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Who is at risk of HIV infection and what are the circumstances in which transmissions occur? This information is important for people working in HIV to know to effectively target HIV prevention messages and inform what these messages say. Historically, those considered at highest risk were HIV-negative partners of people who knew they were HIV positive (diagnosed) and this was reflected in our HIV prevention messaging. However, there has been a gradual paradigm shift in this understanding and we now know that most HIV transmissions – particularly among men who have sex with men (MSM) – may originate from people living with HIV who don’t know that they are HIV positive (undiagnosed).

This article examines this new knowledge, and explores what can be done to improve our HIV prevention messaging to ensure it takes into account these new realities and helps reduce the number of HIV transmissions.

Changing ideas of what is and isn’t risky

New HIV knowledge is changing our idea of who is at risk of infection. Until recently, it was thought that individuals at highest risk of HIV infection were those in sexual partnerships with a person who had been diagnosed as HIV positive and that most HIV transmissions occurred between these people. However, this is not what the most recent evidence tells us.

Studies suggest that a minority of HIV transmissions originate from people living with HIV who have been diagnosed and are engaged in care (see Table 1).1,2,3,4,5,6 There are two reasons for this. First, people living with HIV who are aware of their status – particularly those who are engaged in regular HIV care – are more likely to take measures to reduce their risk of transmitting HIV to others compared to those who are living with HIV but don’t know it.7,8 Second, people living with HIV who are aware of their status and engaged in care can initiate antiretroviral treatment, which research shows  can dramatically reduce the risk of HIV transmission by lowering the viral load (amount of virus) in the bodily fluids to very low levels.9,10  In fact, people with HIV who are on treatment and maintain an undetectable viral load do not transmit HIV to their sexual partners.

Studies also suggest that the majority of HIV transmissions originate from people living with HIV who are undiagnosed, even though they represent a minority (between 14% and 25%) of people living with HIV (see Table 1).1,2,3,4,5,6 Undiagnosed individuals may believe they are HIV negative and be less likely to use prevention methods. They are also more likely to have an elevated viral load in their bodily fluids, particularly if the undiagnosed individual has recently become infected and is in the acute infection phase.7,8 A higher viral load can increase the risk of HIV transmission.

Most HIV transmissions from people living with HIV who are undiagnosed are likely occurring in partnerships where both partners believe that their own – and their partner’s – HIV status is negative or it is unknown. Further, modelling studies of MSM suggest a sizeable proportion of these transmissions (between 68% and 90%) are occurring within long-term relationships (a main sex partner or repeat casual partner) as opposed to more short-term relationships (a one-off sexual encounter or infrequent casual partner).1,11

Table 1. Summary of modelling studies




How many people living with HIV were estimated to be undiagnosed?

How many HIV transmissions were estimated to come from the people living with HIV who are undiagnosed?


Not specified

All populations





All populations





MSM only





MSM only





MSM only





All populations


30% (and 61% originated from individuals who were diagnosed but not in care)

* NA=not available; MSM=men who have sex with men; US=United States; UK=United Kingdom

Our HIV prevention messages and why they may be misleading

An important priority for HIV prevention is the development of messages to reduce the number of HIV transmissions originating from people living with HIV who are undiagnosed. However, examples of effective messages are few and far between.

Early HIV prevention messages encouraged people to avoid serodiscordant relationships and ensure their sex partners had the same HIV status as they did (a strategy sometimes referred to as “serosorting”). These early messages also focused on the importance of disclosure of HIV status as a prevention strategy; some of our prevention messaging continues to do so. Examples of such messages include, “If you and your partner know your HIV status, you reduce your risk of becoming infected with HIV.” and “Protect yourself and your partner from HIV. Talk about your status.”

Unfortunately, disclosure of HIV status is unlikely to be an effective HIV prevention strategy in partnerships where one partner is HIV negative and the other is HIV positive but undiagnosed. Regardless of the status disclosed by the undiagnosed partner (HIV negative or unknown), there is still a high risk of HIV transmission if the couple has unprotected sex (that is, if a condom, post-exposure prophylaxis [PEP] or pre-exposure prophylaxis [PrEP] are not used). In fact, if disclosure does occur, this could lead to the sharing of incorrect information about HIV status, create a false sense of security, and increase the risk of HIV transmission as a result.

Although disclosure of an HIV-negative or unknown status can lead to important discussions about HIV status with sex partners, uncertainty is often the end result any way.  It can be very difficult to be sure of an HIV-negative status, but this nuance is lost in overly broad messages such as “If you and your partner know your HIV status, you reduce your risk of becoming infected with HIV.” These vague messages seem to imply that disclosure of HIV status can, in itself, reduce HIV risk. They provide little guidance on topics that should be discussed or considered such as testing and sexual history, window period, and trust, to be certain – or more certain – of an HIV-negative status.

These messages also support the idea that the most effective HIV prevention strategy is to ensure both partners in a couple are HIV negative. We know that,  in some circumstances, the reverse may be true. It may be safer for an HIV-negative person to know a partner is HIV positive, as this can reduce uncertainties related to HIV status and viral load, and allows for informed decision-making on the use of other prevention interventions such as condoms, antiretroviral treatment and an undetectable viral load, or PrEP.

How can a person be sure they are HIV negative? First, they need to get tested and receive an HIV-negative result. If they haven’t had any potential exposures to HIV in the past three months (the window period), then they can be sure that they are HIV negative. If they have had potential exposures to HIV in the past three months, they need to avoid any additional exposures for up to three months, and then test again. Meeting these criteria can be challenging, particularly for individuals who have frequent unprotected sex (condomless sex or not using PrEP) and are in non-monogamous relationships. In fact, for such an individual whose last HIV test was negative, it can be almost impossible to know if they are actually HIV negative. This is because they may have been in the window period during their last test or may have become HIV infected since that test.

If disclosure of an HIV-negative or unknown HIV status does lead to discussions about HIV testing and sexual history with sex partners – and these discussions facilitate more informed safer-sex decisions – then disclosure may help prevent HIV transmissions. For example, a recent study found that HIV-negative MSM who had conversations about HIV status before sex were at reduced risk of HIV infection.12

However, disclosure is undoubtedly a much more effective HIV prevention strategy in circumstances where one partner is aware they are living with HIV. There are no uncertainties associated with the disclosure of an HIV-positive status, and therefore subsequent safer-sex decisions can be based on correct information.

Developing new HIV prevention messages

There is a clear need to develop new HIV prevention messages and improve public awareness of our new understanding of HIV transmission. Research shows that many people refuse to enter a relationship with a person who is HIV positive even though there is no risk of sexual HIV transmission when the person with HIV is on treatment and maintains an undetectable viral load. For example, in a recent survey of MSM across Canada, 49% said they would not have sex with an HIV-positive man even if they were very attracted to him.13

In addition, evidence suggests some people continue to rely only on HIV prevention strategies that depend on accurate knowledge of an HIV-negative status. For example, in the same survey of MSM across Canada, most men thought serosorting (50%), disclosure/discussion (66%), and regular STI/HIV testing (77%) were effective HIV prevention strategies while only a minority thought that taking antiretroviral treatment (39%), regular viral load monitoring (38%), and pre-exposure prophylaxis (36%) were effective prevention strategies.13

These beliefs contradict what we know about HIV transmission and prevention. Treatment as prevention and PrEP are both highly effective HIV prevention strategies if used consistently and correctly. Low awareness of these relatively new strategies may partly explain why so few MSM thought they were effective. A recent review of Canadian HIV-related websites found that a minority contained information on treatment as prevention and PrEP.14 The evidence to support the assertion that other strategies are highly effective is less strong.

Moving forward, reducing the number of HIV transmissions originating from HIV-positive people who are undiagnosed will require our HIV prevention messaging to:

  • Encourage clients to question their ability to know their own, and their partner’s, HIV status with certainty. If a client is unsure about their HIV status, they should be encouraged to disclose their status as unknown.
  • Encourage clients to avoid making assumptions about their own, or their partner’s, HIV status. Encourage them to discuss important topics such as HIV status, testing and sexual history, window periods, and HIV prevention prior to engaging in sex. Shared responsibility for disclosure and HIV prevention (as opposed to placing this responsibility solely on the shoulders of people living with HIV) should be promoted.
  • Promote the adoption of highly effective HIV prevention strategies that do not rely on accurate knowledge of an HIV-negative status, such as external and internal condoms and pre-exposure prophylaxis (PrEP). These options should be promoted in situations where there is uncertainty with regard to either partner’s HIV status.
  • Improve awareness of the new knowledge that people living with HIV who are on successful treatment do not transmit HIV to their sexual partners. This may help to reduce HIV-related stigma and barriers to the formation of serodiscordant relationships.
  • Promote regular HIV testing to ensure a person living with HIV learns about their status as soon as possible after infection. This can help improve health outcomes and decrease new HIV transmissions. The Public Health Agency of Canada’s HIV Screening and Testing Guide recommends that people involved in “high-risk practices” should be screened for HIV at least annually. The guidelines also recommend more frequent HIV testing for MSM who have multiple partners and/or have sex when they or their partners are using street drugs (particularly methamphetamine).



  1. a. b. c. d. Punyacharoensin N, Edmunds WJ, De Angelis D, et al. Modelling the HIV epidemic among MSM in the United Kingdom: quantifying the contributions to HIV transmission to better inform prevention initiatives. AIDS. 2015;29(3):339–349.
  2. a. b. c. Marks G, Crepaz N, Janssen RS. Estimating sexual transmission of HIV from persons aware and unaware that they are infected with the virus in the USA. AIDS. 2006;20(10):1447–1450.
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  8. a. b. Marks G, Crepaz N, Senterfitt JW, Janssen RS. Meta-analysis of high-risk sexual behavior in persons aware and unaware they are infected with HIV in the United States: implications for HIV prevention programs. Journal of Acquired Immune Deficiency Syndromes 1999. 2005;39(4):446–453.
  9. Cohen MS, Chen YQ, McCauley M, et al. Prevention of HIV-1 infection with early antiretroviral therapy. New England Journal of Medicine. 2011;365(6):493–505.
  10. Rodger A, Bruun T, Valentina C, et al. HIV transmission risk through condomless sex if HIV+ partner on suppressive ART: PARTNER Study. In: Program and abstracts of the 21st Conference on Retroviruses and Opportunistic Infections, March 3 to 6th, 2014, Boston, U.S., abstract 153LB.
  11. Sullivan PS, Salazar L, Buchbinder S, Sanchez TH. Estimating the proportion of HIV transmissions from main sex partners among men who have sex with men in five US cities. [Miscellaneous Article]. AIDS. 2009;23(9):1153–1162.
  12. Santos-Hövener C, Zimmermann R, Kücherer C, et al. Conversation about Serostatus decreases risk of acquiring HIV: results from a case control study comparing MSM with recent HIV infection and HIV negative controls. BMC Public Health. 2014;14(1):453.
  13. a. b. Male Call Canada. Technical Report.; 2013.
  14. Gilbert M, Dulai J, Wexler D, Martin S. Challenges to providing HIV risk and prevention information online to gay, bisexual and other men who have sex with men: preliminary findings from an environmental scan of Canadian agency websites. In: Abstracts of the 24th Annual Canadian Conference on HIV/AIDS Research (CAHR), April 30th to May 3rd, 2015, Toronto, Canada; abstract O047


About the author(s)

James Wilton is the coordinator of the Biomedical Science of HIV Prevention Project at CATIE. James is currently completing his master’s degree of Public Health in Epidemiology at the University of Toronto and has completed an undergraduate degree in microbiology and immunology at the University of British Columbia.