Viral load explained
Viral load is the amount of HIV in the blood.
A viral load test shows how much of the virus is in the body by measuring how many particles of HIV are in a blood sample. The results are given as the number of copies of HIV per millilitre of blood – for example 200 copies/ml.
Why is viral load significant?
For the past 20 years, evidence has been building up to show that the likelihood of passing on HIV is linked to the amount of the virus in the blood.
The landmark PARTNER 1 study (2014) looked at over 58,000 instances of sex without a condom, where one partner was HIV positive and one was HIV negative. There were zero cases of HIV transmission in couples where the HIV positive partner was on effective treatment (‘undetectable’).
The statistical certainty of the PARTNER 1 study was slightly lower for receptive anal sex with ejaculation than it was for vaginal sex though. The PARTNER 2 study (2018), which looked only at instances of condomless anal sex, showed zero transmissions for both same-sex male couples and heterosexual couples.
With the results of both those studies, scientists have concluded that the chance of any HIV-positive person with an undetectable viral load transmitting the virus to a sexual partner is scientifically equivalent to zero.
Combined studies PARTNER 1 and PARTNER 2, with 2017’s Opposites Attract study, together make up about 126,000 occasions of condomless sex between serodiscordant partners, with no transmissions.
This gives us the robust evidence to say, with confidence, that people on effective HIV treatment can’t pass on the virus.
What does it mean to be undetectable?
HIV medication (antiretroviral treatment, or ART) works by reducing the amount of the virus in the blood to undetectable levels. This means the levels of HIV are so low that the virus cannot be passed on. This is called having an undetectable viral load or being undetectable.
It can take up to six months for some people to become undetectable from when they start treatment.
PARTNER 1 and PARTNER 2 studies provide robust evidence for gay couples and heterosexual couples that the risk of HIV transmission with suppressive ART (undetectable) is effectively zero, which supports the message of the international campaign, U=U (Undetectable = Untransmittable).
What is effective treatment?
We are using the term effective treatment to mean that someone is on treatment, taking it as prescribed and has an undetectable viral load. In the UK this is usually classed as a viral load below 20 copies/ml.
The Partners PrEP study found that there remains a transmission risk within the first six months of treatment as the HIV positive partner’s viral load takes time to come down.
Therefore effective treatment means someone has been taking it as prescribed for at least six months and has an undetectable viral load.
Over 95% of people diagnosed with HIV in the UK are on effective treatment and undetectable, and therefore are not infectious (U=U).
How variable is an undetectable viral load? Can this change over time?
HIV specialists have decades of experience managing antiretroviral therapy and are confident that you can remain uninfectious as long as you:
- take your medication every day as prescribed, and
- have your viral load checked regularly.
The presence of other sexually transmitted infections (STIs) can potentially affect viral load, but in the PARTNER study there were no HIV transmissions even when other STIs were present. Those results held through the PARTNER 2 trial as well.
It is however important to remember that HIV treatment can only be successful if you have access to it and are taking it as prescribed.