Explainer: What is the actual risk of contracting HIV?

November 22, 2015

Image: Getty. By Allen Cheng, Monash University.

Charlie Sheen’s recent announcement that he has HIV has raised concerns about whether he may have transmitted the infection to other people.

While we obviously don’t know his specific medical details (and really, it’s none of our business), HIV treatment is effective at reducing transmission. This is known as the “treatment as prevention” strategy.

There have been many advances in HIV care since Rock Hudson was diagnosed in the 1980s. At this time, the prognosis of AIDS was similar to patients with advanced cancer.

Since the mid-1990s, combinations of antivirals have markedly improved the survival of people living with HIV. Modern combinations are simple to take (as few as one tablet, once a day) and associated with much fewer side effects than older drugs.

Charlie Sheen has revealed he is HIV positive. Image via Getty.

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The life expectancies of people living with HIV are now comparable to the general population, both in first world countries and developing countries.

Antiviral drugs work by interfering with the replication of HIV. This results in a drop in the viral load – the concentration of virus detectable in blood.

This doesn’t mean, however, that the virus is eradicated. HIV can hide in sanctuary sites in the body (known as “latency”) and will quickly become detectable if the antivirals are stopped. In many people taking treatment, the concentration of virus is often undetectable, but this doesn’t exclude the possibility of very small amounts of circulating virus that are below the limit of the tests that detect them.

In the era before treatment, it was estimated that the risk of acquiring HIV was around one in 200 sexual encounters, which varied by the type of act and other factors.

There are now two new effective strategies to reduce this risk and prevent HIV infection: treating the person with HIV to suppress their viral load; and giving antivirals to the uninfected partner (“prophylaxis”) to stop them becoming infected. (Post continues after gallery.)

Popular forms of birth control

CondomsThe male condom is a barrier contraception that prevents both pregnancy and the transmission of STDs. They work by collecting semen so it doesn't enter theman's partner's body. Condoms are 95 to 99% effective at preventing pregnancy.

DiaphragmLike the male condom, the diaphragm is a barrier method of contraception. It's a shallow silicone dome that's inserted into the vagina andcovers the cervix, which is the entrance to the uterus. It's held in place by the pelvic muscles, and works by preventing sperm from entering the uterus. After sex, diaphragms need to remain in placefor another six hours, in which time the sperm inside the vagina die. This method is 88 to 94% effective at preventing pregnancy, although this rate increases with experience.

Contraceptive pillThe Pillis the most popular form of birth control. There are two types: the combined pill, containing two hormones, and theprogestogen-only, or mini, pill, which only contains one female hormone. The combined pill works by preventing the ovary from releasing an egg each month. The mini pill changes the mucus at the entrance of the uterus, which stops sperm from entering. Both are 99.7% effective; however, they don't offer protection from STDs.

Intrauterine deviceIUDs are small, T-shaped devices that are inserted into the uterus by a doctor in order to prevent pregnancy. There are two types of IUD: copper, which affects the movement and survival of sperm in the womb while changing the lining of the womb;and hormonal, which releases progestogen (a synthetic version of the naturally-occurring hormone progesterone) into the uterus. Although IUDs have a success rate of more than 99%, only around 2% of Australian women currently use one.

The female condomThe female condom is a pre-lubricated sheath, which loosely fits inside of the vagina. The condom has a removable ring, which helps to insert the sheath and keep the condom in place. A larger ring remains on the outside to cover the opening of the vagina. They are designed to fit all women and can only to be used once. Due to misuse of the product it is only 79 per cent effective.

Birth control implantThis is a 4cm-longimplant that is inserted into a woman's upper arm, just under the skin, by a doctor. It releases a hormone called etonogestrel, which prevents ovulation and thickens the mucus in the cervix, which can stop sperm from entering. Contraceptive implants - usually sold as Implanon - are removed under anaesthetic every three years, and are close to 100% effective.

Vaginal ringVaginal rings are soft plastic rings that contain the same two hormones as the combined pill - oestrogen and progestogen. The ring is inserted into the vagina, where the hormones are absorbed into the body and then prevent the ovaries from releasing eggs. Once inside the ring can't be felt, and it needs replacing every three weeks (with one week's break in between). When used correctly, this method is more than 99% effective.

Depo ProveraDepo is a long lasting hormonal injection. The synthetic form of progesterone is administered every 12 to 14 weeks to woman, preventing pregnancy within this ongoing time period. The injection, which is given in the buttock or muscle of the upper arm, prevents ovulation and thickens the mucus in the cervix, helping to block sperm. The injections are 99 per cent effective but do not protect against STDs. Male contraceptive injections are currently being trialled

The morning after pillEmergency contraception (popularly referred to as the morning after pill) is used to prevent a pregnancy following unprotected sex. Other reasons include missing a contraceptive pill or a condom breaking. It is common in Australia to use the progestogen-only method. This entails a dosage of EC pills containing progestogen or the synthetic version. A doctor can prescribe the pill or they can be bought over the counter at a pharmacy without a prescription. Emergency contraceptionis 85 per cent effective in reducing the risk of pregnancy.

SterilisationSterilisation is a permanent form of contraception. For women there are two types of sterilisation: tubal ligation, in which clips are placed on the fallopian tubes, or tubal occlusion in which a device is inserted into the tubes. These methods work by preventing the sperm from reaching the egg. Tubal litigation is 99.5 per cent effective. The Vasectomy (for men) works by cutting and blocking the tubes called the vas deferens, preventing sperm from leaving the testicles. The chances of impregnation with a vasectomy are 1 in 1000.

It has long been known that the risk of transmission is related to the amount of virus present in the body. A number of trials now show that treatment of people with HIV dramatically reduces the risk of transmission.

A trial of 1,763 couples where one partner was HIV positive and the other was HIV negative recorded only one transmission in the group on antiviral treatment compared to 27 who were not on treatment.

Two other ongoing studies have reported no transmissions in more than 1,300 couples where the HIV-positive partner was successfully treated with antivirals.

It should be noted that the participants of these trials were given the usual advice to prevent HIV transmission in addition to antiviral treatment, including the use of condoms. The initial trial primarily included heterosexual couples, but the subsequent studies have extended the evidence to gay men.

For the uninfected partner, post-exposure prophylaxis (taking antivirals like a “morning after pill”) or pre-exposure prophylaxis (taking antivirals continuously like the oral contraceptive) are also effective options. (Post continues after gallery.)

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In those taking preventive antivirals, the protection against HIV infection appears to be near complete. But as with the oral contraceptive, antivirals only work if they are taken.

The evidence suggests both strategies are highly effective at preventing HIV infection. However, antivirals do have some potential side effects. And in many countries, including Australia, they are not yet approved for prophylaxis.

They also do not protect against other sexually transmitted infections (such as syphilis or gonorrhoea) and need to be combined with other risk-reduction strategies.

Both “treatment as prevention” and pre-exposure prophylaxis rely on the individual knowing his or her HIV status. This reinforces the importance of diagnosing HIV, so that treatment can be offered both for the benefit of the person with HIV, as well as for their uninfected partners.

Like the prevention of pregnancy, there are many potential options to prevent HIV and these should be carefully considered with the person’s treating doctor. But “treatment as prevention” and pre-exposure prophylaxis are clearly major advances in our goal to eliminate HIV transmission.

Allen Cheng, Professor in Infectious Diseases Epidemiology, Monash University. This article was originally published on The Conversation. Read the original article.

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