How Vaccines Work

Your body has a defence system called the immune system. It fights germs that make you sick. The first time it meets a new germ, it is slow. You get ill while your body learns to fight.

A vaccine helps your body learn early, before you meet the real germ. A vaccine contains a weak or dead form of the germ, or just a small piece of it. It cannot make you sick, but your immune system still reacts. It makes special proteins called antibodies, and it remembers the germ.

Later, if the real germ enters your body, your immune system is ready. It fights fast, and you do not get seriously ill.

Vaccines also protect other people. If most people in a town are vaccinated, the disease cannot spread easily. This protects babies and sick people who cannot have the vaccine. This idea is called community immunity.

The immune system learns by experience. The first time it encounters a new pathogen, it takes days to mount an effective response, and during that delay you can become seriously ill. A vaccine removes the delay by teaching the system in advance.

It does this by presenting the immune system with something that looks like the pathogen but cannot cause disease: a weakened or inactivated version, an isolated protein from its surface, or, in newer vaccines, a set of genetic instructions that briefly get your own cells to produce that protein. In every case the immune system responds as if to a genuine threat. It generates antibodies tailored to the pathogen and, crucially, creates long-lived memory cells.

If you later meet the real pathogen, those memory cells recognise it and the response is fast and strong enough to stop an infection developing, or at least to prevent serious illness.

Vaccination also has a collective effect. When a high enough proportion of a population is immune, an infectious disease can no longer find enough susceptible people to sustain a chain of transmission. This "herd immunity" indirectly protects those who cannot be vaccinated — newborns, some people with weakened immune systems — and is how smallpox was eradicated and polio nearly so.

Vaccination exploits a feature of adaptive immunity that would otherwise be a liability: its slowness on first contact. Confronted with a genuinely novel pathogen, the immune system needs several days to select and expand the right responding cells, and it is during that lag that many infections do their damage. A vaccine collapses the lag by staging a rehearsal.

The staging can take several forms. Live-attenuated vaccines use a weakened strain that replicates just enough to provoke a robust response. Inactivated and subunit vaccines present killed pathogen or isolated components — typically a surface protein the immune system can latch onto. Nucleic-acid vaccines go further upstream, delivering mRNA or DNA that instructs the recipient's own cells to manufacture the target protein transiently. What unites them is the outcome: a primed repertoire of antibodies and, more importantly for durability, a population of memory B and T cells that persist for years and enable a rapid recall response.

The individual benefit is straightforward. The population-level benefit is subtler and often underappreciated. Every infectious disease has a threshold — determined by how transmissible it is — above which the fraction of immune individuals is high enough that outbreaks fizzle out rather than propagate. Reaching that threshold confers indirect protection on the unvaccinated minority and can, for pathogens with no non-human reservoir, drive a disease to elimination or global eradication. Smallpox is the completed case; measles and polio are unfinished ones, kept in check only by sustained coverage.

That dependence on sustained coverage is also the system's vulnerability. As a disease becomes rare, its threat becomes abstract, attention shifts to the small risks of the vaccine itself, and coverage can slip below the threshold — at which point the disease, if it still exists anywhere, returns.