HIV update - 29th May 2014

Long-term HIV transmission risks

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 greater than 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.

Cancer

People living with HIV have double the risk of cancer compared to other people, a new study shows. But the increased risk was almost entirely due to higher rates of cancers that are linked to smoking and to viral infections. The risk of other types of cancer was not higher in people with HIV.

This Danish study gives us useful information about cancer rates when people living with HIV have good access to HIV treatment (a similar situation to that in the UK). It’s well known that people living with HIV who have a low CD4 count and are not taking treatment have a much greater risk of cancer. There’s more uncertainty about cancer rates when most people with diagnosed HIV are taking HIV treatment.

The researchers compared around 3500 people living with HIV with almost 13,000 members of the general population.

Overall each year, 84 individuals in every 10,000 people living with HIV had a cancer (0.84%). This was double the rate in the general population, which was 45 in every 10,000 people (0.45%).

Almost half the cancers affecting people living with HIV were linked to viral infections – including anal cancer (linked to human papillomavirus, HPV), Hodgkin’s lymphoma and Kaposi’s sarcoma (each linked to a different type of herpes virus) and liver cancer (linked to hepatitis C). Rates of these cancers were much higher (twelve times higher) in people living with HIV. But the higher rates may partly be explained by differences in behaviour and lifestyles, rather than HIV itself making a person more vulnerable. For example, some of these viruses can be passed on during sex.

Another large group of cancers affecting people with HIV were linked to smoking – cancers of the lung, head and neck. Rates of these were three times higher in people with HIV, even after the researchers took into account whether people smoked or not. One possible explanation is that a number of the HIV-positive smokers were actually very heavy smokers, smoking more than 20 cigarettes a day. Another possibility is that damage to the immune system could make HIV-positive smokers more susceptible to these cancers.

But in relation to ‘other’ cancers – those with no link to smoking or to viral infections – there were no differences in cancer rates according to HIV status. Around a third of the cancers which HIV-positive people had were ‘other’ cancers, but as the rates were exactly the same in the general population, it doesn’t look as if HIV or immune system damage made individuals more vulnerable to cancer. However, this is a scientifically controversial issue and other studies have come to different conclusions.

Pre-exposure prophylaxis

Several recent studies have reported encouraging research findings in relation to pre-exposure prophylaxis (PrEP) in recent weeks. PrEP involves people who don’t have HIV taking anti-HIV drugs in order to prevent infection – the drugs most commonly used are tenofovir, or a combination of tenofovir and emtricitabine (Truvada).

Two weeks ago, the American public health agency, the Centers for Disease Control and Prevention (CDC) announced new guidelines which recommend that PrEP should be considered for a wide range of people. While the CDC’s guidelines aren’t always followed as consistently as the agency would like, this is the strongest endorsement of PrEP so far by an influential health body.

The guidelines recommend that PrEP should be considered for any HIV-negative person who is in an ongoing sexual relationship with a person living with HIV. They also suggest it for gay men who don’t always use condoms, heterosexual people not always using condoms with ‘high risk’ partners and for people who inject drugs.

Critics have expressed a number of concerns about PrEP, including that it will cause side-effects and that there is a risk of drug resistance developing. New studies on these topics are reassuring. It has sometimes been thought that tenofovir can damage the kidneys, but a group of Thai people who took PrEP for five years did not have any long-term problems. And a separate study found that nobody who became HIV positive while taking PrEP had drug-resistant virus.

And another study has confirmed that PrEP works well – as long as people take it regularly, as prescribed. People who took the drug every day had a 90% lower risk of infection.