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.
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.