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

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.

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. www.criminalhivtransmission.blogspot.com, 9 June 2010.