Indirect evidence that treatment is bringing down HIV transmission in Denmark

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The HIV transmission rate amongst gay men and men who have sex with men (MSM) in Denmark appears to be falling even though the number of people living with the virus continues to increase, and despite high levels of unsafe sex, the Eighteenth International AIDS Conference in Vienna was told this week.

Susan Cowan of the National Infections Institute in Denmark said the only explanation had to be that these factors were being overcome by a decline in the infectiousness of the average person with HIV, due to very high rates of viral suppression.

There are about 5250 people in Denmark living with HIV in a population of 5.5 million, so it is still a low-prevalence country (less than one case per thousand). About half of the HIV positive population are men who have sex with men.


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.

Seventy per cent of gay men have ever taken an HIV test, surveys reveal, roughly similar to the UK, with 50 to 59% – depending on which survey one looks at – having taken a test in the last year.

More than 80% of diagnosed MSM are on antiretroviral (ARV) treatment and 85% of them have an undetectable viral load.

The current testing rates still seem to fail to detect both recent infections and a number of more chronic ones. Denmark does not yet do incidence testing which can estimate the date of infection to within three to six months. However, the average CD4 count of diagnosed gay men did not change between 2005 and 2009. Only 30% of new diagnoses had CD4 counts over 500 cells/mm3; fewer than half (47%) had a CD4 count over 350 cells/mm3; and 23% had a CD4 count below 200. Not just recent infections, but chronic untreated ones, could be driving continued new infections seen in gay men.

The number of infections newly diagnosed in gay men had increased per year from 1000 in 1995, to 1500 in 2001 and 2200 in 2009. With a very low death rate, this meant that more and more gay men in the country had HIV.

Furthermore, there are very high levels of unsafe sex amongst gay men. The percentage of HIV-positive men who had had unprotected anal sex with someone who had or might have had a different HIV status was 37% in 2001 and 64% in 2009.

Despite this, the proportion of gay men who transmitted HIV during any one year has steadily declined. Dividing the number of gay men living with HIV by the annual rate of new diagnoses yields a downward slope in the number of new infections per year per MSM living with HIV. This has declined from 0.11 infections per MSM per year in 1995, to 0.7 in 2001, to less than 0.5 now.

The most likely explanation for this paradox is the higher proportion of HIV-positive men who are on HIV treatment and undetectable and who are never or very rarely transmitting HIV.

Further information

Susan Cowan's abstract is available on the official conference website.


Cowan S et al. New paradigm for positive prevention: "Test and treat" testing for and treating HIV has lowered transmission rate in Denmark in spite of increased unsafe sex among MSM. Eighteenth International AIDS Conference, Vienna, abstract MOAC0103, 2010.