Thanks for your query. The question addresses some complex issues so we’ve asked our senior researcher Peter Saxton to provide a response below. We’re also assuming that your question relates to anal sex between males; if this isn’t the case please let us know since HIV transmission risk is heavily influenced by the type of sex occurring e.g. anal sex vs vaginal sex.

Your question has several parts to it so they’ll be addressed one at a time.

First of all, from your posting it looks like you’re aware that oral sex is considered low risk for HIV. In fact, receptive oral sex is estimated to have a per-episode risk of HIV transmission of approximately 1 in 10,000, a risk that has been determined in NZ courts to not present a “significant risk” of HIV transmission. The reasons for this are important to clarify. Mainly, this is because the mouth does not contain high concentrations of cells that are vulnerable to HIV infection, nor does it contain many cells that harbour HIV virus. Also, saliva possesses protective qualities that help keep viruses and bacteria out of the body, in ways that anal mucus doesn’t. This is an important difference that explains why receptive oral sex is so much less risky than receptive anal sex as far as HIV is concerned – if someone ejaculates HIV infected semen into an anus, the risks for the HIV negative receptive man are far greater than if HIV infected semen is ejaculated into a mouth. The normally very low risk of oral sex is of course increased if the body’s immune system in the mouth is compromised, for example if you have bleeding gums, have recently had dental work, or if you have an ulcer. Even then, the risk from anal sex is still greater.

Second, unprotected anal sex is the most efficient way of sexually acquiring HIV. Again, the reason is due to the different biology of the anus. The anus has a very high concentration of cells that are both vulnerable to HIV infection, and which also harbour HIV virus. Sexually transmitted infections (STIs) such as syphilis, herpes, gonorrhoea and anal warts that are present in the anus can substantially heighten the risk of HIV acquisition and transmission, and many of these infections may go unnoticed and/or unchecked. In addition, a person with HIV is much more infectious in the early stages of infection, and when they have other STIs in addition to HIV (which may or may not be noticed/treated). Due to these issues and other biological and genetic factors, although the per contact risk of HIV transmission for unprotected receptive anal sex with an HIV positive man has recently been estimated at 1 in 70 episodes (and presents a “significant risk” of transmission in NZ law), in that study 12 of the 46 men who contracted HIV had experienced fewer than 10 episodes of unprotected receptive anal sex. It is still not well understood why some men become infected very quickly, and for others it takes many episodes. The probability of HIV transmission through unprotected anal sex can therefore be described as both high and variable. Receptive anal sex is also more risky than insertive anal sex, but both carry considerable transmission risks. Condoms effectively reduce this high biological risk by physically interrupting the way HIV is transmitted in the anus. By reducing the per contact risk to “below significant”, a condom also removes any legal risk (i.e. a man who knows he has HIV cannot be prosecuted for having anal sex if a condom is used).

Thirdly, you asked what effect an undetectable viral load might have on transmission risks. Four things are important to emphasise here:

(a) the term “undetectable” merely describes the limits of the viral load tests currently available. HIV is still present, albeit at levels that are below what the current test can detect. The phrase “undetectable” is increasingly being replaced with “low viral load” in order to more accurately communicate that HIV is not absent.

(b) viral load tests are only conducted a few times a year, and in between tests HIV levels can increase (“blip”) for different reasons – sometimes because the HIV positive individual is fighting off another infection (e.g. a cold or an STI) , sometimes for reasons unknown.

(c) standard viral load tests measure the quantity of HIV in blood, to help inform HIV treatment practices. However, it does not measure the quantity of HIV levels in semen, or in anal mucus, which are the main body fluids implicated in sexual practices. This is important because HIV levels in semen and in the anal mucus can differ from those measured in blood, meaning that a recent low blood viral load result may not mean that a person has low HIV viral load in semen or anal mucus. Unfortunately for gay and bisexual men, very little research has investigated HIV viral load in anal mucus, so it is difficult to say precisely in what ways it is different to HIV in semen or blood.

(d) while lower blood HIV viral loads can mean that a person is less infectious than someone with higher blood HIV viral loads, there is therefore mounting evidence that the last blood viral load test may not be a good indicator of current sexual infectiousness. Individuals with low blood viral load at their last test have transmitted HIV to sexual partners, even when that person has been adherent to HIV treatments, and in the absence of STIs.

For these reasons, it is still possible to contract HIV through unprotected anal sex with an HIV positive individual who has had a recent “undetectable” blood viral load test. For a one-off sexual encounter, the risk would be very difficult to quantify, because of the many variables at play – both known and unknown to both partners. Our practical recommendation is therefore to regard all unprotected anal sex as riskier than other sexual practices. It is also important to emphasise that condoms work very well to prevent HIV transmission through anal sex, and help protect both partner’s health in many other ways, while at the same time enabling partners to continue to enjoy having anal sex.

A short review of HIV treatments and their impact on sexual risk for men has recently been published online here which you might also find useful.

We hope this was helpful and answered your concerns, please let us know if you’d like to discuss this more.

- NZAF official response

We'd advise waiting until your treatment is finished. Talk to whoever prescribed your medication or your chemist or GP if you've any questions about your treatment.

Some NZAF Regional Centres offer a full STI screening service, as well as the HIV and syphilis FASTest. Contact us for details on your nearest centre. You can also contact your nearest sexual health clinic.

(NZAF note - this answer was edited on 24 May 2010.)

- NZAF official response

A HIV test doesn't test for the virus in your blood - it tests for antibodies for HIV. It is not possible to tell if somebody has contracted HIV immediately, as sometimes these antibodies can take up to three months to appear in your blood. This is called the window period.

If you're worried about your HIV status, it's important to talk to someone. Your nearest NZAF Centre can talk with you about all the issues you face, and talk to you about testing. The HIV test the NZAF uses is called the FASTest, and gives results within twenty minutes (although we ask you to take an hour out of your schedule for the appointment).

It's really important to use condoms and lube every time you have sex - they're the best way to protect yourself and your partner from the HIV.

- NZAF official response

You can get free condoms from the NZAF.

- NZAF official response

It's important that you use condoms correctly, to make sure they don't break. Firstly, before you put on a condom you should make sure it's use-by date hasn't passed. Condoms should never be stored near a source of heat, as this can weaken the latex. When you're using a condom, you should always use water-based lube. Oil-based lube (for instance Vaseline or lotion) can weaken the latex used to make

HIV can be present in the mucous lining of the anus; if he’s not wearing a condom, the virus can enter a man’s body through the end of his penis or the inner folds of his foreskin. If you're concerned about any sexual act, it's best to contact your nearest NZAF Centre to book a test.

- NZAF official response