Half of all new HIV transmissions in US may originate in undiagnosed individuals

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Almost half of all new HIV transmissions in the US originate in individuals who are unaware that they are HIV-positive, a modelling study published in the online edition of AIDS estimates.

Only 20% of HIV infections in the US are undiagnosed, but the investigators calculated that they were the source of 49% of all new onward transmissions.

“Decreasing the number of persons unaware of their infection must remain a priority goal of HIV prevention efforts,” comment the authors.


retention in care

A patient’s regular and ongoing engagement with medical care at a health care facility. 

virological suppression

Halting of the function or replication of a virus. In HIV, optimal viral suppression is measured as the reduction of viral load (HIV RNA) to undetectable levels and is the goal of antiretroviral therapy.

mathematical models

A range of complex mathematical techniques which aim to simulate a sequence of likely future events, in order to estimate the impact of a health intervention or the spread of an infection.


The prospect of survival and/or recovery from a disease as anticipated from the usual course of that disease or indicated by the characteristics of the patient.

linkage to care

Refers to an individual’s entry into specialist HIV care after being diagnosed with HIV. 

With appropriate treatment and care the prognosis of many HIV-positive patients is now normal. Virologically suppressive antiretroviral therapy also has wider public health benefits, reducing the risk of HIV transmission by 96%.

Late diagnosis is the principal factor underlying much of the HIV-related mortality that continues to be seen in resource-rich countries. Several studies have also suggested that a disproportion amount of new HIV transmissions have their source in undiagnosed individuals.

Testing is therefore a cornerstone of efforts to control HIV. Opt-out HIV testing is now recommended in the US for adolescents and adults aged between 13 and 64 years as part of their routine health care. Screening at least annually is recommended for patients with higher risk of the infection.

Previous research has suggested that HIV transmission rates in the US are 3.5 times higher for undiagnosed individuals compared to patients whose infection has been diagnosed.

Investigators updated the information used to arrive at this estimate, taking into account revised estimates of the proportion of undiagnosed infections and the impact of virologically suppressive HIV therapy on infectiousness.

The model was based on HIV prevalence rates in 2008 and HIV incidence between 2006 and 2009. The proportion of HIV-positive individuals unaware of their infection was 20%, and 39% of patients had a viral load below 400 copies/ml. Other variables included in the model were estimates of the frequency of unprotected sex with HIV-negative partners. The model also took account of the proportion of patients who were successfully linked to HIV care and retained in care.

An estimated 49% of all new transmissions were from individuals who were unaware of their infection status.

 If the proportion of diagnosed patients with a viral load below 400 copies/ml increased to 60%, this increased the burden of new transmissions attributed to undiagnosed individuals to almost 60%.

“Increasing the percentage with viral suppression substantially reduced the percentage of transmissions form persons aware of their infection,” comment the authors. “The impact could be even further strengthened with implementation of revised recommendations for antiretroviral treatment for patients with CD4 cell count below 500 cells/mm3, or even higher.”

The transmission rate for patients aware of their infection status varied between 1.8 and 2.7 per 100 persons. It was between three and seven times higher for undiagnosed individuals (9.2 to 12.6 per 100 persons).

In a scenario when 39% of HIV-positive patients had a viral load below 400 copies/ml, then each increase of 100 in the number of diagnosed infections was estimated to avert eight new transmissions.

However, simply reducing the number of undiagnosed infections is not enough in itself to control the HIV epidemic. The authors stress that patients then have to be promptly connected with a specialist clinic, retained in care and achieve virological suppression.

“While current estimates of the percentage of persons diagnosed with HIV who are linked to care are relatively high (69% to 82%), the percentage of persons retained in care is much lower (45% to 59%) and estimates of the percentage with viral suppression range from 24% to 39% for those who are aware of their infection.”

The authors conclude: “Additional efforts are needed to educe the number of people unaware of their infection…reduce transmission rates, and improve linkage and retention in care and viral suppression.


Hall HI et al. HIV transmissions from persons with HIV who are aware and unaware of their infection, United States. AIDS 26, online edition. DOI: 10.1097/QAD013e328351f73f, 2012 (click here for the free abstract).