have been able to devise vaccines that offer powerful and long-lasting
protection against many viruses cause serious diseases in humans. (See
Dragonfly pages 1041 - 1042). When you were very young, for example,
you were probably vaccinated against measles
and several other diseases. When your parents or grandparents
were young, they were probably vaccinated against polio, which was still
common as recently as the 1950ıs. And when the authors of your text
were young, we were also vaccinated against smallpox before leaving
the United States.
that we bring this subject up, you might wonder why flu shots offer
protection for only one season at a time if they work at all.
The answer can be found in the way our immune system operates, the speed
at which these viruses evolve, and the host species that they infect.
key to understanding this situation is appreciating the way our immune
system identifies and attacks foreign invaders (Dragonfly book section
40-2; Elephant book section 45-2). Remember that our the immune system
doesn't actually recognize an entire virus. Instead, various immuneT-cells
and B-cells "learn" to identify and remember only certain
particular proteins on the outside of the viral coat. If the coat proteins
f a virus never change, an immune system exposed to that virus once
can "remember" it for a long time, and rapidly mount an attack
is the situation with smallpox. The smallpox virus is a genetic stick-in-the-mud.
Its genes, and therefore its coat proteins, don't change much over time.
For that reason, the coat proteins of a smallpox virus collected in
1950 will probably closely resemble the coat of another sample collected
in 1960, 1970 or even 2000. What's more, the smallpox virus infects
only a single host species: Homo sapiens.
these reasons, the World health organization was able to mount a worldwide
smallpox vaccination program that eliminated the disease
by 1980. How was that possible? Because the smallpox virus doesnıt evolve
rapidly, so a single dose of smallpox vaccine protects against infection
for years. When as enough humans around the world were vaccinated, the
smallpox virus could not "find" vulnerable hosts, and therefore
couldn't survive and reproduce in human populations. And because smallpox
doesn't have any animal hosts, once humans were immune, the virus had
nowhere to grow at all. Until the threat of bioterrorism emerged, this virus was effectively out of business.
(That's why almost no one under the age of 30 in this country has been
vaccinated against smallpox which unfortunately makes this virus
attractive as a weapon for terrorists.)
situation with influenza, on the other hand, is different for two reasons.
Influenza viruses evolve rapidly.
viruses mutate and evolve rapidly. (In fact, it has been estimated that
the proteins of the influenza virus evolve as much as a million times
faster than most human proteins!) For this reason,
so new and slightly different viral strains appear all the time.
The differences among these new strains often include changes in coat
proteins. Therefore, any immune system memory cells produced by a vaccination
or previous infection no longer recognize the new strains. When that
happens, the immune response must mount a defense from scratch, and
the viruses have enough time to make the host sick.
you think about this, you can probably see how our immune response creates
a form of natural selection that drives the evolution of these viruses.
Here's why. Strains that don't change their coat protein are stopped
by the immune response before they can spread. These strains, therefore,
have substantially lowered fitness, and are effectively wiped out. Strains
that change their coat proteins, on the other hand, have higher fitness.
These strains delay the immune response long enough to make us sick
which forces us to spread the virus to new hosts by coughing
In this highly simplified schematic diagram, a human antibody formed
form a previous exposure to a viral strain matches a particular coat
protein (the antigen). This kind of match spurs the immune system into
immediate action that prevents the virus from causing seirous illness.
In this schematic, that same antibody encounters a new viral antigen
which it does not recognize. If none of the body's circulating
antibodies recognize this new strain, it will take time for the immune
system to identify and fight the invader.
a result of these selective pressures, the flu strains that appear and
spread successfully each year are different enough from last year's
strains to evade memory cells left over from previous infections. Even
during a single flu season, new strains can emerge with coat proteins
that are slightly different from the first strains in circulation. That
explains both why you can even catch the flu several times in a single
season. The determining factor is whether or not the strains you are
exposed to are different enough from each other to confuse your immune
Influenza viruses infect a wide variety of domestic and wild animal
addition to this constant evolutionary "cat and mouse" game
with human immune systems, several strains of influenza virus can survive
and reproduce in other animal hosts. For that reason, even if all humans
in a given area acquire resistance to the virus, those strains can survive
in other species long enough to mutate and infect humans again later