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A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe, its toxins or one of its surface proteins. The agent stimulates the body’s immune system to recognize the agent as foreign, destroy it, and “remember” it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.

Vaccines may be prophylactic (example: to prevent or ameliorate the effects of a future infection by any natural or “wild” pathogen), or therapeutic (e.g. vaccines against cancer are also being investigated; see cancer vaccine).

The term vaccine derives from Edward Jenner’s 1796 use of cow pox (Latin variola vaccinia, adapted from the Latin vaccīn-us, from vacca, cow), to inoculate humans, providing them protection against smallpox.

Vaccines do not guarantee complete protection from a disease.[2] Sometimes, this is because the host’s immune system simply does not respond adequately or at all. This may be due to a lowered immunity in general (diabetes, steroid use, HIV infection, age) or because the host’s immune system does not have a B cell capable of generating antibodies to that antigen.

Even if the host develops antibodies, the human immune system is not perfect and in any case the immune system might still not be able to defeat the infection immediately. In this case, the infection will be less severe and heal faster.

Adjuvants are typically used to boost immune response. Most often aluminium adjuvants are used, but adjuvants like squalene are also used in some vaccines and more vaccines with squalene and phosphate adjuvants are being tested. Larger doses are used in some cases for older people (50–75 years and up), whose immune response to a given vaccine is not as strong.[3]

Maurice Hilleman’s measles vaccine is estimated to prevent 1 million deaths every year.[4]

The efficacy or performance of the vaccine is dependent on a number of factors:

the disease itself (for some diseases vaccination performs better than for other diseases)
the strain of vaccine (some vaccinations are for different strains of the disease)[5]
whether one kept to the timetable for the vaccinations (see Vaccination schedule)
some individuals are “non-responders” to certain vaccines, meaning that they do not generate antibodies even after being vaccinated correctly
other factors such as ethnicity, age, or genetic predisposition.

When a vaccinated individual does develop the disease vaccinated against, the disease is likely to be milder than without vaccination.[6]

The following are important considerations in the effectiveness of a vaccination program:[citation needed]

careful modelling to anticipate the impact that an immunization campaign will have on the epidemiology of the disease in the medium to long term
ongoing surveillance for the relevant disease following introduction of a new vaccine and
maintaining high immunization rates, even when a disease has become rare.

In 1958 there were 763,094 cases of measles and 552 deaths in the United States.[7][8] With the help of new vaccines, the number of cases dropped to fewer than 150 per year (median of 56).[8] In early 2008, there were 64 suspected cases of measles. 54 out of 64 infections were associated with importation from another country, although only 13% were actually acquired outside of the United States; 63 of these 64 individuals either had never been vaccinated against measles, or were uncertain whether they had been vaccinated.[8]