Demographic characteristics of sources of HIV-1 transmission in Zambia

BACKGROUND In the last decade, universally available antiretroviral therapy has led to reduced HIV incidence in sub-Saharan Africa. Sources of remaining transmission need to be characterised to design effective prevention strategies. METHODS We used phylogenetics to understand the population characteristics of people who are sources of infection. HIV samples from 6,864 individuals from Zambia were deep-sequenced as part of HPTN 071-02 (PopART) Phylogenetics between 2014 and 2018. We identified 300 likely directed transmission pairs and analysed their sources to better understand transmission in the general population. RESULTS After demographic weighting of the recipient population to match the estimated total population infected during the trial period, 59.4% (95% CI: 53.1%-65.8%) of transmissions were male-to-female, with 43.1% (36.6%-49.5%) of transmissions from males aged 25-40. Since the adult HIV prevalence was 2.0 times higher in women than men, the per-capita transmission rate was 2.93 times higher per infected male than per infected female. 25.6% (19.9%-31.3%) of sources were estimated to have themselves been infected less than a year before the transmission event. 16.8% (12%-21.7%) of sources transmitted viruses resistant to first-line ART. 13% (8.6%-17.4%) of transmissions occurred between individuals from different study communities. CONCLUSIONS Our findings suggest that HIV transmission in the study communities took place as part of common sexual mixing, and that there was no outsized contribution of importation, from drug resistance, or recent infection. Men aged 25-40 should be the focus of treatment for prevention interventions, even if linking them to care requires more effort.