Transmission of non-B HIV subtypes in the UK is increasingly driven by large non-heterosexual clusters

BACKGROUND:  In the UK HIV epidemic, historically dominated by subtype B transmission among men who have sex with men (MSM), 50% of diagnoses and prevalent infections are now heterosexual, mainly non-B subtypes. Between 2002 and 2010 non-B diagnoses among MSM increased from 5.4% to 17% and this study has focused on the drivers of this change.

METHODS:  Growth between 2007 and 2009 in transmission clusters among 14,000 subtype A1, C, D and G sequences from the UK HIV Drug Resistance Database was analysed by risk group.

RESULTS:  Of 1148 clusters containing at least two sequences in 2007, >75% were pairs and >90% were heterosexual. Most clusters (71.4%) did not grow during the study period. Growth was significantly lower for small clusters, and higher for clusters ≥7, being highest for clusters comprising sequences from MSM and people who inject drugs (PWID). Risk group (p<0.0001), cluster size (p<0.0001) and subtype (p<0.01) were predictive of growth in a generalized linear model.

DISCUSSION:  Despite the increase in non-B subtypes associated with heterosexual transmission, MSM and PWID are at risk for non-B infections. Crossover of subtype C from heterosexuals to MSM has led to the expansion of this subtype within the UK.