HIV spread most by people with medium levels of virus in blood

People with medium levels of HIV in their blood contribute more to the spread of the virus than people with higher viral loads, according to new research from a team of British and Dutch
scientists. The findings have implications for policies aiming to reduce levels of HIV transmission.

The study, which is published in the journal the Proceedings of the National Academy of Science (PNAS), investigates the factors influencing the transmission potential of people with different
levels of the virus in their blood. The transmission potential refers to the average number of people one person can potentially infect before they develop the symptoms of HIV.

People with a high viral load are highly infectious. However, they also have a shorter life expectancy and so have fewer opportunities to infect other people. At the other end of the scale, the
transmission potential of people with low viral loads is limited by low infectiousness.

This leaves people with intermediate levels of the virus in their blood. These people are moderately infectious but can remain asymptomatic for six to eight years before developing the symptoms
of AIDS. During this asymptomatic period they may be unaware that they even have HIV, and could transmit the virus to a number of sexual partners, thereby contributing more to the epidemic than
people with higher viral loads.

‘Just being highly infectious isn’t enough, you have to live long enough to pass the virus on,’ explained Dr Déirdre Hollingsworth of Imperial College London.

The findings have implications for policies designed to reduce virus transmission, particularly in areas where access to treatment is limited. Current thinking is that treatment should be
targeted at people with higher viral loads, as they are more infectious. However, this study suggests that these policies would be misguided, as it is in fact the people with intermediate viral
loads that are responsible for causing the majority of new infections.

Furthermore, people whose loads are reduced from high to intermediate levels by treatment could see their transmission potential increase, as their improved health gives them the opportunity to
infect more people.

‘An intervention that reduces viral loads from high to intermediate levels and is therefore beneficial to the individual may nevertheless increase overall incidence and thus cause more overall
harm than benefit,’ the scientists write in their paper. They suggest that in areas where resources are limited, other criteria such as clinical need, likelihood of treatment adherence and
sexual behaviour should be used to decide treatment protocols.

The majority of HIV positive people who are not on treatment have intermediate viral loads, and the researchers suggest that this could be due to the evolution of the virus, which needs to
achieve the optimal balance between infectiousness and virulence to maximise its chances of being passed on to another host.

‘It’s certainly very striking that the viral loads we see most in nature are just right to make sure the virus gets transmitted as much as it can before it kills its host, which is what you
would expect from evolution,’ commented Dr William Hanage of Imperial College.

The next step for the researchers is to see whether the virus really has adapted in order to allow it to infect the most people.

‘This would have serious implications for public health policy, because if it is true then some strategies to prevent transmission could end up making the virus more virulent by accident,’
warned Imperial College’s Dr Christopher Fraser. ‘While it is too early to sound the alarm, more research to prove or disprove this theory is urgently needed. That is what we are focusing on
now.’

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