Different transmission patterns of hepatitis A virus for two main risk groups as evidenced by molecular cluster analysis.

Men who have sex with men and traveling children are the most important risk groups for transmission of hepatitis A virus (HAV) in Amsterdam, The Netherlands. Between these two risk groups, different HAV genotypes are found. In this study the patterns of introduction and transmission of HAV were investigated in the two groups. HAV sequences from Amsterdam patients were divided according to risk: (I) travelers and their contacts, (II) homosexual men and their contacts. The sequences in each risk group were then grouped into clusters based on the genetic distances between the sequences. Among travelers many sporadic cases were found, the clusters were small, and introduced frequently into the population, mostly in the second half of each calendar year, indicating a seasonal pattern of introduction and transmission after the summer holidays. Among men who have sex with men the clusters were bigger and remained present for a longer time; sporadic cases were few, and introduction of new strains occurred only occasionally but throughout the year. Our findings indicate that new HAV strains are frequently imported into Amsterdam by travelers, but they are limited in the extent and season of their spread. In contrast, HAV is only occasionally imported into the male homosexual and bisexual population, but remains endemic and spreads to a large number of individuals without a seasonal pattern.