Diagrams of the ‘treatment cascade’ provide a useful way of visualising problems with the implementation of ‘treatment as prevention’ in specific contexts, and of identifying reasons why HIV treatment may not have as great an impact as could be hoped.

In most parts of the world, large gaps exist between the number of people who have HIV, those who know that they have HIV, those attending medical services, and those receiving effective treatment. For example, the treatment cascade for the United States shows that just 28% of those living with HIV have a suppressed viral load. This means that treatment may only have a limited impact on the American epidemic, except in places where barriers to accessing medical care have been removed.

The equivalent diagram for the UK is more encouraging. Around 76,800 adults were living with diagnosed HIV in 2012, of whom 88% (67,600) were receiving antiretroviral treatment. Between 71% (54,800) and 78% (59,900) of adults with diagnosed HIV had an undetectable viral load (<50 copies/ml).

We also know that 97% of people newly diagnosed with HIV were connected with specialist care within three months, and 95% of people who attended during one year were retained in care the following year. Few other countries have comparable results.

Furthermore, there is equality in these results, with similar figures in people of different ages, ethnicities, genders, exposure groups and geographical regions. One exception is that younger people are less likely to take treatment than older people.

But the UK has low rates of HIV testing compared to countries such as France, Australia and the United States. In the UK, one-in-five people who have HIV are undiagnosed and half of all diagnoses are made late: in other words, when HIV treatment is already needed (a CD4 cell count below 350 cells/mm³). Late diagnosis is especially common in heterosexual people and in older people.

As a result, only six in ten of those with HIV have an undetectable viral load. It is clear that for treatment as prevention to achieve its potential in the UK, a priority is for interventions targeting the first step in the treatment cascade – in other words, programmes which reduce the number of people with undiagnosed HIV.

In African countries, the United States and elsewhere, health systems are often so dysfunctional that there is considerable scope to improve the numbers of people who attend medical care and receive HIV treatment. In contrast, HIV care in the UK is already of very high quality. Specifically, there are few bureaucratic or financial barriers to accessing HIV clinics in the NHS, including by people of uncertain immigration status. However, if NHS policies were changed, this could have a negative impact on engagement with care.

Moreover, while overall levels of linkage to care, retention and adherence to therapy are good, some individuals do drop out of care, attend irregularly or have problems taking their medications as prescribed. High quality, personalised support may be needed.

Finally, some individuals living with HIV may not be aware of the preventive benefits of treatment and of BHIVA’s recommendation that any patient wishing to take HIV treatment for that reason may do so. More information for people with HIV and their partners could be provided.

In the UK, there are far more individuals with undiagnosed HIV than individuals with diagnosed HIV who are not taking antiretroviral therapy. One analysis estimated that, in 2010, there were 14,000 HIV-positive MSM (men who have sex with men) with a viral load above 1500 copies/ml. Within this group of ‘infectious’ men:

• 62% (8700 men) were undiagnosed
• 5% (700 men) were on treatment which was not yet fully effective
• 16% (2300 men) were not on treatment and had a CD4 cell count above 500 cells/mm³
• 12% (1600 men) were not on treatment and had a CD4 cell count between 350 and 500 cells/mm³   
• 5% (700 men) were not on treatment and had a CD4 cell count below 350 cells/mm³

Taken together, these men amount to 35% of all HIV-positive MSM in the country. The researchers examined which strategies would be most effective in reducing the size of this group. Getting all diagnosed men with a CD4 cell count below 500 onto treatment would reduce the proportion to 29%, while halving the number of undiagnosed men through increased HIV testing would reduce the proportion to 27%. It would be more effective to combine both approaches – bringing the proportion down from 35 to 21%.