Vaccinations give protection against specific diseases, but the level of protection in a population depends on the proportion of people vaccinated.
Pathogens are microbes that cause diseases. Vaccines contain a dead or altered form of the disease-causing pathogen, which is introduced into the body. These dead or altered pathogens carry a specific antigen. This causes the immune system, specifically the white blood cells, to produce complementary antibodies, which target and attach to the antigen.
The body has two kinds of white blood cells which work together to destroy the pathogens.
The white blood cells which make antibodies remain in the body afterwards. These are called memory cells. They enable a more rapid and larger build-up of antibodies following a second exposure to the pathogen.
During the primary infection the antibodies slowly increase, peak at around ten days and then gradually decrease. A second exposure to the same pathogen causes the white blood cells to respond quickly in order to produce lots of the relevant antibodies, which prevents infection.
Some vaccinations last longer than others, so new vaccines need to be developed regularly. This is because the DNA of some pathogens mutates frequently, while others do not change. Once you are fully vaccinated against measles, you will be protected for at least 20 years. However, the influenza viruses' DNA mutates frequently, producing new antigens. The memory cells will not recognise this year's new strain of flu, and so a different vaccination is needed each year to give immunity.
Vaccination wiped out smallpox. In 1967 there were 10 to 15 million cases of smallpox worldwide. That year WHO began a campaign to vaccinate people all over the world. The last natural case of smallpox was recorded in Africa in 1977.