Vaccines used in China's chickens to combat bird flu have so far
proved effective, with no new cases of the deadly virus reported in
any of the nation's flocks. But a recent report in the UK magazine
New Scientist warns that vaccinations can lead to the
evolution of new bird flu strains, increasing the risk of human
pandemics.?
According to the report, vaccines, especially those for the flu,
are never 100 percent effective.
While such vaccines can prevent animals from falling ill, small
amounts of virus can still replicate inside creatures' bodies and
spread from animal to animal.
Such "silent epidemics," the report said, are very hard to spot,
and can cause new outbreaks if unvaccinated animals are exposed or
if vaccination programs end too early.
"Such possibilities do exist if the quality of vaccines used is
not good enough," admits Chen Hualan, chief of the national Bird
Flu Reference Laboratory of the Veterinary Research Institute in
Harbin, Heilongjiang Province.
But she said the inactivated vaccines used in China have proved
so far to be effective. Continued close surveillance has shown that
vaccinated chickens in China do not carry the bird flu virus.
"Vaccination does not mean everything is okay. We must never
relax our vigilance," said Jia Youlin, chief veterinarian and
spokesman for bird flu control at the Ministry of Agriculture.
According to Chen, surveillance include the testing of waste
samples from vaccinated chickens to see whether there are any bird
flu viruses present.
Chen said the method of putting unvaccinated "sentinel" chickens
among vaccinated chickens has been also adopted. The use of such
chickens is also recommended in the New Scientist
report.
Once bird flu viruses exist, the sentinel chickens will show
symptoms, Chen said.
The surveillance part of the equation is vital, said Ilaria
Capua of the bird flu reference lab of the World Organization for
Animal Health in Legnaro, Italy, according to the New
Scientist report.
"The vaccine used without this monitoring can have a boomerang
effect, and become a tool to spread the virus, not control it," the
report quoted Capua as saying.
In 1995 Mexico stopped an outbreak of severe H5N2 flu by
vaccinating chickens. But the virus was still circulating silently
and Mexico is still vaccinating, the report said.
Normally the bird flu virus mutates little in chickens because
it rarely persists long enough, but in Mexico the virus has been
exposed to vaccinated chickens for years. That encouraged new forms
of the virus to evolve.
China lifted its restrictions on the last two bird flu epidemic
areas on March 16, but Jia, while announcing the news, said no
efforts should be spared to prevent and control the disease.
(China Daily April 5, 2005)