Six in ten US HIV infections are transmitted by people who know they have HIV but are not in care

The overall transmission rate was 3.9 transmissions per 100 individuals per year (100 PYs). The rate was highest in the undiagnosed, with 6.6 transmissions per 100 PYs. Just testing positive was associated with a 19% reduction in the risk of transmission: the transmission rate among people who knew their status but were not in care was 5.3 per 100 PYs.

This meant that, according to the model, the 18% of HIV positive people who are undiagnosed are the source of 30% of HIV infections, and the 45% who are diagnosed, but not attending medical care regularly, are the source of 61% of infections.

So the 37% of patients who are in regular medical care, in short, are the source of just 9% of infections. Just being seen regularly by a physician halved a person’s likelihood of transmitting HIV, to 2.6 infections per 100 PYs. But in fact most people who are in regular medical care in the US are on ART, so these only form a small group of the HIV-positive population (about 4%).

People on ART but not virally suppressed (8% of the HIV-positive population) transmitted 1.8 infections per 100 PYs and people who were virally suppressed at their last test - 25% of the HIV-positive population - transmitted 0.4 infections per 100 PYs. Because there were more virally suppressed than unsuppressed people though, they hardly transmitted any fewer infections: about 2.5% of the total number of infections compared with 3.3% from those on ART but not virally suppressed.

Compared with the undiagnosed, people in regular medical care were 61% less likely to transmit HIV, people on ART but not virally suppressed 73% less likely and people on ART and virally suppressed 94% less likely.

The model was able to estimate the ‘efficacy’ of different steps in the cascade compared with the previous step. Thus, simply retaining people in care more than halved their likelihood of transmitting compared to not being in care. Putting those people on ART cut that likelihood by 30% more. And achieving viral suppression in people on ART cut that likelihood by a further 78%.

The researchers estimated that 58% of HIV was spread through sex between men and while people aged 35 to 44 accounted for the most HIV transmissions – 32% of the total – people aged 25-34 had the highest transmission rate (6.2 transmissions per 100 PYs), because there were fewer of them.