Evidence supporting the inclusion of strains from each of the two co-circulating lineages of H3N8 equine influenza virus in vaccines

Two lineages of antigenically distinct equine influenza A H3N8 subtype viruses, American and European, co-circulate. Experiments were conducted in ponies to investigate the protection induced by vaccines containing virus from one lineage against challenge infection with homologous or heterologous virus. Regression analysis showed that vaccinated ponies with average pre-challenge single radial haemolysis (SRH) antibody levels (i.e. 45-190 mm(2)) had a higher probability of becoming infected if they were vaccinated with virus heterologous to the challenge strain than if they were vaccinated with homologous virus. Field studies in Thoroughbred racehorses also showed that SRH antibody levels of greater than or equal to150 mm(2) induced by vaccines containing a European lineage strain are protective against infection with a virus from the same lineage, but that the same or higher antibody levels may not be protective against an American lineage virus. In conclusion, vaccines should contain virus strains representative of both H3N8 subtype lineages to maximise protection against infection. (C) 2004 Elsevier Ltd. All rights reserved.