Vaccinating more children might help slow the evolution of the
constantly changing flu virus, US government scientists reported.
Influenza is a mighty mutator. Sometimes it makes big changes
that result in never-before-seen strains, like this year's swine
flu. But from season to season, it undergoes subtle shape-shifting,
which is the reason people need a new winter flu vaccine every
This regular shift is what is called antigenic drift. Think of
influenza as wearing a coat, a protein on the virus' surface - the
"H" in the family flu names, for hemagglutinin - that is the major
factor in triggering the immune system to mount an attack. Slight
changes in the appearance of that coat can be enough to confuse
your immune system and allow infection.
But just how do those changes happen? National Institutes of
Health researchers infected lots of mice - vaccinated and
unvaccinated ones - to watch that evolution in action. They report
in the journal Science that influenza responds to immune pressure,
a discovery that supports today's vaccine policy urging more
children to get their flu shots each fall.
How the research was done
Even a vaccine that isn't a perfect match to the virus may
provide some protection and limit the scope of changes in the
"We're giving the virus more wiggle room, more evolutionary
space, by having some naive individuals," explained lead researcher
Dr Jonathan Yewdell of NIH's National Institute of Allergy and
Infectious Diseases. To scientists, "naive" means the previously
Yewdell's team took an old strain of seasonal H1N1 flu that
circulated decades ago and infected groups of mice. After each
infection, they culled virus from mouse lungs and used that to
infect more sets of mice.
After nine such cycles, they checked the hemagglutinin gene for mutations. Unvaccinated mice had none. In vaccinated mice, the virus had mutated to become stickier – it changed the way it binds to cells to clamp on more tightly, which helps it evade an attack from immune cells called antibodies.
But that is a big tradeoff: A stickier virus does not spread as
easily, Yewdell said.
Then the team took the new sticky mutant and infected more
unvaccinated mice with it - and here's the surprise: The virus
mutated back into a less sticky, easier-to-spread version.
That's why there are implications for people, because children
who haven't yet caught or been vaccinated against many flu strains
would offer a similar opportunity for mutation to easier-to-spread
"We want to box flu in as much as we can," Yewdell said. "With
more antibody pressure (from vaccinated people), it's got to bind
tighter and the virus is not so happy."
In a separate study published in Science, a team led by
University of Georgia researchers examined ponies vaccinated
against equine influenza to determine how much antigenic drift it
takes to outwit a particular year's flu vaccine. It's a model that
scientists might use to track a worsening outbreak.
But the take-home message - if enough of a population is vaccinated, even an imperfect vaccine can provide some benefit - is potentially
useful information the next time a worrisome new strain crops up. – (Sapa, October 2009)
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