What is undetectable?

  • Modern anti-HIV drugs mean that most people with HIV on treatment have a very low or undetectable viral load. 95% of people in the UK on HIV treatment are undetectable (1).
  • Someone with an undetectable viral load is very unlikely to infect their sexual partners
  • Someone with an undetectable viral load still has HIV. If they stop taking treatment their viral load will become detectable again (and the risk of transmission will increase)
  • The recent PARTNER study [8] monitored over 1,000 sero-discordant couples (i.e. where one had HIV and the other did not). After more than 58,000 acts of unprotected sex, no one with an undetectable viral load, gay or heterosexual, who took part in the study transmitted HIV to their partners.

Does being undetectable mean safer sex?

All studies suggest that someone who is undetectable on treatment is not infectious.

The PARTNER study found no cases of HIV transmission from an HIV-positive partner on treatment whose viral load was low enough to be considered undetectable. This remained the case even when other STIs were present.

Researchers are cautious about saying that any prevention method is 100% effective however the weight of evidence that undetectable means uninfectious is growing. There is less risk of HIV transmission if someone is undetectable than if they are detectable and use condoms and lube.

People do not become undetectable immediately they go on treatment and viral load may fluctuate, particularly early on. For this reason, the recommendation is that someone should be undetectable for six months before expecting this to mean uninfectious.

Being undetectable for HIV does not mean that someone won’t pass on other STIs. Using condoms and lube will prevent transmission of a wide range of infections.

What is viral load and how does it impact risk?

  • Viral load refers to the amount of HIV a person has in their blood, cum and anal mucus 
  • The lower the viral load, the less infectious the HIV-positive person will be 
  • People who have been diagnosed with HIV are encouraged to have regular blood tests
  • Looking at viral load in the blood helps doctors monitor someone’s progress, e.g. to see if the drugs are working for him
  • If HIV treatment is successful, someone's viral load will become so low that it will be undetectable. 
  • Someone who has not been diagnosed or who is not on treatment is likely to have a higher viral load than someone who is on treatment.

  • Viral load is particularly high just after someone has been infected with HIV and so if someone has only recently been infected, they will be more likely to pass the virus on to their sexual partners if they have unprotected sex.




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(1) PHE: HIV in the UK – Situation Report 2015 Incidence, prevalence and prevention)
(2) Zuckerman RA, Whittington WLH, Celum CL, Collis TK, Lucchetti AJ, Sanchez JL, Hughes JP, Sanchez JL, Coombs RW. Higher concentrations of HIV RNA in rectal mucosa secretions than in blood and seminal plasma, among men who have sex with men, independent of antiretroviral therapy. Journal of Infectious Diseases, 2004;189:156-161.
(3) Rodger A et al. HIV transmission risk through condomless sex if HIV+ partner on suppressive ART: PARTNER study. 21st Conference on Retroviruses and Opportunistic Infections, Boston, abstract 153LB, 2014.