Between 1997 and 2005, large outbreaks of HAV occurred among homosexual men in traditionally "low endemicity" countries of Denmark, Germany, the Netherlands, Norway, Spain, Sweden, and the United Kingdom.

This fueled a collaborative effort among these countries to determine if the outbreak strains were genetically related. Sequencing studies showed that most of the outbreak strains to be related.

"The majority of the strains found among homosexual men from the different European countries formed a closely related cluster, named MSM1, belonging to genotype IA," report Dr. Kathrine Stene-Johansen from the Norwegian Institute of Public Health, Oslo, and colleagues in the April Journal of Medical Virology. There were "very few" nucleotide substitutions within the cluster of MSM1 strains.

"The results of this study suggest that highly similar HAV strains are exchanged among the homosexual communities in Europe," Dr. Stene-Johansen and colleagues note.

"Seen on a more global basis," they write, "the homosexual communities across Europe are probably so large and interconnected that HAV may circulate for years resulting in an endemic situation among homosexual men."

The researchers say vaccination of Europe's homosexual population is needed to prevent further outbreaks, which "may be expected soon." However, they caution that a vaccination policy based on distribution of vaccine to active homosexual men during outbreaks may provide "only limited preventive measures when HAV spreads on a global basis."

J Med Virol 2007;79:356-365.

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