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doi: 10.1097/QAD.0b013e328351f73f
Research Letters

HIV transmission rates from persons living with HIV who are aware and unaware of their infection

Hall, H. Irenea; Holtgrave, David R.b; Maulsby, Catherineb

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aDivision of HIV/AIDS Prevention, National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia

bDepartment of Health, Behavior and Society, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA.

Correspondence to H.I. Hall, PhD, MS E-47, Centers for Disease Control and Prevention, 1600 Clifton Road NE, Atlanta, GA 30333, USA. Tel: +1 404 639 2050; fax: +1 404 639 2980; e-mail:

Received 9 November, 2011

Revised 9 January, 2012

Accepted 30 January, 2012

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Transmission rate modeling estimated secondary infections from those aware and unaware of their HIV infection. An estimated 49% of transmissions were from the 20% of persons living with HIV unaware of their infection. About eight transmissions would be averted per 100 persons newly aware of their infection; with more infections averted the higher the percentage of persons with viral suppression who can be linked to care. Improving all stages of HIV care would substantially reduce transmission rates.

HIV testing is an essential component of a comprehensive strategy to address HIV in the United States. Persons living with HIV who are diagnosed can be linked to care and receive treatment to reduce morbidity [1,2]. In addition, reduced viral loads in persons living with HIV who are on effective treatment greatly reduces the probability of transmission of the virus to others [3], and those diagnosed can benefit from counseling for risk behaviors [4]. The combination of these factors has contributed to the remarkable reduction in transmission rates [5]. However, about 20% of the 1.2 million Americans living with HIV have not been diagnosed and, therefore, represent missed opportunities for interventions [6].

In 2006, Marks et al.[7] quantified the benefits of diagnosing HIV by estimating that sexual transmission from those unaware of their infection was 3.5 times the rate of those aware. Using the same transmission rate model [7,8], we updated the estimated percentage of HIV transmission from persons aware and unaware of their HIV infection using new information, provided additional scenarios in which the percentage with viral suppression (plasma viral load levels) reflects different estimates from the literature, and determined transmission rates by awareness status that might be achieved with the goals of the National HIV/AIDS Strategy [9].

We reviewed the literature to obtain updated parameters for the model given below, using the original parameters [7] in which no updated information was available:

Equation (Uncited)
Equation (Uncited)
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Updated information included the percentage of persons living with HIV who were aware and unaware of their HIV infection in 2008 (Na, 79.9%; Nu, 20.1% [6]) and percentage of persons living with HIV with viral suppression (≤400 copies/ml, 39% [10], Rvl = 1-0.39). Little new information was available on reduction in unprotected sex with at-risk partners among persons aware of their HIV infection (b) [11,12], confirming that the original value was still the best estimate. Similarly, original values of proportion engaging in unprotected sex (U) and number at risk (aware, SPa; unaware, SPu) were retained. We applied different assumptions for three scenarios: first, lower viral suppression than the base scenario [13]; second, the same proportion with viral suppression as the base scenario, multiplied by the proportion of persons linked to care (69%) from a meta-analysis [14]; and, third, the same proportion with viral suppression multiplied by a higher proportion of persons linked to care (82%) based on HIV surveillance data [10] that more closely reflects the goals of the National HIV/AIDS Strategy (85%) [8].

Using the results from the transmission model, we determined transmission rates {number of new infections per 100 persons living with HIV [(incidence/prevalence)×100] [5]} among persons aware and unaware of their HIV infection. For these calculations, we used updated estimates of HIV prevalence in 2008 (1 178 350 [6]) and incidence (average annual number of new infections 2006–2009: 50 125 [15]).

An estimated 51% of transmissions were from persons aware and 49% from persons unaware of their infections, assuming a similar number of at-risk partners between the groups (Table 1). The percentage of transmissions attributed to persons unaware of their HIV infection was higher assuming higher partner ratios of 1.5 or two partners among this group (59.0% and 65.7%, respectively). However, the percentage of transmissions attributed to those unaware varied widely depending on the assumption about the percentage of persons aware of their infections with viral suppression or linked to care: from 44% assuming a low percentage of persons with viral suppression to 59% assuming high viral suppression and linkage to care.

Table 1
Table 1
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Depending on the prevalence of viral suppression, linkage to care, and number of at-risk sex partners of persons unaware of their infection, the transmission rate among persons unaware was three to seven times the rate among those aware of their infections (Table 1). About eight transmissions were averted per 100 persons aware of their infection; with more infections averted the higher the percentage of persons with viral suppression who can be linked to care.

With about half of HIV transmissions attributed to the 20% of persons living with HIV unaware of their infection, decreasing the number of persons unaware of their infections must remain a primary goal of HIV prevention efforts. Centers for Disease Control and Prevention (CDC) recommends routine HIV screening in healthcare settings for persons aged 13–64 years and at least annual testing for those at increased risk for HIV infection [16], and supports state and local health departments to expand testing in their jurisdictions. However, it is equally important that persons diagnosed with HIV are linked to and retained in care, receive risk counseling, and, if appropriate, antiretroviral therapy [13,17]. Although current estimates of the percentage of persons diagnosed with HIV who are linked to care are relatively high (69–82%) [10,14,18], the percentage of persons retained in care is much lower (45–59% [14,18]) and estimates of the percentage with viral suppression range from 24% to 39% for those aware of their infection [10,13,19].

Increasing the percentage with viral suppression substantially reduced the percentage of transmissions from persons aware of their infection. This impact could be even further strengthened with implementation of revised recommendations for antiretroviral treatment for patients with CD4 cell count less than 500 cells/μl, or even higher [17].

Our analyses are subject to several limitations. First, the percentage of persons with viral suppression may vary based on the definition used [10,13,19,20]. Second, little new information was available for some of the parameters in the model, such as the magnitude of reduction in risk behavior or number of partners. Finally, other factors may contribute to HIV transmission not considered in the model.

In summary, persons unaware of their infection contribute disproportionately to the overall transmission of HIV, and this proportion is likely to increase as more efforts focus on retaining persons with HIV in care and provide therapy earlier in the course of the disease. Additional efforts are needed to reduce the number of people unaware of their infection and reach the goals of the National HIV/AIDS Strategy, reduce transmission rates, and improve linkage and retention in care and viral suppression [8].

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The findings and conclusions in this study are those of the authors and do not necessarily represent the views of the CDC.

H.I.H. and D.R.H. contributed to the design, data acquisition, analysis, interpretation, and writing of the manuscript. C.M. contributed to data acquisition and interpretation of the manuscript and provided critical review.

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Conflicts of interest

This manuscript was prepared as part of government work (H.I.H.) and there are no other funding sources to declare (D.R.H., C.M.).

There are no conflicts of interest.

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11. Marks G, Millett GA, Bingham T, Bond L, Lauby J, Liau A, et al. Understanding differences in HIV sexual transmission among Latino and black men who have sex with men: the Brothers y Hermanos Study. AIDS Behav 2009; 13:682–690.

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20. Marks G, Gardner LI, Craw J, Giordano TP, Mugavero MJ, Keruly JC, et al.The spectrum of engagement in HIV care: do more than 19% of HIV-infected persons in the US have undetectable viral load?Clin Infect Dis 2011; 53:1168–1169.

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