Medical professionals are constantly asked to give precise estimates for the likelihood of transmission. Categories like ‘high risk’ and ‘low risk’ can be perceived as evasive, unhelpful and frustrating for people in search of precise, numeric answers.
It isn’t easy for researchers to calculate the risk of transmission from an exposure to HIV through sex. To do this effectively, a group of HIV-negative individuals need to be followed over time and their exposures to HIV—both the number of times they are exposed and the types of exposure.
Researchers ask HIV-negative individuals enrolled in these studies to report how many times they have had sex in a given period of time, what type of sex they had, how often they used condoms and the HIV status of their partner(s). Because a person may have trouble remembering their sexual behaviour or may not want to tell the whole truth, this reporting is often inaccurate. Due to the difficulties of calculating this risk, these studies have produced a wide range of numbers. To come up with a more accurate estimate for each type of unprotected sex, some researchers have combined the results of individual studies into what is known as a meta-analysis.
In 2014 a study was published by Centres for Disease Control and Prevention researchers in the US which gave updated estimates of the risk of HIV acquisition from various types of exposure, including sexual, parenteral (for example, accidental needle-sticks or sharing injection drug equipment), and vertical (mother-to-child). They also looked at the protective effect of various prevention strategies. After looking at over 15 000 studies, 14 studies were found to have relevant data.
Not surprisingly, blood transfusions posed the greatest risk, at 9 250 transmissions per 10 000 exposures. The next highest risk was mother-to-child transmission, at 2 255 per 10 000 exposures when HIV-positive pregnant women and their newborns do not receive preventive anti-retrovirals.
Sharing needles for injecting drugs had a risk of 63 per 10 000 exposures, while the risk from needle-sticks was 23 per 10 000.
Next week, read Part 2 which describes the caution that is needed when interpreting these numbers. (Remixed from betablog.org and www.catie.ca)