HIV incidence fell by three-quarters in Australian gay men, with strong association with treatment as prevention

Denton Callander at CROI 2020.

HIV treatment as prevention, as measured in community-level viral suppression, was strongly associated with a large drop in the number of new HIV infections among gay and bisexual men in Australia, even before PrEP was available. This study was presented by Dr Denton Callander from the University of New South Wales yesterday at the Conference on Retroviruses and Opportunistic Infections (CROI 2020).

Despite the clear benefits of treatment as prevention (or Undetectable = Untransmittable, U=U) at the individual level, no large-scale studies have yet evaluated the community-level effects of treatment as prevention (TasP) on direct measures of HIV incidence among gay and bisexual men. Previous research has indicated a disconnect between the benefits of TasP/U=U at the individual and population levels. Large observational and experimental studies carried out in various sub-Saharan African countries failed to show a strong link between rates of viral suppression and associated reductions in HIV incidence.

One of the challenges with the African studies is that they treated the population as homogenous rather than considering groups with different risk levels and uptake of treatment, including key populations such as men who have sex with men. Another challenge was defining the specific measures that would reveal a relationship between individual level viral suppression and its subsequent impact at a broader community level.

The study

Data for this study came from the Australian ACCESS database: an existing large surveillance system that collects anonymous data on sexually transmitted infections in gay and bisexual men. The study took place in Australia’s two most populous states, New South Wales and Victoria from 2012 to 2017. Data was taken from 67 sites including sexual health clinics, hospitals, general practices and community testing sites. The final longitudinal cohort consisted of a total of 115,982 gay and bisexual men (101,143 of these were HIV negative while 14, 839 were HIV positive).


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.


The presence of virus in the blood.


detectable viral load

When viral load is detectable, this indicates that HIV is replicating in the body. If the person is taking HIV treatment but their viral load is detectable, the treatment is not working properly. There may still be a risk of HIV transmission to sexual partners.

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.

Undetectable = Untransmittable (U=U)

U=U stands for Undetectable = Untransmittable. It means that when a person living with HIV is on regular treatment that lowers the amount of virus in their body to undetectable levels, there is zero risk of passing on HIV to their partners. The low level of virus is described as an undetectable viral load. 

In order to estimate community level viraemia, two measures were used. The first was clinical viral load results (indicating if HIV-positive men had detectable or undetectable viral loads, ≤ 200 copies) from the ACCESS database. Secondly, the estimated number of men with undiagnosed HIV, based on mathematical modelling, with these men assumed to have detectable viral loads and therefore be infectious. Combining values for HIV-positive men with detectable viral loads and the estimated number of men who were living with undiagnosed HIV yielded an estimate of community viraemia.

HIV incidence was established by looking at repeated HIV tests and new annual HIV diagnoses for the period 2012-2017. The correlation between community viraemia and annual HIV incidence was calculated both prior to the widespread availability of PrEP in Australia from 2012-2015 and for the entire period 2012-2017.


There was a large reduction in HIV viraemia among diagnosed gay and bisexual men from 17% in 2012 to 4% in 2017, while the proportion of men living with undiagnosed HIV decreased only slightly from 11% to 9%. Taken together, annual community prevalence of HIV viraemia decreased from 29% in 2012 to 13% in 2017, while HIV incidence decreased from 0.88/100 person-years in 2012 to 0.22/100 person-years in 2017.

There was a strong correlation between the decrease in community-level viraemia and lower HIV incidence for the analyses in both periods: with correlation coefficients of 0.90 prior to the introduction of PrEP and 0.94 for the period 2012-2017 (p < 0.001 for both).


The authors demonstrated that between 2012 and 2017, there had been major increases in treatment uptake for HIV-positive gay and bisexual men, with subsequent drops in viraemia. This decreased level of community viraemia was strongly associated with a reduction in the number of men diagnosed HIV positive in this period. A significant point is that this relationship existed prior to the introduction of PrEP in these Australian states.

“This provides at least one kind of evidence in support of TasP as a public health strategy,” Callander concluded. “It suggests that Australia’s investment in TasP and treatment generally seems to really have had a public health payoff.”


Callendar D et al. Decreasing community viremia is associated with decreasing HIV incidence in Australia. Conference Retroviruses and Opportunistic Infections, abstract 48, March 2020.

View the abstract on the conference website.

Watch the webcast on the conference website.