vaccine vaccine a vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. the safety and effectiveness of vaccines has been widely studied and verified. 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. vaccinesmallpox vaccine and equipment for administering itmeshd014612 part of a series onvaccination general information vaccination vaccinator vaccine vaccine trial covid-19 vaccine card vaccines smallpox polio chicken pox measles/mumps/rubella influenza ebola covid-19 issues national vaccine injury compensation program vaccine hesitancy vaccine line jumping vaccine resistance 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). some vaccines offer full sterilizing immunity, in which infection is prevented completely. the administration of vaccines is called vaccination. vaccination is the most effective method of preventing 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 for twenty-five different preventable infections. the folk practice of inoculation against smallpox was brought from turkey to britain in 1721 by lady mary wortley montagu. 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. in 1881, to honor jenner, louis pasteur proposed that the terms should be extended to cover the new protective inoculations then being developed. the science of vaccine development and production is termed vaccinology. infectious diseases before and after a vaccine was introduced. vaccinations have a direct effect on the diminishment of the number of cases and contributes indirectly to a diminishment of the number of deaths. effects a child with measles, a vaccine-preventable disease there is overwhelming scientific consensus that vaccines are a very safe and effective way to fight and eradicate infectious diseases. 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 agent, 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. sometimes, protection fails for vaccine-related reasons such as failures in vaccine attenuation, vaccination regimens or administration. failure may also occur for host-related reasons if the host's immune system does not respond adequately or at all. host-related lack of response occurs in an estimated 2-10% of individuals, due to factors including genetics, immune status, age, health and nutritional status. one type of primary immunodeficiency disorder resulting in genetic failure is x-linked agammaglobulinemia, in which the absence of an enzyme essential for b cell development prevents the host's immune system from generating antibodies to a pathogen. host–pathogen interactions and responses to infection are dynamic processes involving multiple pathways in the immune system. a host does not develop antibodies instantaneously: while the body's innate immunity may be activated in as little as twelve hours, adaptive immunity can take 1–2 weeks to fully develop. during that time, the host can still become infected. once antibodies are produced, they may promote immunity in any of several ways, depending on the class of antibodies involved. their success in clearing or inactivating a pathogen will depend on the amount of antibodies produced and on the extent to which those antibodies are effective at countering the strain of the pathogen involved, since different strains may be differently susceptible to a given immune reaction. in some cases vaccines may result in partial immune protection (in which immunity is less than 100% effective but still reduces risk of infection) or in temporary immune protection (in which immunity wanes over time) rather than full or permanent immunity. they can still raise the reinfection threshold for the population as a whole and make a substantial impact. they can also mitigate the severity of infection, resulting in a lower mortality rate, lower morbidity, faster recovery from illness, and a wide range of other effects. those who are older often display less of a response than those who are younger, a pattern known as immunosenescence. adjuvants commonly are used to boost immune response, particularly for older people whose immune response to a simple vaccine may have weakened. the efficacy or performance of the vaccine is dependent on several 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 (breakthrough infection), the disease is likely to be less virulent than in unvaccinated cases. important considerations in an effective vaccination program: careful modeling to anticipate the effect 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 thirteen percent 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. vaccines led to the eradication of smallpox, one of the most contagious and deadly diseases in humans. 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 among humans, is targeted by an extensive eradication campaign that has seen endemic polio restricted to only parts of three countries (afghanistan, nigeria, and pakistan). however, the difficulty of reaching all children, cultural misunderstandings, and disinformation 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. the measles vaccine is estimated to prevent a million deaths every year. adverse effects vaccinations given to children, adolescents, or adults are generally safe. adverse effects, if any, are generally mild. 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. mmr vaccine is rarely associated with febrile seizures. host-("vaccinee")-related determinants that render a person susceptible to infection, such as genetics, health status (underlying disease, nutrition, pregnancy, sensitivities or allergies), immune competence, age, and economic impact or cultural environment can be primary or secondary factors affecting the severity of infection and response to a vaccine. elderly (above age 60), allergen-hypersensitive, and obese people have susceptibility to compromised immunogenicity, which prevents or inhibits vaccine effectiveness, possibly requiring separate vaccine technologies for these specific populations or repetitive booster vaccinations to limit virus transmission. severe side effects are extremely rare. varicella vaccine is rarely associated with complications in immunodeficient individuals, and rotavirus vaccines are moderately associated with intussusception. at least 19 countries have no-fault compensation programs to provide compensation for those with severe adverse effects of vaccination. the united states' program is known as the national childhood vaccine injury act, and the united kingdom employs the vaccine damage payment. types vaccines typically contain attenuated, inactivated or dead organisms or purified products derived from them. there are several types of vaccines in use. these represent different strategies used to try to reduce the risk of illness while retaining the ability to induce a beneficial immune response. attenuated main article: attenuated vaccine 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. inactivated main article: inactivated vaccine some vaccines contain inactivated, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, or radiation – "ghosts", with intact but empty bacterial cell envelopes. they are considered an intermediate phase between the inactivated and attenuated vaccines. examples include ipv (polio vaccine), hepatitis a vaccine, rabies vaccine and most influenza vaccines. avian flu vaccine development by reverse genetics techniques toxoid main article: toxoid toxoid vaccines are made from inactivated toxic compounds that cause illness rather than the micro-organism. examples of toxoid-based vaccines include tetanus and diphtheria. not all toxoids are for micro-organisms; for example, crotalus atrox toxoid is used to vaccinate dogs against rattlesnake bites. subunit main article: subunit vaccine 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. one example is the subunit vaccine against hepatitis b, which 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). another example is edible algae vaccines, such as the virus-like particle (vlp) vaccine against human papillomavirus (hpv), which is composed of the viral major capsid protein. another example is the hemagglutinin and neuraminidase subunits of the influenza virus. a subunit vaccine is being used for plague immunization. conjugate main article: conjugate vaccine certain bacteria have a polysaccharide outer coat that is 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. outer membrane vesicle outer membrane vesicles (omvs) are naturally immunogenic and can be manipulated to produce potent vaccines. the best known omv vaccines are those developed for serotype b meningococcal disease. heterotypic main article: heterologous vaccine heterologous vaccines also known as "jennerian vaccines", are vaccines that are pathogens of other animals that either do not cause disease or cause mild disease in the organism being treated. the classic example