Untested HIV-positive individuals account for up to 70% of sexually transmitted HIV infections in US, estimates CDC

Edwin J. Bernard
Published: 28 June 2006

HIV-positive individuals who are unaware of their infection may account for between 54% and 70% of all new sexually transmitted HIV infections in the United States, according to a "conservative" mathematical calculation from the US Centers for Disease Control and Prevention (CDC) published in the June 26th edition of the journal, AIDS. Targeted HIV testing campaigns, as well as broader-based screening, could have a major impact on future HIV incidence and prevalence, they argue.

Understanding who is more likely to transmit HIV can help inform current debates around contentious issues such as universal testing and serosorting as HIV prevention tools, as well as the criminalisation of HIV transmission, which tends to penalise people already aware of their HIV status.

The US Centers for Disease Control and Prevention (CDC) estimates that of the approximately 1.1 million individuals currently infected with HIV in the US, around 75% are aware of their infection and 25% are not.

Last year, a meta-analysis from the CDC concluded that untested HIV-positive individuals were more than twice as likely to engage in high-risk sex than those aware of their HIV-positive status. Now, the same researchers have utilised a mathematical formula in order to estimate the proportion of sexual transmission of HIV attributable to those aware and unaware of their HIV status.

The formula included the following parameters, some of which were based on empirical data and some of which were based on informed assumptions:

  • the number of people aware they are HIV-positive i.e. 75 out of 100
  • the number who are unaware they are HIV-positive i.e. 25 out of 100
  • the reduction in prevalence of high-risk sex (unprotected anal or vaginal intercourse) undertaken by HIV-positive aware individuals compared with HIV-positive unaware individuals, based on last year's meta-analysis which demonstrated a 57% relative reduction (range 52-59%)
  • the ratio of high-risk sex with at-risk partners practised by HIV-positive unaware individuals, compared with HIV-positive individuals, i.e. a calculation based on whether being diagnosed HIV-positive has an effect on the number of at-risk partners with whom HIV-positive aware individuals have high-risk sex. There are no data on this, so several ratios were used based on different assumptions. The lower estimate was based on both groups having high-risk sex with equal numbers of at-risk partners (a ratio of 1 to 1), and the higher estimate was based on HIV-positive unaware individuals having high-risk sex with twice as many at-risk partners (a ratio of 2 to 1).
  • A calculation reflecting the reduced infectivity of HIV-positive aware people on successful anti-HIV therapy, due to their having a viral load below 500 copies/ml. Studies of heterosexual HIV transmission from Rakai, Uganda found that no transmission occurred when the HIV-positive partner had a viral load below 1500 copies/ml: the CDC used a lower cut-off point "to be more conservative".

The CDC estimates that the proportion of sexually transmitted HIV from HIV-positive individuals unaware of their infection ranges from 54% (assuming there is no difference between the average number of at-risk partners with whom HIV-positive aware and HIV-positive unaware individuals have high-risk sex) to 70% (assuming that HIV-positive unaware individuals have high-risk sex with twice as many at-risk partners than HIV-positive aware individuals).

"Our results indicate that the HIV-positive unaware group contributes disproportionately to [sexually acquired new HIV] infections relative to their percentage of the HIV-positive population in the USA", the study's authors write. Even applying their lower estimate of a 54% contribution from HIV-positive people unaware of their results in an HIV transmission rate that is "3.5 times higher in the unaware group".

They add that their estimates are "conservative", since "viral load is not the the only biologic factor affecting transmission risk" and their formula did not take into account sexually transmitted infections (STIs) such as syphilis or gonorrhoea, which may increase the likelihood of transmitting or contracting HIV. Since several studies have found a higher prevalence of STIs amongst HIV-positive unaware individuals, the authors suggest that "the contribution to transmission from the unaware group may be even larger than the estimates given here."

They argue that their results "indicate a need to expand HIV counseling and testing to increase the number of HIV-positive persons who are aware of their infection." They estimate that if everyone who was HIV-positive learned of their infection in a timely manner, and reduced their high-risk sex with at-risk partners by their 57% (based on their data from the previous meta-analysis), then new sexual HIV infections could, theoretically, be reduced by 31% a year.

They suggest that "a combination of targeted HIV testing campaigns and broad-based screening campaigns" are necessary. In particular targeting gay and bisexual men from ethnic minorities will have a major impact, due to the especially high prevalence of unrecognised HIV infections in this risk group.

They also recommend more general screening programmes, such as antenatal testing, which can identify people with HIV who may not consider themselves to be at risk for infection.


Marks G et al. Estimating sexual transmission of HIV from persons aware and unaware that they are infected with the virus in the USA. AIDS 20(10); 1447-1450, 2006.


Although they mention it in their discussion, the CDC calculation did not take into account data from Rakai published last year which suggests that in the five months immediately following seroconversion, HIV transmission risk is approximately ten times greater than during chronic infection. In fact, in this study almost half of all the sexual HIV transmission seen could be ascribed to a sex partner with newly acquired HIV infection.

Taken together, the CDC calculations and the Rakai data suggest that nearly all new HIV infections can be attributed to HIV-positive persons unaware of their HIV status, either during primary HIV infection or during chronic infection with HIV that is untreated.

In addition, the CDC's data are based on one-in-four individuals being unaware of their HIV infection. It should be noted that in in the United Kingdom, it is estimated that one-in-three HIV infections remain undiagnosed, which suggests that the proportion of new HIV infections from undiagnosed HIV-positive individuals in the UK may be even higher than in the US.

Reducing both the fear and experience of stigma and discrimination that is experienced around HIV infection may be of additional benefit here, both for African communities and also for gay men, especially those living outside of London.

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