Life of the influenza virus: replication with variation
Like all organisms, the influenza virus’s main job in life is to make copies of itself. However, the virus is not very good
at it. As it replicates, it does so badly, making many mistakes called mutations.
Most of these mistakes are catastrophic to the organism, making them unable to replicate or killing them. Other mutations
prove to be a boon, letting the virus replicate more easily and more often, allowing it to live longer in more diverse environments.
For example, a mutation may help the virus live in all parts of the lung and not just the top of the lung, or a mutation may
make it easier for the virus to jump from one organism (you) to another (the grocer, the teacher, your child). These minor
but significant "point mutations" are called antigenic drift.
Small mutations: antigenic drift
Because the influenza virus replicates quickly, particularly compared to humans reproduction across generations, it evolves
quickly as the small but beneficial mutations take hold and become more abundant, squeezing out less efficient forms of the
virus. After countless millennia, this process has resulted in many types and subtypes of influenza, circulating around all
parts of the planet and living in a wide range of hosts, from pigs and birds to civet cats and humans. As hosts are infected
and re-infected, they make antibodies to fight the disease and in the process, develop some immunity to the virus.
Most of the virus's copying mistakes are small, and host organisms usually have some immunity to the slightly altered virus.
The antibodies produced to defend the body last year, for example, can still have some ability to fight the new virus this
year if the old and new viruses are genetically close. The result is although people still get sick, fewer get very sick,
and many fewer people die.
| Evolution of a Pandemic Strain of Influenza |
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| Although this explanation is simplified, it is very likely this process, or a similar process, created the new H1N1 virus
strain. Since the new virus is substantially different from the seasonal flu virus, humans have had little time to develop
any immunity, making it potentially dangerous.
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Larger mutations: antigenic shift
Unfortunately, copying mistakes are not always small. Sometime, large mutations occur when genetic material re-assorts or
recombines. This may occur when two types of influenza infect the same organism. For example, a small mutation allows a pig
to be infected with an influenza type normally only seen in birds. At the same time, the pig may be infected with a swine-type
flu. As both types of virus replicate in the pig, genetic material from one virus may combine with the genetic material from
the other. This is called antigenic shift, which can produce very large and very sudden changes, creating a new subtype of
the virus.
If this mutation is also beneficial to the virus, unlucky host organisms like you and me may have no natural immunity to the
new version of the virus. This can cause more people to be infected, more severe illness, and more deaths as our bodies struggle
to create new antibodies to mount an effective defence.
What is pandemic flu?
When a copying mistake is beneficial and large, making the now mutated virus significantly different from the old version,
there is the possibility a pandemic strain will circulate. Pandemic means that the virus is new enough that few people have any immunity and that virus is likely
to spread far and wide.
Pandemic flu can be mild or severe, it may infect many or few, and it may circulate for many months or die out. As much as
science and medicine have learned about the life cycle of influenza and the way it is transmitted from animals to humans and
among humans, predicting what will happen during a pandemic is very difficult. One thing we do know is that this process is
not new. Pandemic strains of influenza have circulated at least three times in the last 100 years.
Most people have heard of the Spanish Flu of 1918, which killed tens of millions of people. But two other pandemic influenzas
have circulated since: Asian Flu in 1957, which killed between one and four million, and Hong Kong Flu in 1968, which killed
approximately one million people around the globe.
So far, the current H1N1 pandemic flu has infected millions of people on the planet, but relatively few have become severely
ill or died. Over time, the virus will continue to mutate, so it is difficult to predict what will happen with H1N1 in the
months and years to come.
| Last Reviewed | Reviewed by |
| October 20, 2009 |
Upton Allen, MBBS, MSc, FRCPC, FAAP
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