Increased testing leads to decrease in viral load and infections in San Francisco, and in late diagnosis in Washington

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The HIV infection rate in San Francisco appears to be falling, and the fall is associated with a reduction in the average viral load in HIV-positive people, due to more people on treatment, the 17th Conference on Retroviruses and Opportunistic Infections (CROI) heard on Wednesday.

Dr Moupali Das from the San Francisco Department of Public Health (DPH) told the conference that the reduction in infections was ultimately due to an increased frequency of HIV testing. It is estimated that only one in six people with HIV in the city is unaware of their infection, one of the lowest undiagnosed rates in the world.

Between 2004 and 2008, Dr Das said, the number of HIV diagnoses in San Francisco fell by 45%, and the average viral load amongst the HIV-positive population by 40%. The DPH also estimated that the actual HIV incidence – the true number of new HIV infections, diagnosed and undiagnosed – fell by one-third between 2006 and 2008.


linkage to care

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

statistical significance

Statistical tests are used to judge whether the results of a study could be due to chance and would not be confirmed if the study was repeated. If result is probably not due to chance, the results are ‘statistically significant’. 


A tentative explanation for an observation, phenomenon, or scientific problem. The purpose of a research study is to test whether the hypothesis is true or not.


A sexually transmitted infection caused by the bacterium Treponema pallidum. Transmission can occur by direct contact with a syphilis sore during vaginal, anal, or oral sex. Sores may be found around the penis, vagina, or anus, or in the rectum, on the lips, or in the mouth, but syphilis is often asymptomatic. It can spread from an infected mother to her unborn baby.


Choosing sexual partners of the same HIV status, or restricting condomless sex to partners of the same HIV status. As a risk reduction strategy, the drawback for HIV-negative people is that they can only be certain of their HIV status when they last took a test, whereas HIV-positive people can be confident they know their status

During the four-year period, the proportion of gay men taking HIV tests who had tested less than a year previously rose from 65% to 72%, and the proportion testing within the last six months from 41% to 53%. It was estimated that during this time the proportion of gay men with HIV unaware of their infection fell from 24% to 14.5%. Gay men represent 85-90% of HIV cases in San Francisco: Dr Das said that "the MSM data reflect almost completely the data for the population at risk in San Francisco".

The proportion of gay men diagnosed with HIV linked to care rose to nearly 80%, and the proportion of those in care who were on antiretrovirals rose from 78% to 90%, with nearly three-quarters having an undetectable viral load (under 75 copies/ml).

San Francisco City’s HIV surveillance system includes mandatory reporting of viral loads, and the DPH was able to calculate two different measures of so-called 'community viral load' (CVL) in the HIV-positive population attending care. They calculated both the mean of the most recent viral load test reported for all individuals in care, and also the cumulative sum of all viral loads.

“Community viral load acts like a virometer,” said Dr Das, “a measure of the temperature of the epidemic.”

They found that the mean CVL was about 23,000 each year between 2002 and 2005, but then started to fall and was around 15,000 by 2008.

At the same time the number of new HIV diagnoses fell from 796 in 2004 to 434 in 2008. The association between reduction in viral load and new diagnoses was statistically significant (p = 0.019). However it is important to note that this is only a measure of the correlation between viral load and diagnoses: it doesn’t prove one caused the other.

A Centers for Disease Control (CDC) algorithm, which calculates the likely true incidence of HIV from the diagnosis and testing-frequency data, enabled the researchers to estimate that the actual number of HIV infections in the city fell in two years by 34%, from approximately 930 in 2006 to 620 in 2008. However, due to the margin of error in this method of calculating incidence, this was not statistically significant (p = 0.3) so cannot be said to prove that a real decline is yet happening.

The study has one significant limitation in that it could not include the viral load from undiagnosed individuals in its calculation of CVL, though a reduction in the undiagnosed proportion would lead indirectly to a reduction in CVL due to more people on treatment.

Interestingly, the reduction in new diagnoses and estimated incidence occurred within a context of significant increases in sexually transmitted infections (STIs) including rectal gonorrhoea and syphilis. Dr Das said that serosorting practices between gay men may be the reason this rise did not appear to impact on new infections.

“Our findings support the hypothesis that wide-scale early antiretroviral therapy can have a preventative effect at population level,” commented Dr Das. She said that CVL was a useful ‘upstream’ predictor of the likely number of new infections, and could therefore be used to calculate future resources and prevention needs.

Testing increases ‘exponentially’ in Washington DC

Meanwhile on the east coast of the USA, a comprehensive drive to increase HIV testing in Washington, DC has led to a significant rise in the CD4 count of those tested and a near-halving of the proportion who develop AIDS symptoms in the first year after diagnosis, though it has not yet led to falls in HIV diagnosis.

Washington DC has the highest HIV prevalence in the general population of any US city – around 3%. From 2006 three separate campaigns were initiated to increase the number of sites that performed opt-out HIV testing in routine medical settings and the number of tests they did; to encourage members of the public to test; and to improve the proportion of people who test positive who access care.

There had been approximately 20,000 HIV tests performed in Washington DC in 2004, and 35,000 at the start of the campaign in 2006. By 2009 this had risen to 93,000.

The number of HIV diagnoses increased by 17% between 2004 and 2007, from approximately 1100 to 1300, and this was significant. An apparent fall to 1100 diagnoses in 2008 reflects reporting delays.

In the DC setting, unlike San Francisco, increasing testing increased the number of new diagnoses, as many testers had never tested before and were late-presenters. But the increased testing rates did shorten the average time between infection and diagnosis.

Median CD4 count at diagnosis increased from 216 in 2004 to 343 in 2008 (p = <0.05), and the proportion who had AIDS-related symptoms within the first year after diagnosis fell from 47% to 28%.

Linkage to HIV care improved. A quarter of patients diagnosed in 2004 had not attended an HIV clinic for a follow-up appointment within a year of diagnosis; by 2008 this fell to just over 5%.

There was no increase in the number of medical providers performing tests, but a significant increase in the number of tests each one performed, and a significant increase in the number of non-medical sites offering tests, such as community-based organisations.

One of the most significant contributors to the increase in both testing and linkage to care, commented presenter Amanda Castel, was the involvement of the Washington DC jail in the testing programme. One-third of the patients who had rapid, as opposed to conventional, HIV tests were tested in prison, and the jail also has an active programme linking released prisoners to care, providing 28 days’ worth of antiretrovirals on release.

Further information

You can view abstract 33 and abstract 34 on the official conference website.

You can also view a webcast and slides of this session on the official conference website.


Das-Douglas M et al. Decreases in community viral load are associated with a reduction in new HIV diagnoses in San Francisco. Seventeenth Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 33. 2010.

Castel A et al. Monitoring the impact of expanded HIV testing in the District of Columbia using population-based HIV/AIDS surveillance data. Seventeenth Conference on Retroviruses and Opportunistic Infections, San Francisco, abstract 34, 2010.