Far fewer HIV infections in Australian PrEP users – consistent use is key

Dr Nicholas Medland at CROI 2024. Photo by Roger Pebody.
Dr Nicholas Medland at CROI 2024. Photo by Roger Pebody.

An impressive 79% reduction in HIV incidence was seen between two directly comparable groups in a whole-population analysis from Australia, presented to the Conference on Retroviruses and Opportunistic Infections (CROI 2024) this week by Dr Nicholas Medland.

People who have received hepatitis C treatment and who were not adherent to PrEP were particularly vulnerable to acquiring HIV within a few years of receiving their first PrEP prescription.


While we know that PrEP is highly effective at preventing HIV transmission at the individual level, questions remain regarding its effectiveness as an intervention in the population. One challenge for researchers is choosing who to compare to people who are eligible for PrEP and use it. It is unhelpful to compare them to people who would not be eligible for PrEP, as behaviours that increase vulnerability to HIV would differ substantially.

Many country-level comparisons have looked at infections before and after the rollout of PrEP. However, there can be problems with this approach as the number of annual new HIV infections in the pre-PrEP era was most likely higher anyway, and reductions have happened over time independent of PrEP.



Studies aim to give information that will be applicable to a large group of people (e.g. adults with diagnosed HIV in the UK). Because it is impractical to conduct a study with such a large group, only a sub-group (a sample) takes part in a study. This isn’t a problem as long as the characteristics of the sample are similar to those of the wider group (e.g. in terms of age, gender, CD4 count and years since diagnosis).

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.

risky behaviour

In HIV, refers to any behaviour or action that increases an individual’s probability of acquiring or transmitting HIV, such as having unprotected sex, having multiple partners or sharing drug injection equipment.

Medland and colleagues considered a direct comparison of people eligible for PrEP: those who continue taking it compared to those who have been prescribed it but have only filled one prescription, and not returned for subsequent refills.

PrEP has been subsidised by the Australian government since 2018 and is available through its national health system. However, prior to that, there were already 18,000 people taking PrEP through participation in implementation trials. It can be prescribed by any doctor in any health setting for up to three months at a time, with a co-payment of US $20 for those who are not eligible, and $5 for those who are.

Overall, in Australia, PrEP is highly accessible, low-cost, and uptake has been high –with equally high rates of HIV testing among those on it. Similarly, for those who test positive, antiretroviral therapy (ART) is also government subsidised, with 95% of people diagnosed starting treatment within six weeks.

The study

Using de-identified government dispensing data, the researchers estimated the number of new HIV infections among people who regularly took PrEP between 2018 and 2023, compared to those who only filled one PrEP prescription during this period. They also considered prescriptions for ART, with the assumption that an individual who had been prescribed both PrEP and ART seroconverted during this period. Additionally, they looked at hepatitis C treatment prescriptions.

Each person is assigned a code within the government data; all prescriptions filled are linked to this code, with information including age, the medication and quantity prescribed, indication, date, and their postcode of residence. Some information, such as race, is not included.


During the study period, 66,206 people were dispensed PrEP. Only a small percentage were women (2%), as the main PrEP users in Australia are gay and bisexual men, amongst whom the epidemic is concentrated. The median age of the sample was 33. While 19% received PrEP once only, the majority (53%) had received PrEP more than once, yet the proportion of total days covered by PrEP for this group was less than 60%. A third group (28% of the sample) were more adherent: they received PrEP more than once and had a proportion of total days covered by PrEP of over 60%. 

Of all the people who were dispensed PrEP, there were 207 new HIV infections. People who only received PrEP once comprised 30% of the infections, while the low-adherence group comprised 55% of new infections. As expected, the high adherence group comprised the least new infections at 15%. Most of those who had only received PrEP once were off it for a long time before being prescribed ART – usually over a year.

The overall HIV incidence for the sample was low at 1.07 per 1000 person-years. Incidence was higher for younger people aged 18 to 29, at 1.33 and for those who only received PrEP once, at 2.61.

Incidence was staggeringly high for those who had received hepatitis C treatment during the study period at 9.83 per 1000 person-years. Medland suggested that this is likely to be due to sexual risk behaviour, rather than transmission through injecting drugs.

Compared to those who had only received PrEP once, incidence was 62% lower in the group that used it more than once but had less than 60% of days covered. However, for the more adherent group (with PrEP coverage over 60% of the time), this jumped to HIV incidence being 79% lower.


Medland concluded that while these data are great news for PrEP programmes – they clearly show the benefits of remaining on PrEP – it is important to call attention to specific groups and consider their ‘intersectionality of risk’. These groups include younger people, those who do not continue to take PrEP, and specifically those who have had hepatitis C treatment: they are at a much higher risk of acquiring HIV and are in greater need of the benefits PrEP has to offer.


Medland N et al. HIV Incidence in Users of HIV Preexposure Prophylaxis in Australia: A Whole-of-Population Analysis. Conference on Retroviruses and Opportunistic Infections, Denver, abstract 166, 2024.

View the abstract on the conference website.