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Going
nuclear to fight polio |
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An important component of
the Atomic Energy Programme may just have the answer to the disconcerting
news that India is still some distance from being free of polio. One of the reasons of
this failure may be the need to keep the polio vaccine under refrigeration.
Scarce refrigeration equipment and uncertain power makes this difficult. But
heavy water, which is manufactured at great cost for Pressurised Heavy Water
Nuclear Reactors, has been found to be effective as a medium for the storage
of the polio vaccine without refrigeration. |
Heavy water differs from
ordinary water in that in place of hydrogen in its composition, it has
deuterium, the ‘heavy isotope’ of hydrogen. Deuterium is exactly like hydrogen
in all its chemical properties, but in its nucleus, which is the tiniest part
of its volume, it has an extra, neutral particle. As this extra particle in the
nucleus is neutral, or without a charge, the rest of the deuterium atom takes
no notice of it and deuterium behaves like hydrogen. But the mass of the atom is
all in the nucleus and an extra particle there makes the deuterium atom twice
as heavy as hydrogen. Now, the water molecule consists of two hydrogen atoms
and one of oxygen. As the oxygen atom is 16 times as heavy as the hydrogen
atom, the mass of the water molecule is about as much as 18 hydrogen atoms. But
if the water molecule had deuterium in place of hydrogen, then the water
molecule would weigh as much as 20 hydrogen atoms. This kind of water is called
‘heavy water’ and one can see that it is about 10 per cent heavier than
ordinary water.
But reverting to polio,
this is a disease that became endemic in Europe and the US after 1850. Right up
to the discovery of vaccines for it, the toll it caused is probably
unprecedented and there are records that suggest that the fear it aroused
parallel the present feelings about AIDS. Ironically, the disease became
widespread with the arrival of piped water, better drainage and better health
care, because infants stopped routinely contracting it in the cradle, when it
passes as a mild intestinal infection but imparts lifelong immunity. When
contracted in later childhood, the virus often affects the nervous system,
leading to paralysis, disability and even death.
The tide turned with the
injected Salk vaccine of 1954, and then the oral, Sabine vaccine a few years
later. The oral vaccine was easy to administer and helped wipe out polio in the
West. It also held great promise of controlling it in Africa and Asia, except
that the need to keep it refrigerated made things difficult. One reason that
the vaccine becomes ineffective at normal temperatures is that the motion of
molecules of the water medium that carries the vaccine is too vigorous and
deforms the vaccine. The vaccine, a weakened form of the polio virus, is a
large, complex molecule, which is easily denatured by high-speed impacts. Hence
the need to keep the medium cool.
This is where using ‘heavy
water’ as the medium becomes useful. As the heavy water molecule is heavier,
the molecular motion is somewhat more ‘sluggish’, even at temperatures where
ordinary water damages the vaccine. Studies by the Enterovirus Research Centre,
Mumbai and the DAE reveal that the oral polio vaccine in heavy water retains
its potency for a week even at 37oC. This could make it possible to double the
coverage of a vaccination programme, with the same effort and infrastructure.