Australian researchers find treatment as prevention (TasP) works in men who have sex with men

HIV treatment (ART) has at least two important benefits. At the level of the individual, when used as directed, ART eventually suppresses the amount of HIV in the blood of the vast majority of people to very low levels that are below the threshold of detection of routinely used tests. Such very low levels are commonly called “undetectable.” This suppression of HIV reduces inflammation and HIV-related injury to the body. Over time, the immune system is able to rebuild itself to the point where the risk of AIDS-related infections and cancers becomes extremely rare. The power of ART is so tremendous that research increasingly finds that many ART users will have near-normal life expectancy.

ART and a suppressed viral load also have a big impact on the sexual transmission of HIV. Nearly a decade ago, well-designed clinical trials were initiated in couples where one partner was HIV positive and the other was HIV negative. After the partner with HIV initiated ART and achieved and maintained a suppressed viral load, both partners underwent regular HIV testing and engaged in condomless intercourse. Researchers found no cases of new HIV infection from partners whose viral load was suppressed. This finding occurred in gay, bisexual and other men who have sex with men (gbMSM) as well as in heterosexual people.

Based on these results, the phrase “U=U” was developed. This phrase is shorthand for undetectable equals untransmittable. U=U is an important part of HIV prevention campaigns and efforts to reduce the stigma that has followed HIV for decades.

Stopping the spread over a large area or population

Researchers are interested in deploying interventions at the level of a city, country or region to help reduce the spread of HIV. In particular, a key part of HIV prevention campaigns is enhancing the availability of HIV testing followed by swift referral of newly diagnosed people into care so that they can be offered treatment. Once in care and on treatment, the vast majority of patients ultimately achieve and maintain an undetectable viral load. This use of treatment (and undetectable viral load) to help prevent HIV is called Treatment as Prevention (TasP).

An Australian study

Researchers in Australia conducted a large study of TasP in more than 100,000 participants—all were gbMSM. There were 90,304 HIV-negative men and 11,468 HIV-positive men. The study took place between January 2010 and December 2019 in two large Australian states—New South Wales and Victoria.

Over the course of the study, the percentage of men with HIV who had a suppressed viral load increased from 69% to 88%. During that time, the rate of new HIV infections fell almost threefold. The researchers estimated that for every 1% increase in the proportion of people with HIV suppression, there was a 6% decrease in HIV infections.

HIV pre-exposure prophylaxis (PrEP) was introduced in Australia in 2016. That year, researchers estimated that about 18% of gbMSM (who were HIV negative) were using it. By 2019, researchers estimated that 36% of gbMSM were using PrEP. However, they noted that the effect of TasP was seen before PrEP became available. The results from Australia are very encouraging. For comprehensive HIV prevention, the researchers recommend deploying both TasP and PrEP.

Study details

Researchers collected de-identified information from sexual health clinics, doctors’ offices, community-based HIV testing sites, hospital clinics and other places that provided services for people with and at risk for HIV.

According to the researchers, they focused on cisgender men because although the clinics had information as to whether a person was transgender, they did not have further information available for the study on whether they were transgender men or transgender women.

Most of the men were of European ancestry and many had been previously diagnosed with rectal sexually transmitted infections.

Although the researchers included more than 90,000 HIV-negative men in the study, they focused their analysis on 59,234 men who were initially HIV-negative and had two or more subsequent HIV test results available during the 10 years of the study.


A total of 1,201 new cases of HIV occurred during the study (about 2% of the 59,234 initially HIV-negative men). However, the rate of new HIV infections fell overall by 66% over a decade.

The decrease in new HIV infections among some age groups was as follows:

  • 30 to 39 years old – a decrease of 89%
  • 16 to 29 years old – a decrease of 63%
  • 40 and older – a decrease of 49%

Over the study period, as mentioned previously, the proportion of men who were virally suppressed with ART increased significantly. There was a statistical association between the increase in men on ART with viral suppression and a decreased risk of other men acquiring HIV.

The researchers found that there was a broad increase of ART initiation and viral suppression across the age groups in the study.

Bear in mind

New South Wales and Victoria are the two most populous states in Australia, with gbMSM concentrated in urban areas. During the study, there was an increased use of ART and viral suppression, and this was associated with a significantly reduced spread of HIV. The decrease in new HIV infections occurred prior to the introduction of PrEP in Australia.

After PrEP was introduced in 2016, new HIV infections continued to fall, suggesting that both TasP and PrEP complement each other’s prevention effects.

However, researchers found that between 2017 and 2019 the rate of decline in new HIV infections became and stayed stable. The researchers stated that this stabilization in infection rates may have occurred because TasP and PrEP had reached a saturation point among some gbMSM. They added that a previous study had found that gbMSM who were migrants to Australia had lower rates of HIV diagnosis, treatment and viral suppression.

Back to Australia. It is possible that TasP and PrEP had not sufficiently been used by gbMSM who had immigrated to Australia. Therefore, the researchers added that in order to achieve the full potential of TasP and PrEP in Australia, better access among migrant gbMSM is needed.

A related finding has been seen in the Netherlands. In that country, a recent study has found that young gbMSM born outside of Western Europe are at heightened risk for HIV compared to young gbMSM born in Western Europe.

The researchers noted that another study found that the use of condoms “declined substantially” between 2010 and 2019 among gbMSM in New South Wales and Victoria.

Another point worth considering is that during the study there were initiatives outside of the study that likely enhanced the impact of TasP. The researchers stated that governments, clinics and community-based organizations “worked to remove ART prescribing restrictions, enabled community pharmacy dispensing, reduced patient treatment costs, and educated those at risk of HIV about the individual and prevention benefits of early and sustained treatment. Furthermore, there have been a range of HIV testing initiatives [focusing] on gbMSM...”

The Australian study was not a randomized controlled trial, so its findings are not definitive. However, its results are strongly encouraging and align with the science of HIV treatment’s effects on transmission potential and PrEP in a well-resourced setting.

The findings from Australia are likely transferrable to other high-income countries that also have a concentrated epidemic of HIV among gbMSM. However, for TasP to be successful in other countries with populations of gbMSM at risk of HIV, a similar increase (as happened in Australia) in the availability of HIV testing, care and access to treatment and PrEP must occur over the long term.

—Sean R. Hosein


  1. Callander D, McManus H, Gray RT, et al. HIV treatment-as-prevention and its effect on incidence of HIV among cisgender gay, bisexual, and other men who have sex with men in Australia: a 10-year longitudinal cohort study. Lancet HIV. 2023 Jun;10(6):e385-e393. 
  2. Rodger AJ, Cambiano V, Bruun T, et al. Sexual activity without condoms and risk of HIV transmission in serodifferent couples when the HIV-positive partner is using suppressive antiretroviral therapy. JAMA. 2016 Jul 12;316(2):171-81.
  3. Cohen MS, Chen YQ, McCauley M, et al. Antiretroviral therapy for the prevention of HIV-1 transmission. New England Journal of Medicine. 2016 Sep 1;375(9):830-9.
  4. Loutfy MR, Wu W, Letchumanan M, et al. Systematic review of HIV transmission between heterosexual serodiscordant couples where the HIV-positive partner is fully suppressed on antiretroviral therapy. PLoS One. 2013;8(2):e55747. 
  5. Baral S, Rao A, Sullivan P, et al. The disconnect between individual-level and population-level HIV prevention benefits of antiretroviral treatment. Lancet HIV. 2019 Sep;6(9):e632-e638. 
  6. Evers YJ, Goense CJD, Hoebe CJPA, et al. Newly diagnosed HIV and use of HIV-PrEP among non-western born MSM attending STI clinics in the Netherlands: a large retrospective cohort study. Frontiers in Public Health. 2023 Jun 19; 11:1196958.