Antigenic and molecular heterogeneity in recent swine influenza A(H1N1) virus isolates with possible implications for vaccination policy

In order to explore the occurrence of antigenic drift in swine influenza A( H1N1) viruses and the match between epidemic and vaccine strains. 26 virus isolates from outbreaks of respiratory disease among finishing pigs in the Netherlands in the 1995/1996 season and reference strains from earlier outb reaks were examined using serological and molecular methods. In contrast to swine H3N2 viruses, no significant antigenic drift was observed in swine H 1N1 viruses isolated from the late 1980s up to 1996 inclusive. However. a m arked antigenic and genetic heterogeneity in haemagglutination inhibition t ests and nucleotide sequence analyses was detected among the 26 recent swin e H1N1 virus strains. Interestingly, the observed antigenic and molecular v ariants were not randomly distributed over the farms. This finding indicate s independent introductions of different swine H1N1 virus variants at the v arious farms of the study and points to a marked difference between the epi demiologies of human and swine influenza viruses. The observed heterogeneit y may hamper the control of swine influenza by vaccination and indicates th at the efficacy of current swine influenza vaccines requires re-evaluation and that the antigenic reactivity of swine influenza viruses should be moni tored on a regular basis. (C) 2001 Elsevier Science Ltd. All rights reserve d.