Vietnam in U-turn over bird flu vaccination

时间:2019-03-02 02:15:02166网络整理admin

By Debora MacKenzie Vietnam says it is going to experimentally vaccinate 600,000 poultry in Ho Chi Minh City against H5N1 bird flu. This is a sharp reversal of the country’s previous vaccine-free policy for controlling bird flu – other methods have failed to halt the disease. The switch could lead to fewer human infections in the short term. But it could also promote further, unpredictable viral evolution. Bird flu has broken out in more than half of Vietnam’s 64 provinces since the start of 2005 – leading to the culling of nearly two million birds – and 41 confirmed human cases, including 16 deaths. Speculation that the H5N1 virus in Vietnam might be spreading more readily in humans has not yet been confirmed, although recent Vietnamese samples are being tested in the US for adaptation to humans, Scott Dowell, head of the International Emerging Infections Program in Bangkok, Thailand told New Scientist. Vietnam’s efforts to contain the virus by destroying all sick and exposed birds have failed, largely because farmers are not fully compensated for culled birds and do not always cooperate, Vietnam’s national steering committee on bird flu reported in April. All poultry in Ho Chi Minh City were supposed to have been destroyed in February. Poultry is key to controlling the virus. Surveys in April found that in Vietnam’s Mekong Delta region, 70% of domestic ducks, and 22% of chickens, harboured H5N1. Vietnam now plans to test vaccines in chickens and ducks from June to August 2005, leading to compulsory vaccination in high-risk areas in October. “Ho Chi Minh City will conduct trial vaccination with 600,000 doses of Trovac avian influenza vaccine, produced by Merial France,” while 2.7 million doses of a related Dutch vaccine will be tested elsewhere in the country, according to the official Vietnam News Agency. Bird flu vaccines in poultry are controversial. Flu can sometimes spread silently among vaccinated birds, both evading detection and coming under novel selection pressures that can produce new strains. The current virulent strain of H5N1 emerged among vaccinated poultry in China. In February 2005, Thailand also reversed its longstanding ban on vaccinating poultry for flu. But vaccination is allowed only for fighting cocks – thought to be a major vector for long-distance spread of the virus as their handlers take them all over the country – and free-range ducks and chickens. “That is a very wise policy,” says Robert Webster, a flu expert at St. Jude’s Children’s Research Hospital in Memphis, US. Ducks can already harbour H5N1 without showing symptoms, so vaccinating them might curb viral spread with limited impact on its evolution. This is because ducks are a more natural host than chickens, where the virus is more likely to come up against novel selective pressures. It is not clear how Thailand plans to monitor hidden infection in vaccinated free-range chickens, however. This has proved impossible in Indonesia, the only country in the region besides China to practice widespread vaccination until now. The UN Food and Agriculture Organization says that, although vaccinated birds must be monitored carefully for hidden infection, even when infected they shed less virus than unvaccinated birds, potentially infecting fewer humans. That might be why Vietnam, home to the majority of human infections so far,