91色情片

THE campaign to eradicate polio is one of the great medical success stories.
The disease used to kill or paralyse hundreds of thousands of people every year.
But since vaccination began in 1955, the virus has been wiped out everywhere but
a few regions of Asia and Africa. The World 91色情片 Organization now plans to
eradicate polio by 2005. Then vaccination can stop.

But some polio experts, while paying lip service to this goal, privately
doubt it will be that easy. The reason for this doubt is the very weapon that
defeated polio. The main vaccine being used to eradicate polio consists of a
live, weakened form of the virus. This vaccine virus could persist after the
disease-causing polio virus is gone鈥攁nd occasionally revert to the
dangerous type.

If that happens after vaccination stops, unvaccinated children will be
defenceless. Once the disease has gone, 鈥渨e should not simply stop vaccinating,鈥�
says Tjeerd Kimman of the National Institute of Public 91色情片 and the
Environment in the Netherlands. Polio experts raised these concerns at a meeting
in Geneva last month.

There is good reason to fear polio. The virus, which is spread by food and
water contaminated with faeces, doesn鈥檛 cause serious symptoms in most people.
But if it spreads to the nervous system, it can be deadly鈥攁nd children are
especially vulnerable. In 1955, it killed 2000 people, and paralysed 16 000, in
the US alone. By 1963, however, the US had fewer than 100 cases a year, and by
1994 polio had been banished from the Americas.

In 1988, when the WHO launched its global eradication campaign, there were 35
000 cases of polio worldwide. In 1998, with 82 per cent of the world鈥檚 children
vaccinated, there were just 3200.

So in theory, eradication should be possible. Only humans carry the virus, so
if enough people are immunised, the virus will no longer be able to find new
victims and will die out, as smallpox did. Then we can stop immunising people,
saving $1.5 billion a year worldwide.

Miscalculation

By far the most common polio vaccine in the world is oral polio vaccine
(OPV), a suspension of the weakened form of the polio virus. The other main
class of vaccine, of which the famous Salk vaccine was the first, consists of
killed virus. But these have to be injected. OPV is cheaper, easier to give and
has the advantage of inducing immunity in the gut, where the virus first
invades. What鈥檚 more, because it is 鈥渓ive鈥�, OPV viruses spread in faeces,
immunising unvaccinated people who come into contact with vaccinated
children.

But OPV viruses differ from their nastier relatives by only a few mutations.
Paul Fine of the London School of Hygiene and Tropical Medicine says conditions
in the gut favour their reversion to pathogenic forms. OPV viruses shed in
faeces are genetically closer to the dangerous forms, and it is well documented
that viruses shed in this way can cause disease.

Usually such reversion happens after the vaccine has induced immunity, so
it鈥檚 very unusual for vaccinated children to become ill. Occasionally, however,
they do鈥攊n around one in a million cases. Because of this, and because the
likelihood of encountering polio is now low, several European countries and the
US now recommend only the inactivated vaccine.

But OPV remains the predominant polio vaccine in the developing world. This
is partly because the killed vaccines now available are not as effective as OPV.
Don Henderson of Johns Hopkins University, who in the 1970s led the WHO鈥檚 drive
to eradicate smallpox, says the organisation mistakenly believed it had all it
needed to banish polio. Writing in Vaccine (vol 17, p S53) last year,
he said the miscalculation threatened polio eradication. The WHO 鈥渋n an
extraordinary act of ignorance, deliberately scrapped promising efforts to
develop an improved vaccine鈥�. New vaccine development effectively stopped in the
1960s after the OPV became available.

So for the immediate future we鈥檙e stuck with large amounts of OPV viruses in
the world. Do they pose a threat? That, says Fine, depends on how long they
persist after vaccination stops鈥攁nd a new generation of susceptible babies
emerges.

Reassuringly, he says, the viruses do not seem to survive long in the
environment or in most people, and seem not to spread 鈥渟ilently鈥� among immunised
people. But they do spread among non-immunised people, such as religious groups
in North America and the Netherlands who reject vaccination. There have been no
epidemics of paralysis in unvaccinated communities, suggesting that viruses
spread in this way are not mutating into the deadly form.

But, Fine cautions, they may simply not have had the chance. Existing
non-vaccinated communities are probably not big enough to maintain
reservoirs of OPV viruses indefinitely. But the longer the viruses persist, and
the more people they infect, the more chance they will have to change.

Ominously, it is also known that OPV viruses can persist for years in people
with impaired immune systems, where they also mutate faster than in normal
populations鈥攕o the millions of HIV-infected people worldwide might provide
a haven for these viruses. Once vaccination stopped, Fine estimates OPV viruses
might have to persist only for three to ten years before a large enough
population of non-immunised people accumulated to start spreading them. Fine
published his calculations at the end of last year in the American Journal
of Epidemiology (vol 150, p 1001).

Besides the threat of reverting into a deadly form, OPV makes it harder to
determine when wild polio virus has been eradicated, Kimman says. The WHO now
tests for polio in two ways. One is by watching for cases of paralysis. But only
one infection in a hundred or even a thousand causes paralysis. The other test
is to look for polio virus excreted into sewage by infected people. But the
large amounts of OPV virus excreted by vaccinated people interfere with the
tests, and make it hard to detect the wild virus.

At the Geneva meeting, experts debated the merits of continuing vaccination
with inactivated viruses for a few years after the last case to eradicate the
OPV viruses. Besides giving OPV viruses more time to die out before there are
large numbers of non-vaccinated people, the use of inactivated vaccine after OPV
vaccination stops would also make monitoring for polio virus easier.

But the cost could be prohibitive. As Kimmans point out, the inactivated
vaccine is made from virulent polio virus. If any escaped the vaccine factories,
it could be spread by babies too young for vaccination or even by people who鈥檝e
been immunised.

So factories would have to use high levels of containment, making the vaccine
very expensive, says Kimman. It is unlikely governments will pay for this, just
to prevent possible problems left behind by OPV. As the spectre of polio fades,
the WHO already has trouble getting governments to cough up for the existing
vaccination programme.

Harry Hull, head of the WHO鈥檚 polio programme, says that the disease will be
eradicated using OPV, but concedes there is no agreement on whether vaccination
should completely stop when there are no more cases of polio.

One solution, he says, might be to use an inactivated vaccine made from the
weakened viruses now used for OPV. These would do less damage than virulent
polio if they escaped the vaccine plant. But that could mean pushing the
deadline for eradication back again. 鈥淚t takes five years to build a vaccine
plant,鈥� says Hull, 鈥渁nd we鈥檙e still at the research stage.鈥�

And another nagging concern is emerging that could make some afraid to stop
vaccinating: bioterrorism. Laboratories all over the world have samples of polio
virus. They are supposed to destroy them one year after the disease is
eradicated. But prospective terrorists need only hide some now and wait for a
generation of unvaccinated children to emerge before they strike. 鈥淭he horror of
polio would make it a good weapon,鈥� says Roy Widdus of the WHO. 鈥淚 would not
want to be the one to stop vaccination, given that risk.鈥�

So it seems that reports of polio鈥檚 imminent demise are premature. The
current verdict on polio should actually be: down, but not out.