Dutch researchers prove effectiveness of avian vaccines in blocking bird flu spread
A team of scientists from the Central Institute for Animal Disease Control in Leystad, and Wageningen University and Research Centre in the Netherlands, have unveiled the results of a study on the effect of immunising poultry against avian influenza, concluding that vaccination could represent a critical form of control, preventing the next human influenza pandemic by eliminating the source of human infections. All of the team's experiments on avian influenza viruses were carried out in the framework of a recently completed EU project, AVIFLU, funded under the Fifth Framework Programme (FP5). The project addressed pathogenesis and improved diagnosis and control of avian influenza infections. According to the study, published in the Proceedings of the National Academy of Sciences, 'vaccination is able to reduce the transmission level to such an extent that a major outbreak is prevented'. It is widely accepted that vaccination offers a potentially attractive measure to control and prevent outbreaks of the highly pathogenic avian influenza (HPAI). In recent years, several vaccines have been developed against H5 and H7 influenza viruses in poultry, and they have proved that vaccination can protect chickens from overt disease (morbidity) and mortality. The reason why outbreaks of HPAI are still controlled by killing and destroying infected poultry - 30 million birds were destroyed in the Netherlands during the 2003 outbreak - instead of simply vaccinating them, is that although vaccination had proved to prevent death and symptoms, asymptomatic viral shedding had also been observed. It was thus uncertain whether, independently of outbreaks, immunised birds could still silently spread the disease, increasing the risk of new outbreaks and posing a severe threat to humans. The ideal vaccine should, therefore, not only stop the disease but also prevent its transmission. Until now, little was however known about the ability of AI vaccines to reduce transmission of HPAI viruses in chickens or the quantification of this reduction: research was exclusively concerned with the efficacy of the vaccine in protecting the chicken from becoming ill. A knowledge gap therefore remained in relation to how well the vaccine reduces the spread of the virus. This represented a critical point in need of clarification before the start of large-scale vaccination campaigns and was becoming urgent as China has announced plans to vaccinate billions of birds, and Indonesia and Vietnam have been urged by the World Organisation for Animal Health to move in the same direction. The Dutch scientists studied the effect of vaccination on the spread of the virus in a population of chickens by using so-called transmission experiments. In a transmission experiment, a number of infected chickens are housed together with a number of uninfected chickens, and the infection chain is monitored. These experiments allow researchers to quantify the effect of vaccination on transmission dynamics. Birds were infected with the H7N7 strain, isolated during the outbreak of avian influenza in the Netherlands in 2003, and vaccinated using two commercially available vaccines: a H7N1 (Italy 99) vaccine and a H7N3 (Pakistan 95) vaccine. The research was carried out as follows: in one of the experiments all birds were vaccinated in advance, then, five chickens were placed in a cage and inoculated with the virus. 24 hours later, five contact chickens were added and the spread of the virus monitored. The experiments were terminated after three weeks. Pair experiments were also performed with vaccinated inoculated chickens and unvaccinated healthy contact chickens. In all cases, strict security measures surrounded the experiments: the H7N7 strain is known to infect people and even killed one person in the Netherlands during the 2003 outbreak. The results show that two weeks after vaccination, both of the tested vaccines were able to completely block the spread of the disease. For shorter periods, however, and in spite of the fact that one week after vaccination the chickens no longer became ill if they were inoculated with the influenza virus, some transmission still occurred. The experiments prove that vaccination not only protects chickens against mortality and morbidity, but also reduces the spread of a virus within a flock to such an extent that a major outbreak can be prevented. However, scientists acknowledge that, since vaccination probably does not completely block transmission of the virus, vaccination programmes should be seen as a complementary measure. The conclusions of a previous study by the same research group warn of the danger of discontinuing the observation of basic measures relating to hygiene, transport restrictions and surveillance once vaccination campaigns take off. Researcher Michiel van Boven explained to CORDIS News that his group has also performed additional experiments on vaccination with ducks and pheasants against H7N7 avian influenza virus, showing that for each specific host-pathogen combination it is perfectly feasible to determine whether vaccination would be an effective tool to prevent outbreaks. However, influenza A viruses are not only highly variable, but may also have different characteristics in different host species, and vaccination should therefore be specific concerning the type of vaccine, the dose and the number of shots. Similar experiments have been carried out in ducks with the H5N1 strain isolated during the outbreak in Pakistan in 2004 using a H5N2 vaccine. The group is now analysing the results and expects to publish the conclusions early next year.
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