Jonas Salk in 1955 holds two bottles of a culture used to grow polio vaccines.

A vaccine is a biological preparation that provides active acquired 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 a threat, destroy it, and to further recognize and destroy any of the microorganisms associated with that agent that it may encounter in the future. Vaccines can be prophylactic (to prevent or ameliorate the effects of a future infection by a natural or "wild" pathogen), or therapeutic (e.g., vaccines against cancer, which are being investigated).[1][2][3][4]

The administration of vaccines is called vaccination. Vaccination is the most effective method of preventing infectious diseases;[5] widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles, and tetanus from much of the world. The effectiveness of vaccination has been widely studied and verified; for example, vaccines that have proven effective include the influenza vaccine,[6] the HPV vaccine,[7] and the chicken pox vaccine.[8] The World Health Organization (WHO) reports that licensed vaccines are currently available for twenty-five different preventable infections.[9]

The terms vaccine and vaccination are derived from Variolae vaccinae (smallpox of the cow), the term devised by Edward Jenner to denote cowpox. He used it in 1798 in the long title of his Inquiry into the Variolae vaccinae Known as the Cow Pox, in which he described the protective effect of cowpox against smallpox.[10] In 1881, to honor Jenner, Louis Pasteur proposed that the terms should be extended to cover the new protective inoculations then being developed.[11]


A child with measles, a vaccine-preventable disease[12]

There is overwhelming scientific consensus that vaccines are a very safe and effective way to fight and eradicate infectious diseases.[13][14][15][16] Limitations to their effectiveness, nevertheless, exist.[17] Sometimes, protection fails because the host's immune system simply does not respond adequately or at all. Lack of response commonly results from clinical factors such as diabetes, steroid use, HIV infection, or age.[citation needed] It also might fail for genetic reasons if the host's immune system includes no strains of B cells that can generate antibodies suited to reacting effectively and binding to the antigens associated with the pathogen.

Even if the host does develop antibodies, protection might not be adequate; immunity might develop too slowly to be effective in time, the antibodies might not disable the pathogen completely, or there might be multiple strains of the pathogen, not all of which are equally susceptible to the immune reaction. However, even a partial, late, or weak immunity, such as a one resulting from cross-immunity to a strain other than the target strain, may mitigate an infection, resulting in a lower mortality rate, lower morbidity, and faster recovery.

Adjuvants commonly are used to boost immune response, particularly for older people (50–75 years and up), whose immune response to a simple vaccine may have weakened.[18]

Maurice Hilleman's measles vaccine is estimated to prevent 1 million deaths every year.[19]

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 others)
  • the strain of vaccine (some vaccines are specific to, or at least most effective against, particular strains of the disease)[20]
  • whether the vaccination schedule has been properly observed.
  • idiosyncratic response to vaccination; some individuals are "non-responders" to certain vaccines, meaning that they do not generate antibodies even after being vaccinated correctly.
  • assorted factors such as ethnicity, age, or genetic predisposition.

If a vaccinated individual does develop the disease vaccinated against (breakthrough infection), the disease is likely to be less virulent than in unvaccinated victims.[21]

The following are important considerations in the effectiveness of a vaccination program:[22]

  1. careful modeling to anticipate the effect that an immunization campaign will have on the epidemiology of the disease in the medium to long term
  2. ongoing surveillance for the relevant disease following introduction of a new vaccine
  3. maintenance of high immunization rates, even when a disease has become rare.

In 1958, there were 763,094 cases of measles in the United States; 552 deaths resulted.[23][24] After the introduction of new vaccines, the number of cases dropped to fewer than 150 per year (median of 56).[24] In early 2008, there were 64 suspected cases of measles. Fifty-four of those infections were associated with importation from another country, although only 13% were actually acquired outside the United States; 63 of the 64 individuals either had never been vaccinated against measles or were uncertain whether they had been vaccinated.[24]

Vaccines led to the eradication of smallpox, one of the most contagious and deadly diseases in humans.[25] Other diseases such as rubella, polio, measles, mumps, chickenpox, and typhoid are nowhere near as common as they were a hundred years ago thanks to widespread vaccination programs. As long as the vast majority of people are vaccinated, it is much more difficult for an outbreak of disease to occur, let alone spread. This effect is called herd immunity. Polio, which is transmitted only between humans, is targeted by an extensive eradication campaign that has seen endemic polio restricted to only parts of three countries (Afghanistan, Nigeria, and Pakistan).[26] However, the difficulty of reaching all children as well as cultural misunderstandings have caused the anticipated eradication date to be missed several times.

Vaccines also help prevent the development of antibiotic resistance. For example, by greatly reducing the incidence of pneumonia caused by Streptococcus pneumoniae, vaccine programs have greatly reduced the prevalence of infections resistant to penicillin or other first-line antibiotics.[27]

Other Languages
Afrikaans: Entstof
العربية: لقاح
aragonés: Vacuna
asturianu: Vacuna
Avañe'ẽ: Tasymombiaha
azərbaycanca: Vaksin
Bân-lâm-gú: E̍k-chu
башҡортса: Вакцина
беларуская: Вакцына
беларуская (тарашкевіца)‎: Вакцына
български: Ваксина
བོད་ཡིག: འབྲུམ་སྨན།
bosanski: Vakcina
català: Vaccí
čeština: Vakcína
dansk: Vaccine
Deutsch: Impfstoff
eesti: Vaktsiin
Ελληνικά: Εμβόλιο
español: Vacuna
Esperanto: Vakcino
euskara: Txerto
فارسی: واکسن
furlan: Vacine
galego: Vacina
한국어: 백신
hrvatski: Cjepivo
Bahasa Indonesia: Vaksin
íslenska: Bóluefni
italiano: Vaccino
עברית: חיסון
Jawa: Vaksin
ქართული: ვაქცინა
қазақша: Вакцина
Kiswahili: Chanjo
Kreyòl ayisyen: Vaksen
kurdî: Kutan
Latina: Vaccinum
latviešu: Vakcīna
lietuvių: Vakcina
magyar: Védőoltás
македонски: Вакцина
മലയാളം: വാക്സിൻ
मराठी: लस
Bahasa Melayu: Vaksin
Mìng-dĕ̤ng-ngṳ̄: Ĭk-mièu
монгол: Вакцин
Nederlands: Vaccin
日本語: ワクチン
norsk: Vaksine
norsk nynorsk: Vaksine
occitan: Vaccin
ଓଡ଼ିଆ: ଟିକା
oʻzbekcha/ўзбекча: Vaksinalar
پنجابی: ویکسین
Patois: Vaxiin
ភាសាខ្មែរ: វ៉ាក់សាំង
polski: Szczepionka
português: Vacina
română: Vaccin
русский: Вакцина
саха тыла: Быһыы
Scots: Vaccine
shqip: Vaksina
සිංහල: එන්නත්
Simple English: Vaccine
slovenščina: Cepivo
کوردی: کوتاو
српски / srpski: Вакцина
srpskohrvatski / српскохрватски: Vakcina
Sunda: Vaksin
suomi: Rokote
татарча/tatarça: Вакцина
తెలుగు: టీకా
тоҷикӣ: Ваксинаҳо
Türkçe: Aşı (tıp)
тыва дыл: Вакцина
українська: Вакцини
اردو: ویکسین
Tiếng Việt: Vắc-xin
Winaray: Bakuna
吴语: 疫苗
ייִדיש: וואקצין
粵語: 疫苗
中文: 疫苗