HIV transmission rate in US has declined enormously

Michael Carter
Published: 16 December 2008

Fewer people with HIV transmitted the infection to others in 2006 than ever before, according to new figures released by US researchers and published in the online edition of the Journal of Acquired Immune Deficiency Syndromes.

Investigators from Johns Hopkins University and the US Centers for Disease Control and Prevention calculated that, in 2006, 95% of people with HIV did not pass on the infection to someone who was HIV-negative. They further calculated that there has been a steady decline in the rate of HIV transmission since the 1980s. They attribute this to the success of HIV prevention initiatives, particularly increased HIV testing.

The investigators calculated the rate of HIV transmission as the number of people with HIV per 100 who infected an HIV-negative individual in a year. Recent revisions in the HIV incidence rate in the US prompted the researchers to look at the rate of HIV transmission between 1977 and 2006.

They estimated that there was a very high rate of transmission in 1977 with, in effect, every HIV-positive person passing on the infection to somebody else. The rate of HIV transmission in 1980 was still above 90, but by 1987 this had fallen to a rate of 15 per 100.

Between 1991 and 1996, the rate of transmission was steady at 6.6 per 100. It increased to 7.5 in 1998, before falling steadily until it reached a rate of 5 per 100 in 2006.

“For every 100 persons living with HIV in the United States, there are 5 HIV transmissions per year. Stated another way, this indicates that in 2006, over 95% of persons living with HIV in the United States did not transmit the virus to a seronegative partner,” write the investigators.

They think that the proportion of people with HIV transmitting the virus may be even lower, given that the virus is often transmitted in clusters.

The researchers also believe that the falling rate of HIV transmission is “a rough measure of prevention success.” Such a success has been achieved against static or declining funding for HIV prevention. The researchers note that people tend to reduce their risk behaviour after an HIV diagnosis, and increased knowledge of HIV status could therefore be an explanation for the fall in the transmission rate.

Although antiretroviral therapy could have had an impact on the transmission rate in recent years, they note that significant falls in the rate of transmission were first notable over a decade before effective HIV treatment became available.


Holtgrave, DR et al. Updated annual HIV transmission rates in the United States, 1977-2006. J Acquir Immune Defic Syndr (online edition), 2009.

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