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)