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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 micro-organism 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 keep a record of it, so that the immune system can more easily recognize and destroy any of these micro-organisms that it later encounters. Vaccines can 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). The administration of vaccines is called vaccination. The effectiveness of vaccination has been widely studied and verified; for example, the influenza vaccine, the HPV vaccine, and the chicken pox vaccine. Vaccination is the most effective method of preventing infectious diseases;〔 *United States Centers for Disease Control and Prevention (2011). (''A CDC framework for preventing infectious diseases.'' ) Accessed 11 September 2012. "Vaccines are our most effective and cost-saving tools for disease prevention, preventing untold suffering and saving tens of thousands of lives and billions of dollars in healthcare costs each year." *American Medical Association (2000). (''Vaccines and infectious diseases: putting risk into perspective.'' ) Accessed 11 September 2012. "Vaccines are the most effective public health tool ever created." *Public Health Agency of Canada. (''Vaccine-preventable diseases.'' ) Accessed 11 September 2012. "Vaccines still provide the most effective, longest-lasting method of preventing infectious diseases in all age groups." *United States National Institute of Allergy and Infectious Diseases (NIAID). (''NIAID Biodefense Research Agenda for Category B and C Priority Pathogens.'' ) Accessed 11 September 2012. "Vaccines are the most effective method of protecting the public against infectious diseases."〕 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 World Health Organization (WHO) reports that licensed vaccines are currently available to prevent or contribute to the prevention and control of twenty-five infections.〔World Health Organization, (Global Vaccine Action Plan 2011-2020. ) Geneva, 2012.〕 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...()...the Cow Pox'', in which he described the protective effect of cowpox against smallpox. In 1881, to honour Jenner, Louis Pasteur proposed that the terms should be extended to cover the new protective inoculations then being developed. ==Effectiveness== Vaccines have historically been the most effective means to fight and eradicate infectious diseases. Limitations to their effectiveness, nevertheless, exist. 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. However 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. 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) * 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, the disease is likely to be less virulent than in unvaccinated victims. The following are important considerations in the effectiveness of a vaccination program: # 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 # 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. After the introduction of new vaccines, the number of cases dropped to fewer than 150 per year (median of 56).〔 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.〔 ==Adverse effects== Vaccination given during childhood is generally safe. Adverse effects if any are generally mild.〔(【引用サイトリンク】url=http://www.cdc.gov/vaccines/vac-gen/side-effects.htm )〕 The rate of side effects depends on the vaccine in question.〔 Some common side effects include: fever, pain around the injection site, and muscle aches.〔 Additionally, some individuals may be allergic to ingredients in the vaccine.〔http://www.cdc.gov/flu/about/qa/flushot.htm〕 MMR vaccine is rarely associated with febrile seizures.〔 Severe side effects are extremely rare.〔 Varicella vaccine is rarely associated with complications in immunodeficient individuals and rotavirus vaccines are moderately associated with intussusception.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「vaccine」の詳細全文を読む スポンサード リンク
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