Model suggests only 9% of new HIV infections due to those with acute HIV infection

Only a small proportion of new, sexually transmitted HIV infections in the United States are due to people with acute, undiagnosed HIV, according to a mathematical model published in the July 31^st edition of AIDS.

Dr Steven Pinkerton from the Center for AIDS Intervention Research at the Medical College of Wisconsin calculated that, although the majority of new HIV infections were due to people who were unaware of their HIV infection status, fewer than 9% of new infections originated in people with acute HIV infection.

This finding is in stark contrast to some estimates which attribute as many as a half of all new HIV infections to patients recently infected with the virus.

The first few weeks after infection with HIV, a phase that is often called primary or acute HIV infection, are characterised by an extremely high HIV viral load. During this period an HIV antibody test is likely to be negative or indeterminate, and some individuals with primary infection, believing themselves to be HIV-negative, may continue to have unprotected sex.

Several studies have demonstrated that HIV-infected individuals who are unaware of their status are more likely to transmit HIV to others than people who know that they are HIV-positive. The high viral load that characterises primary HIV infection could mean that of all individuals who are unaware that they have HIV, those with primary infection are the most infectious.

But the actual impact of acute-phase HIV transmission on the epidemiology of the virus in the US is unknown. Such knowledge is important to the design of appropriate HIV prevention interventions.

Dr Pinkerton designed a mathematical model that included epidemiological data with information about the relative transmission rates for acutely-infected, chronically-infected unaware, and chronically-infected aware HIV-positive individuals.

Of the 32,000 incident infections each year in the US, Dr Pinkerton estimated that 2,760 (9%) were due to people with acute HIV infection. He further estimated that a little over 15,500 (48%) infections were due to people with chronic HIV infection who were unaware of their infection status. Overall, 57% of all incident HIV infections were due to people who were unaware that they were HIV-positive, with 43% of new infections originating in chronically-infected patients who knew that they were HIV infected.

The annual transmission rate for acutely-infected unaware individuals was 51%, but only 2% for chronically-infected individuals who knew of their infection.

Although Dr Pinkerton noted that patients with primary HIV infection have exceptionally high viral loads, the period for which they are highly infectious is relatively short, typically no more than 49 days.

His model was based upon the assumption that individuals who know that they are HIV-positive are 60% less likely to have unprotected sex than individuals who are unaware of their infection status, and that patients taking antiretroviral therapy with a plasma viral load below 500 copies/ml are incapable of transmitting the virus.

“Despite the relatively small predicted impact of acute infection on the USA epidemic, this analysis nevertheless highlights the significant risk of HIV transmission during acute HIV transmission”, writes Dr Pinkerton, adding, “the HIV transmission rate for acutely-infected persons was 16 times larger than the overall transmission rate for people living with HIV in the USA.”

However, because individuals with acute HIV infection only represent 0.5% of all HIV-infected individuals in the US, and because of the brief duration of primary infection, the impact of acute-phase transmission on the overall epidemiology of HIV in the US is minimised.

A study conducted amongst gay men in Amsterdam attributed approximately 11% of new infections to individuals with acute HIV infection, but a study in Uganda found that over 40% of all new infections were attributable to individuals recently infected with HIV. However, Dr Pinkerton draws attention to a modelling study presented to the 2006 Conference on Retroviruses and Opportunistic Infections that suggests that in fact only 11% of infections in Uganda are due to acutely-infected individuals.