The first time the body meets a germ, it is slow. The immune system needs time to build antibodies tailored to that one invader, and the person can fall ill while it catches up. Vaccines, the World Health Organization explains, are a way to run that first meeting without the disease.
Antibodies and memory
The WHO describes antibodies as soldiers, each trained to spot a single antigen, the part of a pathogen that sets off antibody production. The body holds thousands of types. Antibodies against one pathogen generally don’t protect against another, unless the two are close relatives.
After the first infection, the body also makes memory cells. They stay alive after the pathogen is beaten. If the same germ turns up again, those cells are ready to pump out antibodies, so the response is faster and stronger than the first time.
What is in a vaccine
Some vaccines carry weakened or inactive pieces of an organism. Others use a whole weakened or reconstituted virus or bacterium. Newer ones hold the blueprint for making an antigen, in the form of DNA or RNA, and let the body’s own cells produce it. In every case the vaccine does not cause the disease, but it prompts a response much like the one the real pathogen would.
Some vaccines need several doses, weeks or months apart, to produce long-lived antibodies and memory cells.
Protecting people who can’t be vaccinated
Not everyone can get a shot. People whose immune systems are weakened by conditions such as cancer or HIV, or who have severe allergies to a vaccine component, may be unable to receive certain vaccines. They can still gain protection from the people around them.
When enough people in a community are immune, a pathogen struggles to circulate. That is herd immunity. The WHO notes that no single vaccine gives 100 percent protection, and herd immunity doesn’t fully shield those who can’t be vaccinated, but it gives them substantial cover.
The record on polio and measles
The WHO certified smallpox eradicated in 1980. Polio, which paralysed hundreds of thousands of people a year worldwide in the early 1900s, had two effective vaccines by 1950. A global eradication push began in the 1980s. In August 2020 the African continent was certified free of wild poliovirus, leaving Pakistan and Afghanistan as the places where polio had not been eradicated.
Before the measles vaccine arrived in 1963, major epidemics hit every few years and killed an estimated 2.6 million people annually, mostly young children. From 2000 to 2023, measles vaccination prevented an estimated 60 million deaths. Outbreaks still occur where children go unvaccinated.
Consult your doctor for medical advice.
