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HIV treatment and sexual transmission

Roger Pebody
Published: 27 January 2014

If you have HIV, there's a chance that you could pass it on to somebody else. HIV may be present in infectious quantities in blood, semen, vaginal fluids and rectal secretions. Transmission of HIV can happen if one of these fluids gets into somebody else’s body, either directly into the bloodstream, or through unprotected anal or vaginal sex, and very rarely, unprotected oral sex.

Transmission of HIV doesn’t occur each time that a person is exposed to HIV. A number of factors affect the risk of transmission and include:

  • The viral load of the person living with HIV. More virus in the body will increase the risk of transmission.
  • The type of exposure to HIV.  Unprotected anal sex is riskier than unprotected vaginal sex. During anal sex, the receptive partner (bottom) is at greater risk than the insertive partner (top). A small number of HIV infections are due to oral sex performed on a man (fellatio). Ejaculation into the mouth and damage to tissue in the mouth, such as bleeding gums, appear to increase the risk of transmission from fellatio. There are no reliable reports of HIV transmission from oral sex performed on a woman (cunnilingus).
  • The presence of other sexually transmitted infections. These can increase the viral load in the sexual fluids of a person with HIV. They can also cause inflammation or ulcers in the sexual organs of a person who doesn’t have HIV, making it easier for HIV to enter their body.
  • Genetic factors.

Viral load

A person with a high viral load is more infectious – in other words, is more likely to pass on HIV during unprotected sex.

During the first weeks after a person contracts HIV (primary HIV infection), viral load is exceptionally high and there is a considerable risk of passing on HIV during this time. Research suggests that as many as 50% of new HIV transmissions are from people who have themselves only recently contracted HIV.

Viral load is also high when people are ill because of HIV.

However, HIV treatment reduces the amount of HIV in the body. The goal of HIV treatment is an undetectable viral load in the blood, but HIV treatment also reduces the amount of HIV in semen, vaginal fluids and the rectal tissue. When viral load is ‘undetectable’, this means that there is hardly any HIV in a sample of blood.

The preventive benefits of HIV treatment

People with suppressed or undetectable HIV viral loads, especially those on HIV treatment, are much less likely to pass on HIV than people not on treatment.

A large randomised controlled trial called HPTN 052 found that antiretroviral treatment reduced the risk of passing on HIV to a regular partner by 96%. Several observational studies have also shown large reductions in the risk of passing on HIV to a sexual partner.

A position statement by the British HIV Association (BHIVA) and the Expert Advisory Group on AIDS (EAGA) says that successful HIV treatment by the HIV-positive partner is “as effective as consistent condom use” in limiting transmission during vaginal sex.

In relation to anal sex, there is much less research. However, the British experts believe that HIV treatment also lowers the risk of transmission during anal sex.

Both for vaginal and anal sex, the transmission risk will be “extremely low”, provided certain conditions are fulfilled:

  • Neither partner has any sexually transmitted infections. (Regular sexual health check-ups are needed.)
  • The person with HIV has had an undetectable viral load (below 50 copies/ml) for at least six months, including the most recent test.
  • Viral load testing occurs every three to four months (i.e. more regularly than in usual clinical care).

These conditions are similar to those described by Swiss doctors in the so-called ‘Swiss statement’.

The risk is low, not zero

While the risk of HIV transmission is extremely low in the circumstances described above, it cannot be ruled out altogether. Moreover, there are a number of factors which could lead to rises in viral load and so raise the risk of transmission.

For HIV treatment to be effective, all pills need to be taken at the right time, without missing any doses. Also, a person who has been taking HIV treatment for less than six months may not yet have a fully suppressed viral load.

Usually, people who have an undetectable viral load in their blood also have an undetectable viral load in their semen, vaginal fluids and rectal secretions. Occasionally, however, levels may be higher in sexual fluids than in blood. This may be due to a sexually transmitted infection or menstruation. It is also possible that some anti-HIV drugs may suppress HIV less well in the male and female genital tracts and in rectal tissue.

A sexually transmitted infection (in either partner) would raise the risk of transmission, as this can lead to a rise in viral load in sexual fluids or to tissue damage in the genital area.

When meeting casual partners, it’s usually impossible to know whether the other person has a sexually transmitted infection or not (there may be no symptoms).

For a couple, the only reliable way to know that neither partner has an infection is for both partners to have a comprehensive sexual health check-up. If either partner has had sex with a third person since the check-up, it needs to be repeated.

It is worth remembering that HIV treatment does not reduce the risk of the transmission of gonorrhoea, chlamydia, syphilis and other sexually transmitted infections. Condoms remain the most effective way to prevent the spread of other infections.

Getting advice

UK HIV treatment guidelines recommend that healthcare professionals discuss the impact of treatment on sexual transmission with all people living with HIV. An individual who wishes to take treatment in order to protect partners from the risk of HIV infection should be able to do so, even if they have no immediate clinical need for treatment themselves.

HIV-negative people can also get advice from doctors, health advisers or other staff at a sexual health clinic or HIV clinic. If a couple decides to stop using condoms together, it is important to discuss this carefully together, with both partners being comfortable with the decision.

An understanding of the impact of treatment on sexual transmission may be useful for people wishing to have a child. Couples in which one person has HIV and the other does not may consider having unprotected sex on days when the woman is ovulating and at her most fertile.

HIV treatment and sexual transmission

Contact NAM to find out more about the scientific research and information used to produce this factsheet.

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.