New science of transmission

Treatment as prevention

Recent studies provide clear and unmistakable evidence that a person with HIV on effective treatment, which means viral load under 200 copies/ml in blood, cannot sexually transmit HIV.

In four major studies, with 4,244 heterosexual and homosexual couples having over 146,000 sexual encounters where one partner had HIV and the other did not, there were zero transmissions from positive people with suppressed viral load. There were no HIV transmissions in over 4,000 couple-years of follow-up.

The technical name for this approach is ‘treatment as prevention’ (TasP). It is widely known as undetectable = untransmittable (U=U), a pithy and powerful statement popularised by the Prevention Access Coalition.

Note: the technical threshold for undetectability is lower in Australia — around 20 copies per millilitre — and this may decrease further as testing equipment improves. When we are talking about U=U, ‘undetectable’ means under 200 copies/ml, just like in the studies.

As we covered in the previous lesson, HIV treatment works by ‘gumming up’ the ability of HIV to replicate (make new copies of the virus in our cells). This reduces the viral load in blood and other body fluids including semen and the anal and vaginal mucosa (the moist skin lining the anus and vagina). Eventually there is not enough virus in these body fluids to transmit a new infection via sex.

It is not yet clear whether undetectable viral load in blood protects against transmission through injecting drug use and breast- or chest-feeding. More studies are needed to answer these questions.

In order to rely on U=U as their sole prevention method, guidelines say that undetectable status must have been sustained for at least six months. Occasionally, viral load may become detectable again. If this is a ‘blip’ there is no reason to stop relying on U=U. Another prevention method must be used if the viral load remains detectable when re-tested, as this could indicate the emergence of drug resistance.

Pre-exposure prophylaxis

Studies have also shown a strong protective benefit when HIV-negative people take a two-drug combination of HIV medications, known as pre-exposure prophylaxis (‘PrEP’). When people take the medication as directed, the effectiveness is higher than 90%. Indeed, in the iPrEx study, nobody who took at least four tablets per week became infected with HIV.

There are two ways of taking the medication: either a daily pill, or event-based dosing (before and after sex). HIV-negative people can learn a lot from their positive friends and partners about managing adherence and coping with side effects (usually limited, on start-up). Again, this works by stopping HIV from replicating, but this time, it works in the cells of the person exposed to virus during a sexual encounter.


Condoms remain an effective and popular way of preventing HIV transmission when used as directed. Many people continue to use condoms because they are familiar with this approach, it is easy to access and it is simple to verify the sex is protected. Condoms also offer protection against bacterial STIs like gonorrhea and chlamydia, which can be transmitted between a penis and the throat, anus or vagina.

Risk reduction strategies

Not everyone likes using condoms. Prior to U=U and PrEP, some gay and bisexual men adopted strategies, like pulling out before ejaculation, or having positive partners take the receptive role during intercourse. These were called ‘risk reduction’ strategies because they reduced the risk of transmission but were not fully protective against HIV transmission.

If someone is using risk reduction strategies in the absence of a fully protective method like U=U, PrEP or condoms, they should be immediately encouraged to adopt one of these fully protective methods. A person who cannot or does not use a fully protective method may need to be referred to your local department of health for management as a person who places others at risk of HIV acquisition.

Not having anal or vaginal sex

Masturbation, frottage and oral sex present no risk of HIV transmission and can be highly enjoyable and fulfilling. By accident, our focus on penetrative sex in HIV prevention campaigns has created or contributed to the notion that only penetration is ‘real’ sex! But these alternatives to penetration are equally valid. They form a major part of the sexual repertoire of sex workers — who are the acknowledged experts in giving sexual pleasure.

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