Phylogenetic analysis of clinical mumps virus isolates from vaccinated and non-vaccinated patients with mumps during an outbreak, Switzerland 1998-2000

During the past decade mumps outbreaks have occurred in several European countries with universal vaccination programs probably due to poor efficacy of the Rubini vaccine strain. However, the evolution of vaccine escape mutants has also been considered. A phylogenetic analysis was undertaken on 69 clinical mumps isolates obtained from 39 vaccinated and 22 non-vaccinated mumps cases (and six cases with unknown vaccination status) during an outbreak in 1998-2000. Two major strain clusters (SWI-H, SWI-C) with two subgroups each (SWI-H1/2, SWI-C1/2) were identified, which belonged to genotypes C and H. No association between viral clusters and vaccination status or a specific vaccine strain (Jeryl-Lynn or Rubini) was found. Cluster SWI-C1 occurred more frequently in the Western part of Switzerland (P < 0.001). Isolates causing complicated disease tended to cluster more frequently with SWI-H1 (P = 0.11). Wild-type strains homologous or similar to the Rubini vaccine strain (isolated in Switzerland in 1974) were no longer circulating. Therefore, there was no evidence for vaccine escape mutants. Strain redistribution may have occurred during the past decades. Continuous monitoring of circulating mumps virus populations is needed.