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Influenza A virus
subtype H1N1
Influenza A (H1N1) virus is a subtype of
influenzavirus A and the most common cause of
influenza (flu) in humans. Some strains of H1N1
are endemic in humans and cause a small fraction
of all influenza-like illness and a small
fraction of all seasonal influenza. H1N1 strains
caused a few per cent all human flu infections
in 2004-2005 [1]. Other strains of H1N1 are
endemic in pigs (swine influenza) and in birds
(avian influenza).
In June 2009, World Health Organization declared
that flu due to a new strain of swine-origin
H1N1 was responsible for the 2009 flu pandemic.
This strain is often called "swine flu" by the
public media. |
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Types of
vaccinations
All vaccinations work by presenting a foreign
antigen to the immune system in order to evoke
an immune response, but there are several ways
to do this. The four main types that are
currently in clinical use are as follows:
1. An inactivated vaccine consists of virus
particles which are grown in culture and then
killed using a method such as heat or
formaldehyde. The virus particles are destroyed
and cannot replicate, but the virus capsid
proteins are intact enough to be recognized and
remembered by the immune system and evoke a
response. When manufactured correctly, the
vaccine is not infectious, but improper
inactivation can result in intact and infectious
particles. Since the properly produced vaccine
does not reproduce, booster shots are required
periodically to reinforce the immune response.
2. In an attenuated vaccine, live virus
particles with very low virulence are
administered. They will reproduce, but very
slowly. Since they do reproduce and continue to
present antigen beyond the initial vaccination,
boosters are required less often. These vaccines
are produced by passaging virus in cell
cultures, in animals, or at suboptimal
temperatures, allowing selection of less
virulent strains, or by mutagenesis or targeted
deletions in genes required for virulence. There
is a small risk of reversion to virulence, this
risk is smaller in vaccines with deletions.
Attenuated vaccines also cannot be used by
immunocompromised individuals.
3. Virus-like particle vaccines consist of viral
protein(s) derived from the structural proteins
of a virus. These proteins can self-assemble
into particles that resemble the virus from
which they were derived but lack viral nucleic
acid, meaning that they are not infectious.
Because of their highly repetitive, multivalent
structure, virus-like particles are typically
more immunogenic than subunit vaccines
(described below). The human papillomavirus and
Hepatitis B virus vaccines are two virus-like
particle-based vaccines currently in clinical
use.
4. A subunit vaccine presents an antigen to the
immune system without introducing viral
particles, whole or otherwise. One method of
production involves isolation of a specific
protein from a virus or bacteria (such as a
bacterial toxin) and administering this by
itself. A weakness of this technique is that
isolated proteins may have a different
three-dimensional structure than the protein in
its normal context, and will induce antibodies
that may not recognize the infectious organism.
In addition, subunit vaccines often elicit
weaker antibody responses than the other classes
of vaccines.
A number of other vaccine strategies are under
experimental investigation. These include DNA
vaccination and recombinant viral vectors. |
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What Is a Vaccine?
Vaccination is the administration of antigenic material
(the vaccine) to produce immunity to a disease. Vaccines
can prevent or ameliorate the effects of infection by a
pathogen. Vaccination is generally considered to be the
most effective and cost-effective method of preventing
infectious diseases. The material administrated can
either be live but weakened forms of pathogens (bacteria
or viruses), killed or inactivated forms of these
pathogens, or purified material such as proteins.
Smallpox was the first disease people tried to prevent
by purposely inoculating themselves with other types of
infections; smallpox inoculation was started in China or
India before 200 BC. In 1718, Lady Mary Wortley Montagu
reported that the Turks had a habit of deliberately
inoculating themselves with fluid taken from mild cases
of smallpox, and that she had inoculated her own
children. Before 1796 when British physician Edward
Jenner tested the possibility of using the cowpox
vaccine as an immunization for smallpox in humans for
the first time, at least six people had done the same
several years earlier: a person whose identity is
unknown, England, (about 1771); a Mrs. Sevel, Germany
(about 1772); a Mr. Jensen, Germany (about 1770);
Benjamin Jesty, England, in 1774; a Mrs. Rendall,
England (about 1782); and Peter Plett, Germany, in 1791.
The word vaccination was first used by Edward Jenner in
1796. Louis Pasteur furthered the concept through his
pioneering work in microbiology. Vaccination (Latin:
vacca—cow) is so named because the first vaccine was
derived from a virus affecting cows—the relatively
benign cowpox virus—which provides a degree of immunity
to smallpox, a contagious and deadly disease. In common
speech, 'vaccination' and 'immunization' generally have
the same colloquial meaning. This distinguishes it from
inoculation which uses unweakened live pathogens,
although in common usage either is used to refer to an
immunization. The word "vaccination" was originally used
specifically to describe the injection of smallpox
vaccine.
Vaccination efforts have been met with some controversy
since their inception, on ethical, political, medical
safety, religious, and other grounds. In rare cases,
vaccinations can injure people and they may receive
compensation for those injuries. Early success and
compulsion brought widespread acceptance, and mass
vaccination campaigns were undertaken which are credited
with greatly reducing the incidence of many diseases in
numerous geographic regions.
Triggering immune sensitization
In the generic sense, the process of artificial
induction of immunity, in an effort to protect against
infectious disease, works by 'priming' the immune system
with an 'immunogen'. Stimulating immune response, via
use of an infectious agent, is known as immunization.
Vaccinations involve the administration of one or more
immunogens, which can be administered in several forms.
Some modern vaccines are administered after the patient
already has contracted a disease, as in the cases of
experimental AIDS, cancer and Alzheimer's disease
vaccines. Vaccinia given after exposure to smallpox,
within the first four days, is reported to attenuate the
disease considerably, and vaccination within the first
week is known to be beneficial to a degree. The first
rabies immunization was given by Louis Pasteur to a
child bitten by a rabid dog, subsequently post-exposure
immunization to rabies has generally been followed by
survival. The essential empiricism behind such
immunizations is that the vaccine triggers an immune
response more rapidly than the natural infection itself.
Most vaccines are given by hypodermic injection as they
are not absorbed reliably through the intestines. Live
attenuated polio, some typhoid and some cholera vaccines
are given orally in order to produce immunity based in
the bowel.
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