Important factors affecting risk: condoms, viral load, sexually transmitted infections and/or male circumcision

Edwin J. Bernard
Published: 18 July 2010


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In addition to which type of sexual intercourse occurs and which roles both partners take, there are a number of other important factors to consider when assessing the risk of HIV exposure or transmission. Four major factors – condom use, viral load, the presence of sexually transmitted infections (STIs) and the male partner’s circumcision status – are discussed below.

Other potentially relevant factors, including viral characteristics, individual immune responses, individual genetics, pregnancy, use of hormonal contraceptives and use of recreational drugs, are beyond the scope of this book. More information about these topics, and more detailed information regarding all HIV exposure and transmission risks, can be found in NAM's publication HIV Transmission and Testing.

Male and female condoms

Condoms have been the mainstay of HIV prevention since the start of the AIDS epidemic. Although condoms are not always 100% effective in preventing HIV exposure, an extensive body of research has confirmed that male condoms provide a high level of protection when used consistently and correctly.1 As noted in a statement published by UNAIDS, UNFPA and WHO, the male latex condom is the single, most efficient, available technology to reduce the sexual transmission of HIV and other sexually transmitted infections.2

HIV cannot pass through condoms, but condom use does not guarantee protection because of occasional breakage and slippage. Real-life epidemiological studies show that using condoms all of the time, though not necessarily perfectly (i.e., with usual rates of breakage and slippage), reduces the risk of HIV transmission by around 80% compared to not using condoms.1

To put this into perspective, if the risk of acquiring HIV for a woman in a high-income country who has unprotected vaginal sex with a chronically infected HIV-positive man who is not on treatment is estimated to be 1 in 1250 (0.08%), this risk is further reduced by 80% with condom use. In other words, on average, one transmission would be expected to occur for every 6250 instances (0.016%) of protected vaginal sex with a chronically infected HIV-positive man who is not on treatment.

The female condom is a polyurethane pouch that is inserted into the vagina. Female condoms are as impermeable to HIV as male condoms, but are not as widely used, and evidence about their real-life effectiveness is more limited. The small body of research to date suggests that the level of protection female condoms provide against HIV is comparable to that of male condoms.

Viral load

Viral load is the term used to describe the amount of HIV circulating in the body. It is measured in terms of the number of copies of HIV per millilitre of blood, or copies/ml. Sexual transmission is thought to be extremely uncommon when blood viral load is below 1000 copies/ml.3

In high-income countries, HIV viral-load testing is usually undertaken every three to six months in order to help inform antiretroviral treatment decisions. Viral-load testing is seldom undertaken in low-income settlings, however, due to cost and other resource limitations.

Viral-load tests cannot measure extremely low levels of virus, and someone whose viral level is below the threshold of detection (e.g., 50 copies/ml is the current limit of detection of tests in common use globally) is said to have an undetectable viral load. This does not mean the person no longer has any HIV in their body, and may not even mean that the blood sample is completely free of virus. But it usually means that levels of HIV are equally low or lower elsewhere in the body.4,5

Viral load and infectiousness

The association between HIV viral load and the risk of any kind of HIV transmission was first established in studies examining the impact of viral load on mother-to-child transmission.6 More recent studies have found a correlation between the amount of virus measured in the blood and the risk of heterosexual transmission.7 The key principle from an HIV-prevention perspective is that infectiousness increases or decreases in relation to the viral-load level.

Both the stage of infection and the treatment status of someone living with HIV greatly affect the viral-load level. In the first few weeks or months after HIV has entered a person’s body, during a period known as primary or acute infection, the viral load is often extremely high. Once the immune system begins working to suppress HIV, the viral load typically drops. It rises again when the immune system can no longer keep HIV under control. A recently infected individual is estimated to be around nine times more infectious during primary infection and around seven time more infectious during the final stage of HIV disease (although increasingly fewer people with HIV reach that stage if they are diagnosed and treated with antiretrovirals) than during chronic infection.8 Chronic infection is the period of living with HIV that can last for many years,  often without any obvious symptoms.

There is some debate regarding the most important stage of HIV infection for onward transmission. One study examining new infections in the province of Quebec, Canada, found that half of all transmissions emanated from the 10% of people in the study who had been infected within the previous six months.9 However, it has been pointed out that the most infectious period is very short, typically no more than 49 days,10 and may, over a person's lifetime, account for fewer infections than the much longer period of chronic infection.11

A 2009 analysis of all studies to date in heterosexual couples where one partner was HIV-positive – but where most were not on antiretroviral therapy (ART) – has confirmed that a high viral load can significantly increase the risk of transmission, and that a low viral load (which is possible to achieve in a minority of people who do not receive ART) significantly reduces the risk.3 The authors calculated that out of 1000 HIV-positive individuals with a viral load below 400 copies/ml regularly engaging in vaginal sex with an HIV-negative partner, only one transmission could be expected to occur in the course of a year. In contrast, among 1000 HIV-positive individuals with a viral load above 50,000 copies/ml, at least 90 transmissions could be expected to occur in the course of a year.

Impact of treatment on viral load and infectiousness

Since HIV treatment lowers viral load, someone who responds well to treatment would logically be expected to have a lower likelihood of transmitting HIV to another person. In fact, the association between HIV viral load and the risk of transmission was first established in 1994 in a study examining the impact of antiretrovirals on mother-to-child transmission.12 The same association for sexual transmission was first observed amongst heterosexual couples in 2000.13

However, there is less consensus regarding the impact of treatment on individual infectiousness. This is because studies so far have only examined heterosexual transmission risk amongst long-term couples in low-income settings;3 it is unknown whether this translates to other kinds of sex (for example, anal sex; or sex between short-term partners) in other settings14 and whether there is a threshold of viral levels below which transmission is not possible.15

The Swiss statement was produced on behalf of the Swiss National AIDS Commission (EKAF). It was authored by four of Switzerland’s foremost HIV experts: Professor Pietro Vernazza, of the Cantonal Hospital in St. Gallen, and President of EKAF; Professor Bernard Hirschel from Geneva University Hospital; Dr Enos Bernasconi of the Lugano Regional Hospital; and Dr Markus Flepp, President of the Swiss Federal Office of Public Health’s Sub-committee on the clinical and therapeutic aspects of HIV/AIDS. The statement discussed the impact of successful treatment on the individual risk of transmission, and the related implications for doctors; for HIV-positive people; for HIV prevention; and the legal system. The same information was provided in Swiss AIDS Federation (Aids-Hilfe Schweiz) guidance for healthcare workers which meant that all people with HIV in Switzerland were given information about how to reduce their risk of HIV transmission in ways that did not only include using condoms. They are now told that under certain circumstances – on antiretroviral treatment, with an undetectable viral load for six months, with no other sexually transmitted infections, and under regular follow-up – unprotected sex with their primary partner is as safe as using condoms.

Swiss HIV experts generated a great deal of debate16 following their 2008 consensus statement,17 which proposed that a person’s risk of acquiring HIV from an HIV-positive sexual partner with an undetectable viral load who remains on successful HIV treatment and who has no other sexually transmitted infection is no greater than it would be if condoms were used in the absence of HIV treatment. The most recent study to report on the prevention effect of HIV treatment looked at the number of HIV transmissions between long-term heterosexual couples in Africa where one partner was HIV-positive at the start of the study.18 It found that when the HIV-positive partner was on antiretroviral therapy it prevented nine out of ten transmissions. In comparison, condoms are thought to prevent eight out of ten transmissions.1

Although no studies have empirically examined the effects of HIV treatment on transmission risk for anal sex, this has been explored in a mathematical model.14 Using this mathematic model to compare the risk over 100 sex acts of HIV transmission during anal sex between men where one partner is HIV-positive: with condoms; with an undetectable viral load on HIV treatment; with both; and with neither; illustrates that an undetectable viral load on HIV treatment is likely to be as effective as 100% condom use.

Although UNAIDS and the World Health Organization (WHO) initially responded to this Swiss statement by noting that not enough evidence exists to fully support this position regarding advice for individuals,19 there has since been recognition by both organisations of the significant impact of treatment on infectiousness on an individuali and population level.20  Recent evidence also suggests that large-scale viral-load decreases appear to correlate with falling HIV-incidence rates in communities with good access to treatment.21,22

i. For example, UNAIDS Executive Director, Michel Sidibé, recently stated: "We now know that treatment not only saves lives, it is one of the most potent prevention tools we have." See also Bernard EJ ‘UNAIDS/UNDP supports Swiss statement, announces new Global Commission on HIV and the Law’ Criminal HIV Transmission., 9 June 2010.

Risk of HIV transmission over 100 sex acts during anal sex between men where one partner is HIV-positive

Source: Garnett GP and Gazzard B Risk of HIV transmission in discordant couples The Lancet 372: 270-271, 2008.

Sexually transmitted infections

Some sexually transmitted infections (STIs) can increase the transmission risk during sex with an HIV-positive individual by inducing higher levels of HIV in their genital fluids, as well as by causing inflammation and ulcerations on their external and internal genital organs.

As for people exposed to HIV, having an STI can greatly increase vulnerability to infection. A large analysis of multiple studies found that being infected with the virus that causes genital herpes (HSV-2) doubles the chances of a person becoming infected with HIV following sexual exposure, even in the absence of obvious symptoms.23 In addition, any STI that causes genital inflammation or ulcerations will increase the risk that HIV exposure results in infection. Studies in men and women have correlated the presence of chlamydia, gonorrhoea and syphilis with a higher risk of acquiring HIV.24

However, it is currently not known whether STIs can significantly increase the risk of transmission from a person who is under antiretroviral therapy and has an undetectable viral load in the blood. A recent study from Africa, where all HIV-positive partners also had HSV-2, but where the chance of transmission was reduced by at least 90% if the HIV-positive partner was on antiretroviral therapy, suggests that there may not be a significant increase in transmission risk in such situations.18

Male circumcision

There is strong biological and epidemiological evidence that circumcised men have a lower risk of acquiring HIV infection via heterosexual intercourse than uncircumcised men.25 Two studies published in 200726,27 found that HIV-negative men who are circumcised are between 50 and 65% less likely than uncircumcised men to become infected with HIV following sexual exposure via unprotected vaginal intercourse. The studies, undertaken in Africa, followed men who chose to undergo circumcision in adulthood as an HIV-risk reduction strategy. The studies were actually stopped early because the benefits of circumcision were so clear.

The association between circumcision and anal sex between men is less clear. A recent Australian study suggested that men who were exclusively insertive partners were less likely to become infected with HIV during unprotected anal sex, but the study had many limitations.28


  1. Weller SC and Davis-Beaty K Condom effectiveness in reducing heterosexual HIV transmission. Cochrane Database of Systematic Reviews Issue 1, 2002
  2. UNAIDS, UNFPA, WHO Condoms and HIV prevention: position statement by UNAIDS, UNFPA and WHO. March, 2009
  3. Attia S et al. Sexual transmission of HIV according to viral load and antiretroviral therapy: systematic review and meta-analysis. AIDS 23:1397-1404, 2009
  4. Kalichman SC et al. Human immunodeficiency virus load in blood plasma and semen: review and implications of empirical findings. Sexually Transmitted Diseases 35:55-60, 2008
  5. Letendre S et al. Correlates of CSF viral loads in 1221 volunteers in the CHARTER Cohort. 17th Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 172, 2010
  6. Weiser B et al. Quantitation of human immunodeficiency virus type 1 during pregnancy: relationship of viral titer to mother-to-child transmission and stability of viral load. Proc Natl Acad Sci U S A 91(17): 8037-41, 1994
  7. Wawer MJ et al. Declines in HIV Prevalence in Uganda: Not as Simple as ABC. Twelfth Conference on Retroviruses and Opportunistic Infections, Boston, abstract LB27, 2005
  8. Boily MC et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infect Dis 9(2): 118-129, 2009
  9. Brenner BG et al. High rates of forward transmission events after acute/early HIV-1 infection. J Infect Dis 195: 951-959, 2007
  10. Pinkerton SD et al. How many sexually acquired HIV infections in the USA are due to acute-phase HIV transmission? AIDS 21:1625-1629, 2007
  11. Fraser C et al. Variation in HIV-1 set-point viral load: Epidemiological analysis and an evolutionary hypothesis. Proc Natl Acad Sci epub Oct 22, 2007
  12. Connor EM et al. Reduction of maternal-infant transmission of human immunodeficiency virus type 1 with zidovudine treatment. N Engl J Med 331: 1173-1180, 1994
  13. Quinn TC et al. Viral load and heterosexual transmission of human immunodeficiency virus type 1. N Engl J Med 342(13): 921-929, 2000
  14. Wilson DP et al. Relation between HIV viral load and infectiousness: a model-based analysis. The Lancet 372: 314-320, 2008
  15. Wilson DP Data are lacking for quantifying HIV transmission risk in the presence of effective antiretroviral therapy. AIDS 23 (11): 1431-1433, 2009
  16. Bernard EJ Swiss statement that “undetectable equals uninfectious” creates more controversy in Mexico City. Available online at:, 5 August 2008
  17. Vernazza P et al. Les personnes séropositives ne souffrant d’aucune autre MST et suivant un traitment antirétroviral efficace ne transmettent pas le VIH par voie sexuelle. Bulletin des médecins suisses 89 (5), 2008
  18. Donnell D et al. ART and risk of heterosexual HIV-1 transmission in HIV-1 serodiscordant African couples: a multinational prospective study. 17th Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 136, 2010
  19. UNAIDS Antiretroviral therapy and sexual transmission of HIV. 1 February, 2008
  20. WHO Antiretroviral therapy for HIV prevention. (accessed 30 July 2010), 2010
  21. Das-Douglas M et al. Decreases in community viral load are associated with a reduction in new HIV diagnoses in San Francisco. 17th Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 33, 2010
  22. Montaner J et al. Association of expanded HAART coverage with a decrease in new HIV diagnoses, particularly among injection drug users in British Columbia, Canada. 17th Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 88LB, 2010
  23. Wald A et al. Condoms protect men and women against herpes simplex virus type 2 (HSV-2) acquisition. National STD Prevention Conference, San Diego, abstract B9E, 2002
  24. Pebody R HIV Transmission and Testing. NAM, London, 2009
  25. Siegfried N et al. Male circumcision for prevention of heterosexual acquisition of HIV in men. Cochrane Database Syst Rev (2): CD003362, 2009
  26. Bailey R et al. Male circumcision for HIV prevention in young men in Kisumu, Kenya: a randomised controlled trial. Lancet 369: 643-56, 2007
  27. Gray RH et al. Male circumcision for HIV prevention in men in Rakai, Uganda: a randomised trial. The Lancet 369(9562):657-66, 2007
  28. Templeton DJ et al. Circumcision and risk of HIV infection in Australian homosexual men. AIDS 23(17): 2347-2351., 2009

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A writer and advocate on a range of HIV-related issues, Edwin has a particular specialism in HIV and the criminal law. He works with national and international HIV organisations, including the International AIDS Society, GNP+ and UNAIDS, as well having as a long association with NAM as a writer on this topic and as the former editor of HIV Treatment Update. To visit Edwin's blog and respond to posts click here.

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

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We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

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The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.