The Role of Migration and Domestic Transmission in the Spread of HIV-1 Non-B Subtypes in Switzerland

Background. By analyzing human immunodeficiency virus type 1 (HIV-1 ) pol sequences from the Swiss HIV Cohort Study (SHCS), we explored whether the prevalence of non-B subtypes reflects domestic transmission or migration patterns. Methods. Swiss non-B sequences and sequences collected abroad were pooled to construct maximum likelihood trees, which were analyzed for Swiss-specific subepidemics, (subtrees including ≥ 80% Swiss sequences, bootstrap > 70%; macroscale analysis) or evidence for domestic transmission (sequence pairs with genetic distance < 1.5%, bootstrap ≥ 98%; microscale analysis). Results. Of 8287 SHCS participants, 1732 (21%) were infected with non-B subtypes, of which A (n = 328), 21% (A), 16% (C), 24% (CRF01_ AE), and 28% (CRF02_ AG) belonged to Swiss-specific subepidemics. The microscale analysis identified 26 possible transmission pairs: 3 (12%) including only homosexual Swiss men of white ethnicity; 3 (12%) including homosexual white men from Switzerland and partners from foreign countries; and 10 (38%) involving heterosexual white Swiss men and females of different nationality and predominantly nonwhite ethnicity. Conclusions. Of all non-B infections diagnosed in Switzerland, < 25% could be prevented by domestic interventions. Awareness should be raised among immigrants and Swiss individuals with partners from high prevalence countries to contain the spread of non-B subtypes.