What Viral Load is Uninfectious?
posted: 23/11/2009
A new study has found more evidence that people with low viral loads are less infectious.
US researchers found that very few HIV transmissions come from people with a viral load below 1000 copies/ml. The rate of transmission more than doubles when the viral load reaches 10,000 copies/ml.
There’s been a lot of debate about HIV treatment, viral load and infectiousness. Evidence is mounting up to show that successful HIV treatment significantly reduces infectiousness.
'The Partners' study recruited 3408 people and monitored HIV sero-discordant (= mixed HIV status) couples every three months to look for new HIV infections and to measure viral load in the HIV-positive partner. If HIV infection was found in previously HIV-negative partners, viral sequencing of both partners was carried out to check that the partner was the source of HIV. This analysis revealed 108 linked infections, people who had infected their partners.
Viral load logs
Viral load figures are often given on a logarithmic scale, because viral load rises so very steeply. A ‘one log increase’ means that viral load has increased by ten times. A viral load of 10,000 copies/ml is therefore one log higher than a viral load of 1000 copies/ml. Similarly, a viral load of 1,000,000 copies/ml is one log higher than a viral load of 10,000 copies/ml, and so on.
Transmission halves as log falls
Researchers found that a fall in viral load of 0.74 log reduced the risk of HIV transmission by 50%.
It’s hoped that the findings of this study will assist further research into the use of HIV treatment in prevention.
Viral load is not the only thing which affects whether HIV is transmitted. If either partner has any sexually transmitted infection (often these show no symptoms), these STIs make HIV transmission far more likely, even with a low viral load. Recent Swiss, German and French statements have broadly confirmed that an undetectable viral load usually means HIV cannot be transmitted. But where there are STIs, HIV transmission becomes likely.
Viral Load information
There is more information on viral load in NAM’s new patient information booklet CD4, viral load & other tests. You can download it here, or order it free from NAM's online bookshop.
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