A vaccine is an injection of a specialized protein designed to create an immunity to a particular disease (see antigen). The primary mechanism that a body uses to identify infections is a protein molecule called an antibody. The ingredients of vaccines used today are available from the CDC.
Traditionally a few highly tested vaccines were mandatory for children to protect against deadly or severe diseases, such as smallpox and polio. In the 1990s, vaccine manufacturers began introducing and persuading states to require numerous additional vaccines for non-deadly and even sexually transmitted diseases. About one-third of the states have philosophical exemptions entitling parents to decline the vaccines; 48 out of 50 states have religious exemptions; all states have medical exemptions, but they are hardest to obtain.
Since the adoption of mass vaccination programs, many diseases have been curbed. Smallpox, a deadly disease responsible for the deaths of millions, has been entirely eliminated through vaccination, excepting some strains maintained for research. Responsible for an estimated 300 to 500 million deaths worldwide in the 20th century, vaccination programs of the 19th and 20th century were effective at curbing the disease, and in 1979 the World Health Organization declared smallpox to be eradicated.
Polio, for example, killed over 3,000 people in the United States in 1952 , and afflicted such noted persons as President Franklin Delano Roosevelt, actor Alan Alda, and musician Itzak Perlman. When doctors began using a vaccine to fight polio, it practically eradicated the disease in the developed world.
If the body has a particular antibody for a particular disease it can identify the infection early on and the immune system can destroy it. Vaccines work by exposing the body to a form of the disease in such a way that it will not cause illness but allow antibodies to develop. This confers a level of immunity to the organism vaccinated.
There are four major types of vaccines and they are differentiated by what antigen is used. Live attenuated vaccines are created from the actual disease in question, however before injection the pathogen is attenuated (weakened) by growing successive generations of lines in poor nutrient conditions. It is possible for these vaccines to still cause an infection. Inactivated vaccines are created from pathogens that have been killed, usually with the use of a preservative formaldehyde. Toxoid vaccines are created by using toxins produced by the pathogen, before injection these toxins are exposed to heat and chemicals that eliminate their effect, antibodies will still be produced to these deactivated toxins. Finally, component vaccines use pieces of the pathogen in the injection. These pieces often of identifying protein segments that allow antibodies to be produced.
Only live attenuated vaccines can still cause infection. All vaccines can cause illness, but this is commonly a hypersensitivity reaction (similar to an allergic response), not due to the disease itself. Very rarely a serious disease known as eczema vaccinatum can emerge. Vaccines that use pieces of pathogens often use a carrier virus in the injection. Those with very weak immune systems that are exposed to such a vaccine can become infected with the carrier virus and develop eczema. A recent case of this was reported in the United States after a military man was vaccinated with small pox and his two year old son became ill. 
A number of vaccination reactions are deemed serious enough to contraindicate further vaccinations of the same type. These include extensive swelling and redness at the injection site, prolonged fever, bronchospasm and laryngeal edema(narrowing of the airways causing difficulty in breathing), and collapse/seizures with or without unconsciousness and/or encephalitis.
Published Adverse Reaction Rates
- The Hepatitis B Vaccine (HBV) has a .000167% chance of causing anaphylaxis upon vaccination.
- There is no correlation between the HBV and multiple sclerosis.
- The Measles, Mumps, Rubbela (MMR) vaccine has a 25% chance of causing arthralgia (joint pain) and 10% chance of causing arthritis-like symptoms in women who are past puberty. This pain and stiffness begins 1-3 weeks after vaccination, persisting for 1 day to 3 weeks.
- The MMR vaccine has a .00484% chance of causing thrombocytopenia. However, the measles and rubella both have much higher chances of causing thrombocytopenia.
The flu vaccine is an example of modern vaccination. Due to the change in allele frequency over time in the population of the influenza virus, new influenza vaccines are created each year. According to the CDC ,
Influenza viruses are always changing. Therefore, influenza vaccines are updated every year, and an annual vaccination is recommended.
Since production of the vaccine must begin many months before the influenza season starts models of the population genetics of the virus must be produced.  The success of vaccinations programs is dependent on the accuracy of these predictions. The National Institute of Health recently assigned $26 million dollars to establish a research center to better develop molecular models of influenza. 
Herd immunity and mandatory vaccination
A concept in vaccine planning is "herd immunity". When a certain percentage of a population is vaccinated, those within the population who are not vaccinated are protected from the disease. If 90 percent of a population is immune, then the chances that a carrier of the disease will come into contact with a non-immune member of the population (or "herd") is low. . Herd immunity is the goal of most vaccination programs as it is the point where a disease can start to be eliminated altogether from a population.
For example, in 1962 Japan instituted mandatory vaccination against influenza in school children. This continued until 1987 when the program was changed to optional; it was abandoned in 1994. Approximately 85 percent of children were estimated to have been vaccinated. Several studies examined the mortality rates due to influenza during this period of time and revealed that the death rates due to influenza in all population groups was drastically decreased, including older adult populations. Since there was no increased rate of vaccination in these populations, the decreased mortality was caused by herd immunity effects of vaccinating so many children.  Many researchers have written that this and other evidence argues strongly for mandatory vaccination programs world wide as a means of controlling many diseases and preventing a pandemic.
The USA has mandatory vaccination according to an official CDC schedule. Enforcement is left to the states. Most states have medical and religious exemptions. Some states make it very easy to opt out of vaccines, while other states strictly enforce compliance for school attendance. The difference in vaccination rates between these states is actually very slight, and usually not nearly enough to affect herd immunity.
As of February 2008, the CDC noted that "fewer than 100 measles cases are now reported annually in the United States and virtually all of those are linked to imported cases" in which someone is infected during overseas travel. The USA maintains herd immunity for measles, even in states where it is easy to opt out of vaccination requirements. Measles can be serious; according to an article in The Times, "About one in every 15 children has complications that can include meningitis, pneumonia, fits, encephalitis (swelling of the brain), blindness and brain damage." In the England and Wales, there were 1,348 cases of measles in 2008; two children died of the disease. 
The CDC notes that when a measles outbreak occurs, the cost to taxpayers can be substantial. Public health responsibilities include "identification of cases, isolation of patients and vaccination, administration of immune globulin, and voluntary quarantine of contacts who have no evidence of measles immunity...In Iowa, the public health response to one imported measles case cost approximately $150,000."
Other diseases besides the measles have also started not only to reappear but also to kill babies. Hib (Haemophilus influenzae type B) recently killed an infant in Minnesota by causing meningitis in the baby. The child who died had not been vaccinated. Two others who had been either partially vaccinated, but not fully protected, or had an immune deficiency were seriously sickened. The vaccine for Hib is less than twenty years old, but this is the first death due to the disease in the state of Minnesota since the vaccine was introduced.
Herd Immunity in Britain
Due to a decrease in vaccination, measles has become endemic in Britain. This means that measles does not need to be brought in by travelers, but instead has a large enough base of unvaccinated individuals for it to continuously spread inside the country. In London, only 71 percent of children receive the first dose of the vaccine and only 51 percent the second dose. Due to this decline, there were 461 cases in Britain in the first half of 2008. These cases have had tragic consequences, with the death of a 17 year old reported in 2008. This immunization failure is due to parents opting out of the MMR vaccine program in the wake of the false claims concerning links between the vaccine and autism. 
Vaccines are not completely harmless, and many have a significant morbidity and mortality associated with them. Postvaccinial encephalitis is a known hazard of smallpox vaccine, for example. A recently-introduced vaccine against human papilloma virus, the causative agent of cancer of the uterine cervix, has now been linked to a number of sudden deaths reported within days of its administration.
Bulging or swollen fontanelles have been linked with many vaccines commonly given to infants, such as the diptheria-tetanus (DPT) vaccine. Many families, out of concern for their child's health, have sought legal justice for the potential health damage.
Common Misconceptions Surrounding Vaccines
Many people assume that official vaccine recommendations are based on recommendations of independent panels of experts after reviewing risk-benefit and cost-benefit analyses. They are not. The government panels are dominated by vaccine industry representatives and they have no consumer representatives. The meetings are not open to the public. They do not usually do much analysis, and tend to recommend whatever vaccines are available[Citation Needed]
Misinformed rumors about vaccines do not happen only in the developed world. The world wide polio eradication came to a halt in 2003 when Muslim Imams in Nigeria began spreading rumors that the vaccines would give boys HIV and sterilize girls. The vaccine was banned for over a year in the country. The ban has since been lifted, but the vaccination rate remains low.
- http://www.howstuffworks.com/immune-system7.htm How stuff works: Antibodies
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- http://www.associatedcontent.com/article/183349/soldier_infects_toddler_son_with_lifethreatening.html AP article on eczema vaccinatum
- General Practice Notebook - vaccine reactions
- E Miller et al., Idiopathic thrombocytopenic purpura and MMR vaccine in Archives of Disease In Childhood, 84:227-229
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- Glezen 2004
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- The Sunday Times, How the MMR scare led to the return of measles, February 8, 2009
- The Sunday Times, MMR doctor Andrew Wakefield fixed data on autism, by Brian Deer, February 8, 2009 
- Centers for Disease Control and Prevention, Morbidity and Mortality Weekly Report, Outbreak of Measles --- San Diego, California, January--February 2008, February 29, 2008 
- Hib Outbreak Kills Unvaccinated Child
- Health Direct NHS watch blog
- Cockcroft, Lucy, "Cervical cancer drug Gardasil linked to deaths," London Daily Telegraph, October 29, 2007. Retrieved November 10, 2007.
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- HPV vaccine
- Vaccine Injury Compensation Program
- Vaccine Adverse Event Reporting System
- Acellular vaccine
- Vaccine FAQ