrete example: for measles, a vaccination rate of around 95% would be high enough to stop the disease from spreading within a community, while the NHS notes that coverage needs to stay above roughly 90% to prevent outbreaks from resurfacing. The required threshold varies by disease, depending on how easily each pathogen spreads.
Vaccinating not only protects yourself but also protects those in the community who are unable to be vaccinated.
— World Health Organization
Measured public health impact
The WHO states that vaccination has nearly eradicated polio worldwide and that measles vaccination alone is estimated to have prevented about 60 million deaths between 2000 and 2023. The NHS similarly notes that since vaccines were introduced in the UK, diseases such as smallpox and polio have been eliminated or become extremely rare.
Safety monitoring
According to the NHS, vaccines undergo rigorous testing before approval and continued monitoring for side effects after they are introduced. The NHS lists common, temporary side effects such as arm soreness, mild fever and fatigue lasting one to three days, and notes that serious allergic reactions are rare and typically occur within minutes of vaccination, which is why vaccination sites have staff trained to respond immediately.
Questions
How quickly do vaccines start working?
The immune system needs time after the first dose to produce antibodies and memory cells, which is why full protection is not immediate; the WHO notes that multiple doses spaced over weeks or months help build stronger, longer-lasting immunity.
Can a vaccine give you the disease it protects against?
Inactivated, subunit, mRNA and viral-vector vaccines do not contain a live, disease-causing pathogen, so according to the CDC they cannot cause the disease; even live-attenuated vaccines use a weakened form designed not to cause illness in healthy people.
What is herd immunity and why does it matter?
Herd immunity is the indirect protection that occurs when enough people in a community are immune, making it harder for a pathogen to spread; the WHO says this protects people who cannot be vaccinated, while the NHS cites roughly 95% coverage as the threshold needed to stop measles transmission.
Do vaccines contain mercury?
According to the CDC, most vaccines contain no mercury at all; only some multi-dose flu vaccines and certain tetanus vaccines contain the preservative thimerosal, which the CDC states is safe for use.
What is the difference between mRNA and traditional vaccines?
Traditional vaccines deliver a weakened, inactivated, or partial version of the pathogen itself, while mRNA vaccines deliver genetic instructions that let the body's own cells temporarily produce a piece of the pathogen, such as a viral protein, to trigger the same immune response.
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