EVOLUTION OF THE HA1 DOMAIN OF HUMAN INFLUENZA-A (H1N1) VIRUS - LOSS OF GLYCOSYLATION SITES AND OCCURRENCE OF HERALD AND CONSERVED STRAINS

Thirty-one strains of human influenza A (H1N1) viruses isolated in Eur ope, mostly in Finland, from 1978-1999 were compared with respect to t heir nucleotide sequences coding for the HA1 portion of haemagglutinin . In 1984, at least two sublineages of H1N1 subtype viruses co-circula ted in Finland. The viruses isolated after 1986 formed three sequentia l phylogenetic clusters. Loss of glycosylation sites, on the globular head of the HA1 portion suggests that oligosaccharides at these sites are not necessarily advantageous for the human virus. Isolation of a h erald strain in Finland in June 1988 raised the question as to whether the virus was able to survive in Europe throughout the non-epidemic s ummer period. Demonstration of highly conserved strains, found over tw o continents in 1988, is further evidence of the existence of infectio n chains whose viruses have not been subjected to random sampling or s election events.