Vaccine
Jonas Salk in 1955 holds bottles of a culture used to grow polio vaccines.
A vaccine is a biological preparation that improves immunity to a particular disease. 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 foreign, destroy it, and "remember" it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.
Vaccines may 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; see cancer vaccine).
The term vaccine derives from Edward Jenner's 1796 use of cow pox (Latin variola vaccinia, adapted from the Latin vaccīn-us, from vacca, cow), to inoculate humans, providing them protection against smallpox.
"With the exception of safe water, no other modality, not even antibiotics, has had such a major effect on mortality reduction and population growth."
Effectiveness
Maurice Hilleman's measles vaccine is estimated to prevent 1 million deaths every year.
Vaccines do not guarantee complete protection from a disease. Sometimes, this is because the host's immune system simply does not respond adequately or at all. This may be due to a lowered immunity in general (diabetes, steroid use, HIV infection, age) 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 immediately. In this case, the infection will be less severe and heal faster.
Adjuvants are typically used to boost immune response. 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. Larger doses are used in some cases for older people (50–75 years and up), whose immune response to a given vaccine is not as strong.
- 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, age, 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 i
mportant considerations in the effectiveness of a vaccination program: [citation needed]
- 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 and
- maintaining high immunization 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 associated with importation from another country, although only 13% were actually acquired outside of the United States; 63 of these 64 individuals either had never been vaccinated against measles, or were uncertain whether they had been vaccinated.
Importance
The importance of vaccines has been noted:
"The impact of vaccination on the health of the world's peoples is hard to exaggerate. With the exception of safe water, no other modality, not even antibiotics, has had such a major effect on mortality reduction and population growth."
Types
Avian flu vaccine development by reverse genetics techniques. Vaccines are dead or inactivated organisms or purified products derived from them. There are several types of vaccines in use. These represent different strategies used to try to reduce risk of illness, while retaining the ability to induce a beneficial immune response.
Killed
Some vaccines contain killed, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, radioactivity or antibiotics. Examples are the influenza vaccine, cholera vaccine, bubonic plague vaccine, polio vaccine, hepatitis A vaccine, and rabies vaccine.
Attenuated
Some vaccines contain live, attenuated microorganisms. Many of these are live viruses 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. Although most attenuated vaccines are viral, some are bacterial in nature. They typically provoke more durable immunological responses and are the preferred type for healthy adults. Examples include the viral diseases yellow fever, measles, rubella, and mumps 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. They have the capacity of transient growth so they give prolonged protection, and no booster dose is required. But they may get reverted to the virulent form and cause the disease.
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. Toxoid vaccines are known for their efficacy. Not all toxoids are for micro-organisms; for example, Crotalus atrox toxoid is used to vaccinate dogs against rattlesnake bites.
Subunit
Protein 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. 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), the virus-like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsid protein, and the hemagglutinin and neuraminidase subunits of the influenza virus. Subunit vaccine is being used for plague immunization.
Conjugate
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.
Experimental
A number of innovative vaccines are also in development and in use:
Dendritic cell vaccines combine dendritic cells with antigens in order to present the antigens to the body's white blood cells, thus stimulating an immune reaction. These vaccines have shown some positive preliminary results for treating brain tumors.
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 called DNA vaccination, created from an infectious agent's DNA, has been developed. It works by insertion (and expression, triggering immune system recognition) of viral or bacterial DNA into human or animal cells. 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.
T-cell receptor peptide vaccines are under development for several diseases using models of Valley Fever, stomatitis, and atopic dermatitis. These peptides have been shown to modulate cytokine production and improve cell mediated immunity.
Targeting of identified bacterial proteins that are involved in complement inhibition would neutralize the key bacterial virulence mechanism.
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.
Valence
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. In certain cases a monovalent vaccine may be preferable for rapidly developing a strong immune response.
Developing immunity
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 (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.
When two or more vaccines are mixed together in the same formulation, the two vaccines can interfere. This most frequently occurs with live attenuated vaccines, where one of the vaccine components is more robust than the others and suppresses the growth and immune response to the other components. This phenomenon was first noted in the trivalent Sabin polio vaccine, where the amount of serotype 2 virus in the
vaccine had to be reduced to stop it from interfering with the "take" of the serotype 1 and 3 viruses in the vaccine. This phenomenon has also been found to be a problem with the dengue vaccines currently being researched,[ where the DEN-3 serotype was found to predominate and suppress the response to DEN-1, -2 and -4 serotypes.
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 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 (Afghanistan, India, Nigeria and Pakistan). The difficulty of reaching all children as well as cultural misunderstandings, however, have caused the anticipated eradication date to be missed several times.
Schedule
For country-specific information on vaccination policies and practices, Vaccination policy refers to the health policy a government adopts in relation to vaccination. Vaccinations are voluntary in some countries and mandatory in some countries as part of the public health system. Some governments pay all or part of the costs of vaccinations for vaccines in a national vaccination schedule.
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 Centers for Disease
Control and Prevention, recommends routine vaccination of children against: hepatitis
A, hepatitis B, polio, mumps, measles, rubella, diphtheria, pertussis, tetanus,
HiB, chickenpox, 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., Pneumococcal
conjugate vaccine and MMRV vaccine), 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 recommended in the U.S. (as of 2011) and UK (as of 2009). 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 chickenpox virus, which usually affects the elderly.
Immunization Schedule in India
Immunizations are critical to
protect babies and children from various illnesses. Here is some basic
information on Indian immunizations and schedules, that Indian parents can
learn from. Ultimately, your paediatrician is the right resource for you to discuss
immunizations for your child.
Here is a typical immunization schedule recommended by the Indian Academy of
Pediatrics.
|
VACCINE |
DISEASE(S) |
|
BCG |
Tuberculosis (Bacillus Calmette Guerin) |
|
OPV |
Oral Polio Vaccine |
|
Hepatitis B |
Hepatitis B |
|
DTP |
Diphtheria, Tetanus, Pertussis |
|
Hib |
Pneumonia, Meningitis, Bacteremia (Haemophilus Influenzae Type B) |
|
Measles |
Measles |
|
MMR |
Measles, Mumps, Rubella |
|
Typhoid |
Typhoid |
|
Td |
Tetanus |
|
PCV |
Bacterial diseases causing pneumonia, meningitis etc., (Pneumococcal Conjugate Vaccine) |
|
Varicella |
Chicken Pox |
|
Hepatitis A |
Hepatitis A |
|
HPV |
Human Papilloma Virus causing cervical cancer |
|
Rotavirus |
Rotavirus |
|
AGE |
VACCINES |
|
Birth |
BCG, OPV, Hepatitis B |
|
6 weeks |
DTP, OPV+IPV, Hepatitis B, Hib, PCV |
|
10 weeks |
DTP, OPV+IPV, Hib, PCV |
|
14 weeks |
DTP, OPV+IPV, Hepatitis B, Hib, PCV |
|
9 months |
Measles |
|
1 year |
Varicella |
|
15 months |
MMR, PCV Booster |
|
16 months |
Hib Booster |
|
18 months |
DTP Booster, OPV+IPV Booster |
|
2 years |
Typhoid |
|
2 years 1 month |
Hepatitis A |
|
2 years 7 months |
Hepatitis A |
|
5 years |
DTP Booster, OPV Booster, Typhoid |
|
10 years |
Tdap, HPV |
Edward Jenner
Prior to vaccination, inoculation was practised, and brought to the West in 1721 by Lady Mary Wortley Montagu, who showed it to Hans Sloane, the King's physician. Sometime during the 1770s Edward Jenner heard
a milkmaid boast that she would never have the often-fatal or disfiguring
disease smallpox, because she had already had cowpox, which has a very mild
effect in humans. In 1796, Jenner took pus from the hand of a milkmaid with
cowpox, inoculated an 8-year-old boy with it, and six weeks later variolated
the boy's arm with smallpox, afterwards observing that the boy did not catch
smallpox. Further experimentation demonstrated the efficacy of the procedure on
an infant. Since vaccination with cowpox was much safer than smallpox
inoculation, the latter, though still widely practised in England, was banned
in 1840. Louis Pasteur generalized Jenner's idea by developing what he called a
rabies vaccine, and in the nineteenth century vaccines were considered a matter
of national prestige, and compulsory vaccination laws were passed.
The twentieth century saw the introduction of several successful vaccines, including those against diphtheria, measles, mumps, and rubella. Major achievements included the development of the polio vaccine in the 1950s and the eradication of smallpox during the 1960s and 1970s. Maurice Hilleman was the most prolific of the developers of the vaccines in the twentieth century. As vaccines became more common, many people began taking them for granted. However, vaccines remain elusive for many important diseases, including malaria and HIV.