vaccine: see vaccination.

Preparation containing either killed or weakened live microorganisms or their toxins, introduced by mouth, by injection, or by nasal spray to stimulate production of antibodies against an infectious agent. This confers immunity to that agent, since the B lymphocytes remain sensitized to it and respond to later infection by producing more antibodies. The first vaccine, against smallpox, was introduced by Edward Jenner in 1798. Vaccines have been developed against diseases caused by bacteria (e.g., typhoid, whooping cough, tuberculosis) and by viruses (e.g., measles, influenza, rabies, poliomyelitis). Effectiveness varies, and a small percentage of people have adverse reactions. Those with immunodeficiency disorders should not receive live vaccines.

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A vaccine is a biological preparation which is used to establish or improve immunity to a particular disease.

Vaccines can be prophylactic (e.g. 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; see cancer vaccine).

Name and history

The term "vaccine" derives from Edward Jenner's 1796 use of cowpox (Latin variolæ vaccinæ, adapted from the Latin vaccīn-us, from vacca cow), which, when administered to humans, provided them protection against smallpox. The earliest vaccines were based on the concept of variolation originating in China, in which a person is deliberately infected with a weak form of smallpox as a form of inoculation. Jenner realized that milkmaids who had contact with cowpox did not get smallpox. The process of distributing and administrating vaccines is thus referred to as "vaccination" .

Jenner's work was continued by Louis Pasteur and others in the 19th century. Since vaccination against smallpox was much safer than smallpox inoculation, the latter fell into disuse and was eventually banned in England in 1849.

The 19th and 20th centuries saw the introduction of several successful vaccines against a number of infectious diseases. These included bacterial and viral diseases, but not (to date) any parasitic diseases.

Types

Vaccines may be dead or inactivated organisms or purified products derived from them.

There are four types of traditional vaccines:

• Vaccines containing killed microorganisms - these are previously virulent micro-organisms which have been killed with chemicals or heat. Examples are vaccines against flu, cholera, bubonic plague, and hepatitis A.
• Vaccines containing live, attenuated virus microorganisms - these are live micro-organisms that have been cultivated under conditions that disable their virulent properties or which use closely-related but less dangerous organisms to produce a broad immune response. They typically provoke more durable immunological responses and are the preferred type for healthy adults. Examples include yellow fever, measles, rubella, and mumps. The live tuberculosis vaccine is not the contagious strain, but a related strain called "BCG"; it is used in the United States very infrequently.
• Toxoids - these are inactivated toxic compounds in cases where these (rather than the micro-organism itself) cause illness. Examples of toxoid-based vaccines include tetanus and diphtheria. Not all toxoids are for micro-organisms; for example, Crotalis atrox toxoid is used to vaccinate dogs against rattlesnake bites.
• Subunit - rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a "whole-agent" vaccine), a fragment of it can create an immune response. Characteristic examples include the subunit vaccine against HBV that is composed of only the surface proteins of the virus (produced in yeast) and the virus-like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsid protein.

A number of innovative vaccines are also in development and in use:

• Conjugate - certain 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.
• Recombinant Vector - by combining the physiology of one micro-organism and the DNA of the other, immunity can be created against diseases that have complex infection processes
• DNA vaccination - in recent years a new type of vaccine, created from an infectious agent's DNA called DNA vaccination, has been developed. It works by insertion (and expression, triggering immune system recognition) into human or animal cells, of viral or bacterial DNA. Some cells of the immune system that recognize the proteins expressed will mount an attack against these proteins and cells expressing them. Because these cells live for a very long time, if the pathogen that normally expresses these proteins is encountered at a later time, they will be attacked instantly by the immune system. One advantage of DNA vaccines is that they are very easy to produce and store. As of 2006, DNA vaccination is still experimental.

While most vaccines are created using inactivated or attenuated compounds from micro-organisms, synthetic vaccines are composed mainly or wholly of synthetic peptides, carbohydrates or antigens.

Vaccines may be monovalent (also called univalent) or multivalent (also called polyvalent). A monovalent vaccine is designed to immunize against a single antigen or single microorganism. A multivalent or polyvalent vaccine is designed to immunize against two or more strains of the same microorganism, or against two or more microorganisms.

Developing immunity

The immune system recognizes vaccine agents as foreign, destroys them, and 'remembers' them. When the virulent version of an agent comes along the body recognises the protein coat on the virus, and thus is prepared to respond, by (1) neutralizing the target agent before it can enter cells, and (2) by recognizing and destroying infected cells before that agent can multiply to vast numbers.

Vaccines have contributed to the eradication of smallpox, one of the most contagious and deadly diseases known to man. Other diseases such as rubella, polio, measles, mumps, chickenpox, and typhoid are nowhere near as common as they were just a hundred years ago. 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 four countries. The difficulty of reaching all children as well as cultural misunderstandings, however, have caused the eradication date to be missed several times.

Schedule

See also: Vaccination policy

In order to provide best protection, children are recommended to receive vaccinations as soon as their immune systems are sufficiently developed to respond to particular vaccines, with additional 'booster' shots often required to achieve 'full immunity'. This has led to the development of complex vaccination schedules. In the United States, the Advisory Committee on Immunization Practices, which recommends schedule additions for the Center for Disease Control, recommends routine vaccination of children against: hepatitis A, hepatitis B, polio, mumps, measles, rubella, diphtheria, pertussis, tetanus, HiB, chicken pox, rotavirus, influenza, meningococcal disease and pneumonia. The large number of vaccines and boosters recommended (up to 24 injections by age two) has led to problems with achieving full compliance. In order to combat declining compliance rates, various notification systems have been instituted and a number of combination injections are now marketed (e.g., Prevnar and ProQuad vaccines), which provide protection against multiple diseases.

Besides recommendations for infant vaccinations and boosters, many specific vaccines are recommended at other ages or for repeated injections throughout life -- most commonly for measles, tetanus, influenza, and pneumonia. Pregnant women are often screened for continued resistance to rubella. The human papillomavirus vaccine is currently recommended in the U.S. and UK for ages 11–25. Vaccine recommendations for the elderly concentrate on pneumonia and influenza, which are more deadly to that group. In 2006, a vaccine was introduced against shingles, a disease caused by the chicken pox virus, which usually affects the elderly.

In Australia, a massive increase in vaccination rates was observed when the federal government made certain benefits (such as the universal 'Family Allowance' welfare payments for parents of children) dependent on vaccination. As well, children were not allowed into school unless they were either vaccinated or their parents completed a statutory declaration refusing to immunize them, after discussion with a doctor, and other bureaucracy. (Similar school-entry vaccination regulations have been in place in some parts of Canada for several years.) It became easier and cheaper to vaccinate one's children than not to. When faced with the annoyance, many more casual objectors simply gave in.

Efficacy

Vaccines do not guarantee complete protection from a disease. Sometimes this is because the host's immune system simply doesn't respond adequately or at all. This may be due to a lowered immunity in general (diabetes, steroid use, HIV infection) or because the host's immune system does not have a B-cell capable of generating antibodies to that antigen.

Even if the host develops antibodies, the human immune system is not perfect and in any case the immune system might still not be able to defeat the infection.

Adjuvants are typically used to boost immune response. Adjuvants are sometimes called the dirty little secret of vaccines in the scientific community, as not much is known about how adjuvants work. Most often aluminium adjuvants are used, but adjuvants like squalene are also used in some vaccines and more vaccines with squalene and phosphate adjuvants are being tested. 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 other diseases)
• the strain of vaccine (some vaccinations are for different strains of the disease)
• whether one kept to the timetable for the vaccinations (see Vaccination schedule)
• some individuals are 'non-responders' to certain vaccines, meaning that they do not generate antibodies even after being vaccinated correctly
• other factors such as ethnicity or genetic predisposition

When a vaccinated individual does develop the disease vaccinated against, the disease is likely to be milder than without vaccination.

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

1. careful modelling to anticipate the impact that an immunisation 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 and
3. maintaining high immunisation rates, even when a disease has become rare.

In 1958 there were 763,094 cases of measles and 552 deaths in the United States. With the help 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. 54 out of 64 infections were acquired outside of the United States, and 63 of 64 either had never been vaccinated against measles, or were uncertain whether they had been vaccinated.

Types of vaccine developments

• Rational attenuation Specific modifications or deletions of genes that confer virulence removes the pathogenicity of the microbe while still allowing an immune response to be generated. This type of rational attenuation can be viewed as creating a "live" attenuated vaccine.
• Vector-mediated subunit delivery Introducing a non-infectious, non-pathogenic subunit into a live vector can prompt an immune response without presence of the pathogen. This is called vector-mediated subunit delivery. For example, rabies surface protein gene has been inserted into vaccinia virus.
• Virus-like particles Capsid proteins of icosahedral viruses assemble without the presence of a genome. These virus-like particles are antigenically authentic, but non-infectious. This has been used for HPV-16 and HPV-18 vaccines.

Controversy

Opposition to vaccination, from a wide array of vaccine critics, has existed since the earliest vaccination campaigns. Disputes have arisen over the morality, ethics, effectiveness, and safety of vaccination. The mainstream medical opinion is that the benefits of preventing suffering and death from serious infectious diseases greatly outweigh the risks of rare adverse effects following immunization. Some vaccination critics say that vaccines are ineffective against disease or that vaccine safety studies are inadequate. Some religious groups oppose vaccination as a matter of doctrine, and some political groups oppose mandatory vaccination on the grounds of individual liberty.

Economics of development

One challenge in vaccine development is economic: many of the diseases most demanding a vaccine, including HIV, malaria and tuberculosis, exist principally in poor countries. Pharmaceutical firms and biotech companies have little incentive to develop vaccines for these diseases, because there is little revenue potential. Even in more affluent countries, financial returns usually minimal and the risks are great.

Most vaccine development to date has relied on 'push' funding by government, universities and non-profit organizations. Many vaccines have been highly cost effective and beneficial for public health. The number of vaccines actually administered has risen dramatically in recent decades. This increase, particularly in the number of different vaccines administered to children before entry into schools may be due to government mandates and support, rather than economic incentive.

Many researchers and policymakers are calling for a different approach, using 'pull' mechanisms to motivate industry. Mechanisms such as prizes, tax credits, or advance market commitments could ensure a financial return to firms that successfully developed a HIV vaccine. If the policy were well-designed, it might also ensure people have access to a vaccine if and when it is developed.

Intellectual property

Intellectual property can also be viewed as an obstacle to the development of new vaccines. Because of the weak protection offered through the patent of the final product, the protection of the innovation regarding vaccines is often made through the patent of processes used on the development of new vaccines as well as the protection of secrecy.

Preservatives

Many vaccines need preservatives to prevent serious adverse effects such as the Staphylococcus infection that in one 1928 incident killed 12 of 21 children inoculated with a diphtheria vaccine that lacked a preservative. Several preservatives are available, including thiomersal, 2-phenoxyethanol, and formaldehyde. Thiomersal is more effective against bacteria, has better shelf life, and improves vaccine stability, potency, and safety, but in the U.S., the European Union, and a few other affluent countries, it is no longer used as a preservative in childhood vaccines, as a precautionary measure due to its mercury content. Controversial claims have been made that thiomersal contributes to autism; no convincing scientific evidence supports these claims.

Delivery systems

There are several new delivery systems in development, which will hopefully make vaccines more efficient to deliver. Possible methods include liposomes and ISCOM (immune stimulating complex).

Plasmids

The use of plasmids has been validated in preclinical studies as a protective vaccine strategy for cancer and infectious diseases. However, the crossover application into human studies has been met with poor results based on the inability to provide clinically relevant benefit. The overall efficacy of plasmid DNA immunization depends on increasing the plasmid's immunogenicity while also correcting for factors involved in the specific activation of immune effector cells.

Use in nonhumans

Vaccinations of animals are used both to prevent their contracting diseases and to prevent transmission of disease to humans. Both animals kept as pets and animals raised as stock are vaccinated. In some instances, wild populations may be vaccinated. This is sometimes accomplished with vaccine-laced food spread in a disease-prone area and has been used to attempt to control rabies in raccoons.

Where rabies occurs, rabies vaccination of dogs may be required by law. Other canine vaccines include canine distemper, canine parvovirus, canine hepatitis virus, adenovirus-2, leptospirosis, bordatella, canine parainfluenza virus, and Lyme disease among others.

See also

References

External links

• Immunization. MedlinePlus. U.S. National Library of Medicine. Retrieved on 2008-01-14..
• VIOLIN: Vaccine Investigation and Online Information Network