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

People in medical care only transmit one in twelve cases

The majority of HIV in the USA is transmitted by people who know they have HIV but are not in care, a study recently published in the Journal of the American Medical Association shows.

The study also found that being virally suppressed equated to a 94% reduction in the likelihood of transmission compared to people who were undiagnosed. However, it found that even in people who were not taking antiretroviral therapy (ART), simply being retained in medical care was associated with a 60% reduction in the likelihood of transmitting HIV.

Another study found that about one in four to one in five people with HIV in medical care in the US had a viral load above 1500 copies/ml, indicating they could potentially infect another person.

The study

The first study was a mathematical model. It combined data from the US’s National HIV Surveillance System and its Medical Monitoring Project to estimate the proportions of people with HIV who were diagnosed; diagnosed and attending regular medical care; attending medical care and on ART; and on ART and virally suppressed.


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.

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.

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.


A tentative explanation for an observation, phenomenon, or scientific problem. The purpose of a research study is to test whether the hypothesis is true or not.

mathematical models

A range of complex mathematical techniques which aim to simulate a sequence of likely future events, in order to estimate the impact of a health intervention or the spread of an infection.

There are an estimated 1,148,200 people with HIV in the US right now. Of these, the researchers estimated, 82% are diagnosed; 37% diagnosed and attend medical care regularly; 32.5% in care and on ART; and 25% on ART and virally suppressed. This is very similar to previous estimates.

It then estimated the likelihood of each person in this total population transmitting HIV. This was based on data from the Medical Monitoring Project and from a third data source, the National HIV Behavioural Surveillance System, which conducts 5-yearly cycles of random sampling of groups of people with HIV to ask about sexual and injecting risk behaviour.

It combined all these data to calculate the likelihood of any one individual in the database transmitting HIV.

Results in detail

The overall transmission rate was 3.9 transmissions per 100 individuals per year (100 PYs). The rate was highest in people with undiagnosed HIV, 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.

Risk behaviour and care status

One very interesting finding of the model was that people’s risk behaviour fell dramatically once they were put on ART. This contradicts the hypothesis that people on ART, and especially those virally suppressed, might increase their risk behaviour because they feel safer or are healthier.

The reverse appeared to be the case: 16% of people on ART and 10.5% of those on ART and virally suppressed reported unprotected sex with HIV-negative or unknown-status partners in the last year, compared to 51% of those who knew they had HIV but were not in care and 62% of the undiagnosed.

In terms of other risks, those who were diagnosed but not in care actually had more total sexual partners in the previous year (8.8) than the undiagnosed (8.0). This compares with three partners in people in care and two in those on ART.

People who were diagnosed but not in care had double the likelihood of injecting drugs and multi-person use of injecting equipment than the undiagnosed: 12.2% versus 6.3% of the undiagnosed – and only 0.9% of those in medical care. This may be due to a combination of factors: people who inject drugs are more likely to get tested for HIV than others because of drug treatment programmes, but in the US those not in regular medical care may have difficulty accessing sterile injecting equipment, especially as a fairly high proportion of this population may be in prison.

It is important to emphasise that this study is only a model and that errors in surveillance and recording will produce inaccurate outputs. The researchers particularly examine the most salient fact in the whole study – the huge proportion of people in the US who are diagnosed but not in care. The National HIV Behavioural Surveillance System counts someone as in regular care if they visit a medical care provider at least once in the first four months of the year, so could miss a number of visits; and, as the researchers point out, “individuals can cycle in and out of care and on and off ART”.

Nonetheless, the fact that nearly half (45%) of people diagnosed with HIV in the US are not in regular medical care is in stark contrast to countries like the UK, where an estimated 95% of people diagnosed with HIV have been in regular care for more than a year and 88% are on ART.

About 23% of people in HIV care may be infectious at any one point

This study estimates that 31% of HIV-positive people in care in the US may have a detectable viral load (on or off ART) at any one time. But detectable may not mean infectious, or only marginally so, if the viral load is only a few hundred copies/ml.

Another recent paper estimated the proportion of US HIV-positive patients in care who had a viral load above 1500 copies/ml – conventionally regarded as the threshold for infectiousness (a figure based on a 15-year-old cohort study in Uganda, where no-one with a viral load below 1500 copes/ml was observed to transmit HIV).

This study looked at a specific group of patients at six HIV clinics over five years from April 2009 to March 2014. Of these, 90% were either already on ART at the study start or were prescribed it during the study period (though did not necessarily stay on it).

It found that, on average, patients spent 23% of their time with viral loads over 1500 copies/ml. Just being on ART at the start of the study period did not make much difference to time spent with a viral load over 1500 copies/ml – these patients spent 21% of their time with a viral load over 1500 copies/ml. However, patients who already had a viral load below 1500 copies/ml at the start of the study period only spent 10% of their time with a viral load over 1500 copies/ml over the whole five years.

Although a UK study with the same methodology as the US one has not been done, UK figures are roughly comparable. In 2012, it was estimated that 14,600 UK patients out of 74,500 who were diagnosed and had attended at least one medical appointment post-diagnosis were not virally suppressed – about 20%. This would include patients attending regularly but not on ART, patients on ART but not virally suppressed, and patients who attended once or twice but were then lost to care.

Again, however, the 80% of UK patients who are virally suppressed in care represent a far higher proportion of the total number of people with HIV in the UK than the US 77% does of the US HIV-positive population, simply because so few diagnosed UK patients are not in care.


Skarbinski J et al. Human immunodeficiency virus transmission at each step of the care continuum in the United States. JAMA Intern Med.175(4):588-596. 2015.

Marks G et al. Time above 1500 copies: a viral load measure for assessing transmission risk of HIV-positive patients in care. AIDS 29: early online publication. 2015.