Genetic variability of measles viruses circulating in the Benelux

In Europe measles incidence remains high and in some parts the disease is likely to be still endemic due to insufficient vaccination. Luxembourg experienced an outbreak with at least 110 cases in 1996, and cases continued to be reported throughout 1997. We used molecular epidemiology to investigate this apparent endemicity. On the basis of their N gene sequences, the isolates were assigned to the typical European C2 and D6 genotypes. Sequence diversity within the outbreak was 0.2%. The nucleotide distance between the C2-viruses of the outbreak and the other C2 isolates was at least three or four times higher, suggesting an independent origin of the latter viruses. Similarly, the four D6 viruses found in Luxembourg were thought to be of at least two or three origins. Thus, we propose here to use intra-outbreak sequence diversity to differentiate between sporadic endemic cases and a "pseudo-outbreak" of multiple unrelated imported cases.