A group of American researchers have published an analysis which attempts to estimate the long-term risks of HIV being passed on within a couple.

This was a mathematical modelling study – in other words, it does not report new findings from a study of couples in the real world. Instead, modelling studies use previous research findings, assumptions and mathematical techniques to simulate a sequence of likely future events.

Their findings show that even if the risk of HIV being passed on during one sexual act is relatively small, the risk can accumulate over time for a couple who have sex regularly for a number of years.

The researchers wanted to get a rough idea of the long-term benefits and risks of using different methods to reduce the likelihood of HIV transmission. They were interested in couples where one person is HIV-positive and the other person HIV-negative, who were assumed to have penetrative sex six times a month.

For example, for an HIV-positive woman and HIV-negative man, who only have vaginal sex:

• If no protective measures are taken, 6% risk of transmission after one year, which adds up to 44% after ten years.
• If the negative partner takes pre-exposure prophylaxis (PrEP), 2% risk after one year, 15% after ten years.
• If condoms are used, 1% risk after one year, 11% after ten years.
• If the positive partner takes HIV treatment, 0.2% after one year, 2% after ten years.
• If condoms and PrEP are used, 0.3% after one year, 3% after ten years.
• If condoms and HIV treatment are used, 0.05% after one year, 0.5% after ten years.
• If condoms and PrEP and HIV treatment are used, 0.01% after one year, 0.1% after ten years.

The risks are much greater for couples who practise anal sex, whether they are heterosexual or gay. (The risk of HIV transmission during receptive anal sex is 18 times greaterthan that during vaginal sex).

For a gay male couple:

• If no protective measures are taken, 52% risk after one year, which adds up to 99.9% after ten years.
• If the negative man takes PrEP, 34% risk after one year, 98% after ten years.
• If condoms are used, 13% risk after one year, 76% after ten years.
• If the positive man takes HIV treatment, 3% after one year, 25% after ten years.
• If condoms and PrEP are used, 8% after one year, 59% after ten years.
• If condoms and HIV treatment are used, 1% after one year, 6% after ten years.
• If condoms and PrEP and HIV treatment are used, 0.3% after one year, 3% after ten years.

Some mathematical models that we have reported on in the past could be criticised for being based on unrealistically optimistic assumptions. This one can be criticised for including some rather pessimistic assumptions.

In particular, the estimates for pre-exposure prophylaxis (PrEP) and condoms assume that people who plan to use them don’t always manage to. Both PrEP and condoms are likely to be much more effective when they genuinely are used consistently.

The figures produced are rough estimates and there is much that scientists can debate. Nonetheless, the study brings to light four key points:

• A small risk of transmission in a single act of sex translates into a much larger risk during a sexual relationship that lasts several years.
• The risks of transmission through anal sex, especially in the long term, are much greater than through vaginal sex.
• Current research shows that effective HIV treatment provides more protection than other strategies, including condoms.
• The safest strategy is to use several prevention methods in combination, such as HIV treatment, PrEP and condoms.