Increasing HIV testing frequency, and giving everyone antiretroviral therapy (ART), would not in themselves reduce HIV prevalence in US gay men, a mathematical model suggests.
These measures would produce, in the model’s baseline scenario, a 34% reduction in the cumulative number of new infections and a 19% reduction in cumulative deaths by the year 2023. This would lead to the annual number of new HIV infections in gay men almost declining to the annual number of deaths, but not quite.
The model therefore predicts that HIV prevalence would continue to grow in US gay men, albeit very slowly. This remains the situation under a number of different scenarios; even if every gay man took an HIV test every year, and everyone diagnosed with HIV started treatment within six months of infection, infections would still slightly outstrip mortality.
The model also finds that universal treatment would lead to a doubling in the prevalence of multidrug-resistant HIV, although this would not lead to an increase in deaths or progression to AIDS.
However this particular output from the model derives from data on the prevalence of primary HIV drug resistance that is more than seven years old and ‘MDR’ means any resistance to two of the three main classes of HIV drug that were well-established at this point, not resistance to all options currently available.
The model’s assumptions
The model was devised by researchers at the University of Southern California. It includes a number of parameters regarding the gay male population in Los Angeles, such as HIV incidence, the proportion of people in primary infection, the proportion diagnosed, the proportion diagnosed on treatment, and the proportion who progress to AIDS – although it does not directly input a figure for the proportion who have an undetectable viral load (virally suppressed).
It also inputs variable figures for the per-partner risk of HIV transmission in gay men, the frequency of HIV testing in gay men, the adherence rate in people taking ART, and the rate at which people acquire drug resistance.
These figures are all derived from observed trends in HIV infection in Los Angeles gay men between 2000 and 2010.
The researchers perfected their model by testing different combinations of inputs against the observed figures and repeatedly discarding ones that came out with results that didn’t match what actually happened over the previous decade, until they achieved the best fit.
The researchers than tested what would happen to the rates of new infections, the proportion of people with HIV who are not yet diagnosed, the proportion on ART, deaths, progression to AIDS and multidrug resistance, if they increased the frequency of testing and/or reduced the gap between infection and treatment in gay men.
Currently, in Los Angeles, gay men have HIV tests, on average, every 4.4 years. The researchers used the model to find out what would happen if this frequency was increased to every three years, every two years, and every year.
The average time between HIV infection and starting ART in gay men has been calculated as 2.5 years. The researchers modelled what would happen if this was reduced to one year or to six months.
Other figures fed into the model included the cumulative number of new HIV diagnoses, AIDS diagnoses and deaths that would occur by 2023 if nothing changed: 54,000 new infections, 49,000 AIDS diagnoses and 42,000 deaths. The current proportion of people with HIV who are undiagnosed was set at 20%.
The proportion who enter treatment with ‘multidrug-resistant HIV’ (MDR-HIV) was set at 3.1% which would have increased by 2023, even if nothing else changes, to 4.8%. This figure derives from data that was becoming out of date when it was published, and in addition, the model’s definition of ”MDR-HIV’ includes virus that today would be sensitive to a wide range of new drugs. See below for more on the model’s assumptions about MDR-HIV.
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