ONCE there was talk of eradicating measles. Yet for now the World Health
Organization is aiming only to cut deaths by half. But virologists warn that
measles could stage a murderous comeback if we do not vaccinate more of the
world鈥檚 children soon.
In poor countries where vaccination is limited, measles is still a big
killer, of children especially. Nearly a million people die of the disease every
year, and survivors may be left with permanent brain damage. In 1990, the UN
called for 90 per cent of the world鈥檚 children to be vaccinated against measles
by 2000. Though vaccination costs just 26 cents, the figure is still only 75 per
cent.
Only three years ago the WHO hoped measles would follow smallpox into
oblivion. Yet this month it launched a campaign with a more modest aim鈥攖o
cut the death toll in half by 2005.
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But Claude Muller of Luxembourg鈥檚 National Health Laboratory, who advises the
WHO on measles, says we should make a big effort to vaccinate as many people as
we can now. Immune people can still harbour the virus without showing symptoms,
he points out, and there is evidence they can also spread it.
Because vaccination produces a more limited immune response than infection,
vaccinated people are more likely to harbour the virus sub-clinically than those
who鈥檝e had the disease. So if vaccination is rapidly expanded in a country, when
cases of measles stop many people will be immune because they鈥檝e actually had
measles. This makes it less likely that measles will circulate
subclinically.
If health officials are slow to vaccinate, however, by the time measles is
eliminated most people will only have the limited immunity induced by
vaccination. That makes it more likely the virus will circulate
unnoticed鈥攁nd that鈥檚 dangerous, Muller warns. The more this happens, the
greater the chance a resistant strain will emerge and make even vaccinated
people sick.
In the latest issue of Vaccine (vol 19, p 2258), Muller鈥檚 team report that
some circulating strains of measles virus, especially in Africa, are already
resistant to half the antibodies induced by the vaccine, which contains only one
strain of live, weakened virus. For now the vaccine is still effective, Muller
says. 鈥淏ut we know this family of viruses mutates readily.鈥�
Resistance is less likely to evolve if we immunise people as fast as we can
now. 鈥淲e have a window of opportunity,鈥� says Muller.
Bj酶rn Melgaard, head of vaccines at the WHO, says that by targeting the most
vulnerable groups, it should be possible meet the WHO鈥檚 new target. 鈥淚t was
deliberately chosen to be feasible,鈥� he says. 鈥淏ut we may not be able to
eradicate measles, and it might not even be worth it to try,鈥� he admits.