In 2003 an outbreak of highly pathogenic avian influenza virus (H7N7) struck poultry in The Netherlands. A European Commission directive made vaccination of valuable species in zoo collections possible under strict conditions. We determined pre- and post-vaccination antibody titres in 211 birds by haemagglutination inhibition test as a measure of vaccine efficacy. After booster vaccination, 81.5% of vaccinated birds developed a titre of > or =40, while overall geometric mean titre (GMT) was 190 (95% CI: 144-251). Birds of the orders Anseriformes, Galliformes and Phoenicopteriformes showed higher GMT, and larger percentages developed titres > or =40 than those of the other orders. Antibody response decreased with increasing mean body weight in birds > or =1.5 kg body weight. In the vicinity of the outbreak, H7N7 was detected by RT-PCR in wild species (mallards and mute swans) kept in captivity together with infected poultry, illustrating the potential threat of transmission from poultry into other avian species, and the importance of protecting valuable avian species by means of vaccination.