Evaluation of vaccination to support control of H5N1 avian influenza in Hong Kong

Abstract

In 1997, 2002 and 2003 highly pathogenic avian influenza (HPAI) was diagnosed on chicken farms in Hong Kong. Following the February-April 2002 outbreak, vaccination using a killed oil-adjuvanted H5N2 avian influenza vaccine was evaluated as an additional control measure on 22 farms within a 2-km radius of the four farms that were depopulated following infection with HPAI H5N1 virus. Vaccination produced satisfactory flock antibody responses. The serological response was improved following a second dose of vaccine and the response to vaccination was poorer when delivered to older birds compared to birds first vaccinated at 8 days of age. Infection with field virus was not detected in any of these vaccinated flocks so the protective effect of the vaccine was tested under secure laboratory conditions on vaccinated and unvaccinated chickens challenged with HPAI H5N1 virus. Vaccinated birds were protected from disease, virus excretion was not detected in eight of ten vaccinated birds and the two birds that did excrete virus excreted much less virus than unvaccinated controls (> 1000 fold reduction). In December 2002 HPAI H5N1 outbreaks in 2 waterfowl parks and deaths in wild water birds in Hong Kong were followed by outbreaks on five previously unvaccinated chicken farms. Vaccination used in the face of outbreaks on three of these farms, coupled with selective culling, resulted in elimination of H5N1 virus infection from these farms. These investigations showed that the killed H5N2 vaccine, used in conjunction with enhanced biosecurity measures on chicken farms and in poultry markets, reduced the risk of H5N1 avianinfluenza outbreaks in Hong Kong and consequently the risk of spread to humans.