Recently infected HIV-positive Quebec residents cause more than half of new cases

Mark Mascolini for

Most new HIV cases in Quebec, Canada, can be traced to recently infected but still untreated people in the province, according to an 859-person analysis [1]. The study traced genetic links among 56% of viruses from newly infected people, a slight increase from the clustering rate reported in a previous study by the same research group [2].

Mark Wainberg (McGill University, Montreal) analysed HIV-1 pol gene sequences of subtype B virus from 859 people infected within 6 months of first viral sampling from 1998 through 2007. Comparing the sequences through phylogenetic analysis, he classified the viruses as unique, parts of small clusters (2 to 4 linked viruses), or parts of large cluster (5 or more linked viruses). The proportion of viruses that could be assigned to clusters rose from 49% in December 2005 to 56% in June 2007. Of course all HIV infections are linked to at least one other infection, but the source virus cannot always be identified.

Of the 423 unique and small-cluster viruses identified before 2006, 403 (95%) represented dead-end viruses for which no further transmission could be identified. In contrast, viruses in 21 large clusters were parts of growing transmission cascades: The cluster rate grew from 6.6 to 10.3 viruses per cluster over the course of the study.

Viruses carrying mutations that make HIV resistant to nucleosides were less frequent in large clusters (1.2%) than in small clusters (3.4%) or in unique viral samples (7.9%). In contrast, the nonnucleoside mutations K103N/R and G190A were significantly more frequent in large clusters (12.1%) than in small clusters (3.3%) (P < 0.0001).

One immense cluster included 27 viruses harboring the G190A mutation, which made the viruses more than 50-fold resistant to nevirapine. But those viruses remained susceptible to efavirenz and were more than 10-fold hypersusceptible to etravirine (TMC125), the newest NNRTI.

Wainberg concluded that in the population studied, most new HIV infections “arise from untreated persons at early stages of infection, often unaware of their serostatus.” He cautioned that newly infected people with a high viral load who are unaware of their HIV status pose a special risk of onward HIV transmission.



  1. Wainberg M, Brenner B, Roger M, Routy J, Moisi D. Impact of clustering on the transmission of HIV-1 variants harbouring drug resistance. 3rd International Workshop on HIV Transmission: Principles of Intervention. July 31-August 2, 2008, Mexico City. Abstract 36.
  2. Brenner BG, Roger M, Routy JP, et al. High rates of forward transmission events after acute/early HIV-1 infection. J Infect Dis. 2007;195:951-959.

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