China is expanding its effort to vaccinate poultry in an effort to stop the spread of the highly virulent form of bird flu virus known as H5N1. It is one of four Asian nations that have adopted the inoculation tactic. The United Nations' top influenza control official says there are drawbacks to the practice, biological and economic, but emphasizes that it is necessary for reducing the amount of virus circulating in birds.

Bird flu vaccine

A major goal of the emerging international strategy to prevent bird flu from becoming a global human pandemic is to stop the virus at its source before it adapts to human transmission. Part of this effort involves vaccinating birds.

In Asia, the epicenter of the bird flu epidemic, poultry vaccination has been widening as a tactic since September of last year when two U.N. agencies, the World Organization for Animal Health and the Food and Agriculture Organization, strengthened a previous recommendation encouraging it in conjunction with other control methods.

In line with this, China has announced plans to vaccinate all of its many billions of chickens, ducks, and geese. It is an ambitious undertaking. The director of the country's Department of Disease Control, Qi Xiaoqu said on a recent visit to Washington that China has the world's largest population of domestic poultry.

"In China, we are raising 14.2 billion domestic poultry, accounting for 20 percent of the global total," he said.

Chickens sit in cages at a Changji farm, in China's western Xinjiang region

Mr. Qi says the government policy is to kill all birds within three kilometers of an avian flu breakout and vaccinate those between three and five kilometers away. It is also boosting vaccination of birds in other key areas.

"For all the areas that are the key areas, namely water networks and also big farmers of poultry and also the area where there used to be outbreaks of avian influenza, we require a 100 percent vaccination for the poultry there, and also the government funds all the vaccination," he said.

Veterinarians in Vietnam, meanwhile, have been engaged in a poultry vaccination campaign using Chinese vaccines with a goal of inoculating more than 200 million birds by the end of this month. Indonesia and Pakistan are two other Asian nations vaccinating poultry.

Vaccine does not always prevent infection but inoculated birds do not get infected as easily. Those that do become infected do not get sick and shed far less virus than unvaccinated ones.

The U.N.'s Coordinator for Avian and Human Influenza, Dr. David Nabarro, told a Washington gathering that the practice is a necessary part of a bird flu containment program. He notes, however, that it has a major disadvantage.

"You still end up, if you've got a vaccinated chicken that is exposed to the virus, with the chicken being a virus carrier. It's not a very high titer [strength, amount], so you're dampening down the titer of virus that is coming out of the chicken, but still the chicken is a carrier," he said.

Dr. Nabarro says that as a result, the practice has economic consequences. Europe and Japan will not import vaccinated chickens.

"So vaccination as a strategy for use when you are dealing with transnational chicken trade is probably not the best strategy. It is, however, xtremely important for dampening down the virus," he said.

Veterinary experts warn that a poultry vaccine, like those for humans, must be tested to ensure quality and effectiveness, must be targeted at the virus in circulation, and must be the proper dosage.

But a study in the Journal of Virology last year pointed out long-term problems with bird vaccines. Mexican farmers have been inoculating their poultry since the late 1990s to control an avian flu strain much less virulent than H5N1. With such consistent use of the same vaccine, the virus has mutated over time so that it matches the vaccine strain less and less. Vaccinated birds who become infected still avoid disease, but they shed more and more virus into the environment.

A report in the journal Science quotes the author of the study as saying he believes the widespread vaccination probably contributed to the virus becoming endemic not only in Mexico but also neighboring Guatemala and El Salvador.

To avoid this, public health experts say the virus must be monitored and the vaccine updated periodically.