People with medium levels of HIV in their blood are likely to contribute most to the spread of the virus, according to new research published in the journal Proceedings of the National Academy of Sciences.
The study, by researchers from Imperial College London, looked at several groups of HIV-positive people in Europe, the USA and sub-Saharan Africa. It found that those with a high viral load are the most infectious group, but have only limited time to infect others, because they generally progress to AIDS quite quickly.
Viral load - a count of how many viral particles are in a person's blood – varies hugely between individuals. The higher the viral load, the more infectious a person is but the shorter their life expectancy. As a result, the study found, those with a high viral load do not contribute the most in the long run to the spread of HIV.
Those with a medium viral load are moderately infectious but remain asymptomatic for a period of about six to eight years before progressing to the symptoms of AIDS. This means they can be unaware that they have HIV for a long period of time, during which they can transmit the virus to a number of different sexual partners, and hence contribute most to the epidemic.
Dr Déirdre Hollingsworth, one of the authors of the paper from the Department of Infectious Disease Epidemiology at Imperial College, said: “Just being highly infectious isn't enough, you have to live long enough to pass the virus on. This long-term view should inform public health policy.”
Despite much recent progress, effective treatment is still not widely available across sub-Saharan Africa, where most infected individuals live. One idea which has been put forward is that treatment should target the most infectious people, with high viral loads, in order to limit transmission. The results of the new study suggest that this would not be an effective plan, as the largest number of new infections is caused by people with medium viral loads.