The
disease
Influenza, often referred to as 'the flu', is a respiratory infection
caused by a distinct group of viruses, the influenza viruses. Infections with
influenza are usually more severe than those with other respiratory viruses
and involve a combination of respiratory (cough, sore throat) and constitutional
(fever, headache, muscle aches) symptoms. In older adults and people with a
range of pre-existing medical conditions, influenza infections can lead to serious
and even life threatening complications. A notable feature of influenza is that
repeated infections can occur throughout life.
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The
spread of influenza
Influenza is transmitted by tiny droplets of moisture
spread from the respiratory tract of infected people by coughing, sneezing or
even talking. When these are breathed in by a susceptible person the viruses
they contain can enter the cells of the respiratory tract and multiply. The
person will usually become ill within 2-3 days, however, it is not unusual for
them to start shedding virus for up to a day before symptoms are noticed.
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Influenza
viruses
Two types of influenza viruses are of public health
concern; they are designated as Type A and Type B and can be readily distinguished
from each other by laboratory tests but not by the clinical symptoms they produce.
A third type of influenza virus, Type C, is of no public health
significance.
The surface of influenza viruses are covered with two types of proteins, known as the haemagglutinin (H) and neuraminidase (N), which, in electron microscope images (see here), appear as a fringe or spikes projecting from the surface. These two proteins are continually changing by mutation to produce new strains of the virus, a process referred to as 'antigenic drift'. In addition, there are a number of different subtypes of influenza A with quite different haemagglutinins and neuraminidases and occasionally a new subtype of influenza A becomes established in the human population - this is known as 'antigenic shift'. The subtypes of influenza A currently circulating in the population are known as A(H1N1) and A(H3N2).
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Immunity to influenza
Infection with influenza virus, or immunisation, induces the body to produce
antibodies to the strain or strains of virus encountered. As the circulating
viruses change by mutation these antibodies become less effective at neutralizing
the virus and within a relatively short time they are no long protective. Therefore
infections can occur repeatedly throughout life and protection by vaccination
requires annual dosing with updated vaccines. When a new subtype of influenza
A occurs the whole population is highly susceptible and infection rapidly spreads
worldwide producing a pandemic.
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Tracking
changes in the virus
The World Health Organization initiated a worldwide
network in 1947 to monitor the changes that occur in influenza viruses and to
update the formulation of influenza vaccines. This network now involves over
100 national influenza centres, worldwide, and four Collaborating Centres (located
in Atlanta USA, London UK, Melbourne Australia, and Tokyo Japan) which analyse
viruses from around the world to provide information for vaccine strain updating.
The information generated by this network is available on the WHO FluNet web
site (http://oms2.b3e.jussieu.fr/flunet/).
WHO convenes two meetings each year, in February and September, at which the information generated by the network is reviewed and recommendations for vaccine formulations are made for the Northern and Southern hemisphere respectively.
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Influenza
vaccines
The
influenza vaccines currently used in most parts of the world are prepared from
influenza virus which has been highly purified and then inactivated. In most
cases the virus is then broken apart chemically to produce what are known as
split-product vaccines and, in some cases, further purified by removal of some
of the virus components to give purified subunit vaccines. Because of the inactivation
process and the disruption of the virus the vaccines can not cause infection
or influenza symptoms. When injected, however, the body's immune system recognizes
the fragments of virus as foreign and responds by producing antibodies which
will prevent or modify an infection with strains similar to those in the vaccine.
Vaccines usually contain three strains of virus selected by WHO and national authorities as being the most appropriate to protect agains the recent variants in the three families of human influenza viruses - Type A(H1N1), Type A(H3N2) and Type B. The vaccine formulations are determined with reference to laboratory strains of influenza virus and are prepared from viruses which must be 'like' the reference strains.
Groups
most seriously affected
The young generally have the greatest risk of being infected because they have
not developed immunity to influenza. Very young infants may become seriously
ill.
Older adults are at increased risk of developing complications such as pneumonia as they may have underlying conditions and weakened immune response.
Others at increased
risk are people of all ages who have respiratory, cardiac, metabolic or immune
system diseases as all of these can be worsened by infections. Women in the
second and third terms of pregnancy are also likely to suffer more severe infections.
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Influenza
vaccination
Vaccination recommendations vary from country to country but usually include:
Reagents
for influenza diagnosis 2008
Each
year the Centre prepares and distributes a kit of influenza reagents so that
participating laboratories may conduct their own preliminary identification
tests on samples. The kits contain reagents which are appropriate for recently
circulating viruses of influenza types A(H1N1), A(H3N2) and B. In the case of
A(H1N1) there are reagents which represent two distinct evolutionary lineages
of virus. The reagents consist of inactivated egg-grown antigens and antisera
prepared in rabbits and are suitable for identification of both egg-grown and
cell-grown influenza isolates by the HAI test. Please
email us if you are a laboratory performing influenza diagnosis and would like
a kit here.
For a copy of our 2008 Reagent Kit Pamphlet which accompanies our kit reagents click here.
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For articles published on influenza by members of staff please click here