Viral load may be more important than condom use.
Viral load determines how infectious bodily fluids are. Levels are highest in someone who is recently infected (up to 40 million copies in a millilitre of blood). By comparison, someone on treatment with an undetectable viral load has less than 50 copies/mL (see Figures 2 and 3 below).
When viral load is very high, it increases the risk from low risk activities like giving oral sex.
The risk of heterosexual transmission from vaginal sex without a condom is dramatically reduced when an HIV positive person has an undetectable viral load.
One study reported the risk to drop by 92%  and another by 96%. . But the single transmission in each study occurred when the positive partner had only just started treatment when viral load was still high. This means that having an undetectable viral load might reduce risks by more than 96%.
Some researchers think that the risk with an undetectable viral load might gets close to 100% protection. For more details see the footnote below about the PARTNER sutdy. [4, 5, 6]
When viral load is undetectable in blood, there are less than 50 copies/mL and often less than 5 copies/mL.
About 90% of times that viral load is undetectable in blood it is also undetectable in genital fluids.
This changes how we think of traditional risks and safety. So sex without a condom (with undetectable viral load) might become a lower risk than oral sex (with a man who is been recently infected).
Figure 2 – Viral load levels as HIV infection progresses
Figure 3 – The relationship between viral load and HIV transmission in Rakai Study 
NOTE: One person year in research refers to 12 months of data in a study. For example, 100 person years could be 100 people all followed for one year or 200 people all followed for 6 months. This is used when calculating risks. It allows for all the time that different people contribute to a study, even when this might be years for one person and only months for another.
In March 2014 the first results from the European PARTNER study in serodifferent couples were important for reporting that no HIV transmissions occurred after 44,500 times that couple had sex without a condom, when the HIV positive person had an undetectable viral load. [4, 5, 6] See this report for more details.
- Donnell D and others. Heterosexual HIV-1 transmission after initiation of antiretroviral therapy: a prospective cohort analysis. Lancet. 2010;375(9731):2092-8.
See report from first conference presentation here.
- The HPTN 052 Study: Preventing Sexual Transmission of HIV with Anti-HIV Drugs. Cohen M and others. Antiretroviral treatment to prevent the sexual transmission of HIV-1: results from the HPTN 052 multinational randomized controlled trial. 16th IAS Conference on HIV Pathogenesis, Treatment and Prevention, 17–20 July 2011, Rome. Oral abstract MOAX0102. Webcast
See report here.
- Quinn TC and others. Viral load and heterosexual transmission of HIV type 1. Rakai Project Study Group. NEJM 2000; 342: 921-929.
- Rodger A et al. HIV transmission risk through condomless sex if HIV+ partner on suppressive ART: PARTNER Study. 21st CROI, 3-6 March 2014, Boston. Oral late breaker abstract 153LB.
- Q&A fact sheet and resource on the interim results from the PARTNER Study.
- An online interview with the principal investigator of the PARTNER study is posted on the community IFARI youtube channel.
Updated March 2014.
1 February 2013