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A vaccine is a biological preparation that provides active acquired immunity to a particular infectious disease.[1] 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 (to fight a disease that has already occurred, such as cancer).[2][3][4][5]

The administration of vaccines is called vaccination. Vaccination is the most effective method of preventing infectious diseases;[6] 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,[7] the HPV vaccine,[8] and the chicken pox vaccine.[9] The World Health Organization (WHO) reports that licensed vaccines are currently available for twenty-five different preventable infections.[10]

The terms vaccine and vaccination are derived from Variolae vaccinae (smallpox of the cow), the term devised by Edward Jenner (who both developed the concept of vaccines and created the first vaccine) to denote cowpox. He used the phrase in 1798 for 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.[11] In 1881, to honor Jenner, Louis Pasteur proposed that the terms should be extended to cover the new protective inoculations then being developed.[12]

Infectious diseases before and after a vaccine was introduced. Vaccination has a direct effect on the diminishment of the number of cases and contributes indirectly to a diminishment of the number of death

A vaccine is a biological preparation that provides active acquired immunity to a particular infectious disease.[1] 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 (to fight a disease that has already occurred, such as cancer).[2][3][4][5]

The administration of vaccines is called vaccination. Vaccination is the most effective method of preventing infectious diseases;[6] 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,[7] the HPV vaccine,[8] and the chicken pox vaccine.[9] The World Health Organization (WHO) reports that licensed vaccines are currently available for twenty-five different

The administration of vaccines is called vaccination. Vaccination is the most effective method of preventing infectious diseases;[6] 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,[7] the HPV vaccine,[8] and the chicken pox vaccine.[9] The World Health Organization (WHO) reports that licensed vaccines are currently available for twenty-five different preventable infections.[10]

The terms vaccine and vaccination are derived from Variolae vaccinae (smallpox of the cow), the term devised by Edward Jenner (who both developed the concept of vaccines and created the first vaccine) to denote cowpox. He used the phrase in 1798 for 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.[11] In 1881, to honor Jenner, Louis Pasteur proposed that the terms should be extended to cover the new protective inoculations then being developed.[12]

There is overwhelming scientific consensus that vaccines are a very safe and effective way to fight and eradicate infectious diseases.[14][15][16][17] The immune system recognizes vaccine agents as foreign, destroys them, and "remembers" them. When the virulent version of an agent is encountered, the body recognizes the protein coat on the virus, and thus is prepared to respond, by first neutralizing the target agent before it can enter cells, and secondly by recognizing and destroying infected cells before that agent can multiply to vast numbers.

Limitations to their effectiveness, nevertheless, exist.[18] Sometimes, protection fails because of vaccine-related failure such as failures in vaccine attenuation, vaccination regimes or administration or host-related failure due to host's immune system simply does not respond adequately or at all. Lack of response commonly results from genetics, immune status, age, health or nutritional status.[19] 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.[citation needed]

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.[20]

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)[21]
  • 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.[22]

The following are important considerations in the effectiveness of a vaccination program:[23]

  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.[24][25] After the introduction of new vaccines, the number of cases dropped to fewer than 150 per year (median of 56).[25] 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.[25]

Vaccines led to the eradication of smallpox, one of the most contagious and deadly diseases in humans.[26] 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).[27] 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.[18] Sometimes, protection fails because of vaccine-related failure such as failures in vaccine attenuation, vaccination regimes or administration or host-related failure due to host's immune system simply does not respond adequately or at all. Lack of response commonly results from genetics, immune status, age, health or nutritional status.[19] 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.[citation needed]

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.[20]

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

If a vaccinated individual does develop the disease vaccinated against (breakthrough infection), the disease is likely to be less virulent than in unvaccinated victims.[22]

The following are important considerations in the effectiveness of a vaccination program:[23]

  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]

In 1958, there were 763,094 cases of measles in the United States; 552 deaths resulted.[24][25] After the introduction of new vaccines, the number of cases dropped to fewer than 150 per year (median of 56).[25] 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.[25]

Vaccines led to the eradication of smallpox, one of the most contagious and deadly diseases in humans.[26] Other diseases such as rubella, po

Vaccines led to the eradication of smallpox, one of the most contagious and deadly diseases in humans.[26] 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).[27] 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.[28]

The measles vaccine is estimated to prevent 1 million deaths every year.[29]

Vaccination given to children, adolescents, or adults is generally safe.[30][31] Adverse effects, if any, are generally mild.[32] The rate of side effects depends on the vaccine in question.[32] Some common side effects include fever, pain around the injection site, and muscle aches.[32] Additionally, some individuals may be allergic to ingredients in the vaccine.[33] MMR vaccine is rarely associated with febrile seizures.[31]

Severe side effects are extremely rare.Severe side effects are extremely rare.[31] Varicella vaccine is rarely associated with complications in immunodeficient individuals and rotavirus vaccines are moderately associated with intussusception.[31]

At least 19 countries have no-fault compensation programs to provide compensation for those suffering severe adverse effects of vaccination.[34] The United States’ program is known as the National Childhood Vaccine Injury Act and the United Kingdom employs the Vaccine Damage Payment.

Vaccines contain dead or inactivated organisms or purified products derived from them.

There are several types of vaccines in use.[35] These represent different strategies used to try to reduce the risk of illness while retaining the ability to induce a beneficial immune response.

Inactivated

Some vaccines contain inactivated, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, or radiation.[36] Examples include the IPV polio vaccine, hepatitis A vaccine, rabies vaccine and most influenza vaccines.[37]

Attenuated

Some vaccines contain live, attenuated microorganisms. Many of these are active There are several types of vaccines in use.[35] These represent different strategies used to try to reduce the risk of illness while retaining the ability to induce a beneficial immune response.

Some vaccines contain inactivated, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, or radiation.[36] Examples include the IPV polio vaccine, hepatitis A vaccine, rabies vaccine and most influenza vaccines.[37]

Attenuated

So

Some vaccines contain live, attenuated microorganisms. Many of these are active viruses that have been cultivated under conditions that disable their virulent properties, or that use closely related but less dangerous organisms to produce a broad immune response. Although most attenuated vaccines are viral, some are bacterial in nature. Examples include the viral diseases yellow fever, measles, mumps, and rubella, and the bacterial disease typhoid. The live Mycobacterium tuberculosis vaccine developed by Calmette and Guérin is not made of a contagious strain but contains a virulently modified strain called "BCG" used to elicit an immune response to the vaccine. The live attenuated vaccine containing strain Yersinia pestis EV is used for plague immunization. Attenuated vaccines have some advantages and disadvantages. Attenuated, or live, weakened, vaccines typically provoke more durable immunological responses. But they may not be safe for use in immunocompromised individuals, and on rare occasions mutate to a virulent form and cause disease.[38]

Toxoid

Tox

Toxoid vaccines are made from inactivated toxic compounds that cause illness rather than the micro-organism.[39] Examples of toxoid-based vaccines include tetanus and diphtheria. Toxoid vaccines are known for their efficacy.[37] Not all toxoids are for micro-organisms; for example, Crotalus atrox toxoid is used to vaccinate dogs against rattlesnake bites.[40]

Subunit

Rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a "whole-ag

Rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a "whole-agent" vaccine), a subunit vaccine uses a fragment of it to create an immune response. Examples include the subunit vaccine against hepatitis B virus that is composed of only the surface proteins of the virus (previously extracted from the blood serum of chronically infected patients, but now produced by recombination of the viral genes into yeast)[41] or as an edible algae vaccine, the virus-like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsid protein,[42] and the hemagglutinin and neuraminidase subunits of the influenza virus.[37] A subunit vaccine is being used for plague immunization.[43]

ConjugateCertain bacteria have polysaccharide outer coats that are poorly immunogenic. By linking these outer coats to proteins (e.g., toxins), the immune system can be led to recognize the polysaccharide as if it were a protein antigen. This approach is used in the Haemophilus influenzae type B vaccine.[44]

Heterotypic