Background:  Increase or stability in transmission of drug-resistant variants has been reported in recent years. We have systematically monitored transmission of drug-resistant variants in Switzerland between 1996 and 2002.

Methods:  Systematic collection of patients with recent infection (<1 year) in 7 AIDS centers in Switzerland between January 1996 and December 2002. Sequencing and subtyping of reverse transcriptase and protease regions were performed. Mutations were analyzed according to ANRS algorithm version 10. Phylogenetic analyses were performed using the PHYLIP package.

Results:  We have identified 48/453 (10.6%) recently infected patients with drug-resistant variants over 7 years. The prevalence of drug-resistance transmission varied from 5.8 to 12.6% over years. Resistant variants were detected in 10% of MSM, 6.8% of heterosexuals and 25% of IDU. Non-B subtypes were detected in both Caucasians (28%) and non-Caucasians (79%). Several factors influence the prevalence of drug-resistance transmission including small numbers, changes in the prevalence of non-B subtypes, variations in the frequency of potential transmitters with virological failure during sub-optimal therapy or with treatment interruption and detection of a cluster of 5 IDU infected by the same B resistant variant in 2000/2001. The resistance pattern revealed a predominance of RTI-resistance transmission including 11 patients with revertants at position 215 and a low prevalence of variants resistant to 2 or 3 classes of drug.

Keywords: primary infection; resistance; transmission