Australia: Treatment as prevention and PrEP reduce new HIV cases in gay and bisexual men by two-thirds

Domizia Salusest |

A 27% increase in the number of gay and bisexual men taking antiretroviral treatment in Australia and having undetectable viral load resulted in a 66% reduction in HIV infections between 2010 and 2019, showing that treatment as prevention is highly effective as a public health measure, Australian researchers report in the journal Lancet HIV.

The relationship between the increased number of people who are virally suppressed and reductions in HIV transmission became even stronger after the introduction of PrEP, so that by 2019, every 1% increase in the prevalence of viral suppression corresponded to a 20% reduction in HIV incidence.

"Our results suggest that although effective on its own, treatment-as-prevention most effectively reduces HIV incidence when combined with PrEP distribution and low levels of undiagnosed HIV (ie, high rates of testing)," comment the authors.


virological suppression

Halting of the function or replication of a virus. In HIV, optimal viral suppression is measured as the reduction of viral load (HIV RNA) to undetectable levels and is the goal of antiretroviral therapy.

treatment as prevention (TasP)

A public health strategy involving the prompt provision of antiretroviral treatment in people with HIV in order to reduce their risk of transmitting the virus to others through sex.

multivariable analysis

Statistical technique often used to reduce the impact of confounding factors, in order to attempt to identify the real association between a factor of interest and an outcome. 

person years

In a study “100 person years of follow-up” could mean that information was collected on 100 people for one year, or on 50 people for two years each, or on ten people over ten years. In practice, each person’s duration of follow-up is likely to be different.

longitudinal study

A study in which information is collected on people over several weeks, months or years. People may be followed forward in time (a prospective study), or information may be collected on past events (a retrospective study).

Several large studies have shown that at an individual level, viral suppression on antiretroviral treatment eliminates the risk of HIV transmission to sexual partners. However there has been less data showing its effect at the population level, especially in high-income countries.

Preliminary results of a study of the impact of treatment as prevention in Australia were presented at AIDS 2020 by Dr Denton Callendar of the Kirby Institute, University of New South Wales.

Now, using data collected between January 2010 and December 2019, the same research group has assessed the ten-year impact of increasing antiretroviral treatment coverage and providing PrEP on HIV transmission in the states of New South Wales and Victoria. The study is the first comprehensive analysis of the effectiveness of treatment as prevention in gay and bisexual men.

The longitudinal study examined changes in HIV status, treatment uptake, viral suppression and PrEP use in 101,772 gay and bisexual men receiving health services at 69 clinics that provide HIV testing in New South Wales and Victoria. Participants could enter the cohort at any time during the follow-up period.

The primary analysis looked at HIV incidence in men who tested at least twice during the follow-up period and who were HIV-negative at their first test. This cohort comprised 59,234 men, of whom 1201 tested HIV-positive during follow-up, an incidence of 0.4 cases per 100 person-years of follow-up. HIV incidence declined by 66% between 2010 and 2019, from 0.64 to 0.22 cases per 100 person-years. The decline was greatest in men aged 30-39 years (88%).

Among men without HIV, the number of PrEP users increased from 17% in 2016 (the year of introduction) to 36% in 2019, when 12,189 men were taking PrEP.

Repeat HIV testing increased during the study period and the investigators calculated that the prevalence of undiagnosed HIV fell from just under 11% in 2010 to 9% in 2019.

The primary analysis also calculated population prevalence of viral suppression and used this as a variable in the multivariable analysis of factors associated with HIV incidence. The cohort with HIV consisted of 12,554 men who were either living with HIV at baseline or tested HIV-positive during the follow-up period and had at least one viral load test after diagnosis. The proportion of men living with HIV who were virally suppressed grew from 69% in 2010 to 88% in 2019, an increase of 27%.

The primary multivariable analysis, which adjusted for sociodemographic characteristics and PrEP use, showed that each 1% increase in the prevalence of viral suppression below 200 copies/ml was associated with a 6% reduction in the incidence of HIV (incidence rate ratio 0.94), p<0.001). The incidence of HIV was 70% lower in men who used PrEP (IRR 0.30, p<0.001) compared with non-PrEP users.

HIV incidence fell most sharply between 2015 and 2017 and levelled off after this point, suggesting that expanding PrEP use reinforced the impact of declines in undiagnosed HIV and unsuppressed viral load on HIV incidence.

The study investigators say that increases in testing, treatment coverage and viral suppression reflect comprehensive efforts by clinics and community organisations to make services more accessible, as well as targeted education campaigns.

In an accompanying editorial comment, two Thai HIV experts stress the importance of designing services around their users to maximise the impact of treatment as prevention and PrEP. Drs Nittaya and Praphan Phanuphak of the Institute of HIV Research and Innovation in Bangkok emphasise the importance of communities in co-implementing services to achieve trust and equity. They point out that rapid actions by state governments and communities to achieve large-scale implementation of treatment as prevention and PrEP “have brought Australia closer to ending AIDS a decade before many other countries.”