Almost zero risk of HIV transmission at low viral loads on ART

People living with HIV who maintain low — but still detectable — levels of the virus and adhere to their antiretroviral regimen have almost zero risk of transmitting it to their sexual partners, according to new analysis published in The Lancet. Findings from the systematic review indicate the risk of sexual transmission of HIV is almost zero at viral loads of less than 1000 copies of the virus per millilitre of blood — commonly referred to as having a suppressed viral load.

Taking daily medicine to treat HIV (antiretroviral therapy, or ART) lowers the amount of the virus in the body, which preserves immune function and reduces morbidity and mortality associated with the virus and helps reduce HIV progression. Without ART, people living with HIV can have a viral load of 30,000 to more than 500,000 copies/mL, depending on the stage of infection. Previous research has shown people living with HIV with viral loads below 200 copies/mL have zero risk of sexually transmitting the virus; however, the risk of transmission at viral loads between 200 and 1000 copies/mL was less well defined.

Researchers from the Global Health Impact Group and the World Health Organization (WHO) filled this knowledge gap by searching databases for all research studies published between January 2000 and November 2022 on sexual transmission of HIV at varying viral loads. In total, eight studies were included in the systematic review, providing data on 7762 serodiscordant couples — in which one partner was living with HIV — across 25 countries.

Of the 323 sexual transmissions of HIV detected across all eight studies, only two involved a partner with a viral load of less than 1000 copies/mL. In both cases, the viral load test was performed at least 50 days before transmission, suggesting individuals’ viral load may have risen in the period following the test. In studies that provided the full range of viral loads in partners with HIV, at least 80% of transmissions involved viral loads greater than 10,000 copies/mL.

“The ultimate goal of antiretroviral therapy for people living with HIV is to maintain undetectable viral loads, which will improve their own health and prevent transmission to their sexual partners and children,” said study co-author Dr Lara Vojnov, from the WHO. “But these new findings are also significant as they indicate that the risk of sexual transmission of HIV at low viral loads is almost zero. This provides a powerful opportunity to help destigmatise HIV, promote the benefits of adhering to antiretroviral therapy, and support people living with HIV.”

While using lab-based plasma sample methods provides the most sensitive viral load test results, such tests are not feasible in many parts of the world. The new findings support the greater use of simpler testing approaches, such as using dried blood spot samples, as they are effective at categorising viral loads for necessary clinical decision-making.

“These findings are important as they indicate that it is extremely rare for people who maintain low levels of HIV to transmit it to their sexual partners,” said lead author Dr Laura Broyles, from the Global Health Impact Group. “Crucially, this conclusion can promote the expansion of alternative viral load testing modalities that are more feasible in resource-limited settings. Improving access to routine viral load testing could ultimately help people with HIV live healthier lives and reduce transmission of the virus.”

The authors acknowledge some limitations to their study. Some of the data analysed were imprecise due to variations across the studies in the definitions of ‘low viral load’, and in the timing and frequency of viral load testing and patient follow-up. Today, HIV treatment is recommended for everyone living with HIV and very large sample sizes would be needed to develop more precise estimates given the extremely low number of transmissions.

Furthermore, the findings do not apply to HIV transmission from mother to child, as the duration and intensity of exposure (during pregnancy, childbirth and breastfeeding) is much higher. Differences also exist in the way the virus is passed from mother to child as compared with sexual transmission. Ensuring pregnant and breastfeeding women have undetectable viral loads throughout the entire exposure period is key to preventing new childhood HIV infections.

The study’s findings were presented at an official satellite session ahead of the 12th International AIDS Society Conference on HIV Science (IAS 2023). A new policy brief from the WHO, published alongside the paper, provides updated sexual transmission prevention and viral load testing guidance based on this analysis, which aims to further prevent the transmission of HIV and ultimately support global efforts to achieve undetectable viral loads.

Image credit: iStock.com/Viktoriia Hnatiuk