Two studies presented at last month’s 18th Conference on Retroviruses and Opportunistic Infections (CROI 2011) both found that gay men diagnosed with HIV considerably reduce the amount of sex they have that could pass on their infection.
However the two studies disagreed on how much men reduced their risk behaviour, and for how long they sustained this reduction.
The two prospective cohort studies, from Amsterdam and San Francisco, did not look at gay men’s behaviour over the same time frames so cannot be directly compared.
The Amsterdam study followed a group of 206 initially HIV-negative gay men and monitored the sexual risk behaviour of those who acquired HIV from four years before the date of diagnosis to four years afterwards. They were thus able to determine what the men’s baseline behaviour had been before diagnosis.
As a long-established cohort, it was also able to compare the behaviour of men diagnosed before HIV combination therapy came along (1984-1995) and after (1996-2008).
The San Francisco study only followed the risk behaviour of its 237 subjects from the date of diagnosis, but followed behaviour out to twelve years after diagnosis.
In addition, while the Amsterdam study only monitored the prevalence in unprotected anal intercourse (UAI) regardless of partners’ HIV status, the San Francisco study looked at rates of anal intercourse (AI) with all partners and partners of negative or unknown HIV status as well as rates of unprotected insertive anal intercourse (UIAI – the kind of sex most likely to transmit HIV) with partners of known and unknown status.
It also conducted a second analysis in which it took account of the effect of viral suppression by regarding unprotected sex by men with viral loads below 500 copies/ml as representing zero risk of transmission.
In Amsterdam, it was found that HIV diagnosis produced an immediate fall in the amount of unprotected anal intercourse men had. Before diagnosis 68% of men had had UAI (defined as “not (always) using a condom during anal sex”) in the previous year. One year after diagnosis this had reduced to 38% and thereafter the proportion of men who had UAI continued to decline more slowly: four years after diagnosis it was 32%.
In the post-HAART era, however, post-diagnosis UAI rates decreased a lot less and started to increase again after the first post-diagnosis year. The proportion of men who had had UAI in the previous year was 61% four years before diagnosis, 72% at diagnosis, 53% one year post-diagnosis, then back to 61% four years after diagnosis.
The San Francisco study presented an apparently very different pattern of behaviour. This study counted the number of partners men had had in the previous three months. This was ten at diagnosis but then declined to seven two years after diagnosis. It then increased again to 8.5 five years after diagnosis and then declined again, reaching 3.5 ten years after diagnosis.
This absolute decline in the number of partners could be accounted for by age and the acquisition of primary partners, but it was notable that the proportion of sex with partners of negative or unknown HIV status followed a similar pattern of initial decline, then resurgence, then final decline. At diagnosis the men in the study had averaged six partners in the last three months of negative or unknown HIV status (60% of all partners); two years after diagnosis it was 2.5 partners (36% of all partners); five years after diagnosis it was back to 6.25 partners (69% of all partners), but by year ten it was down to one partner (29% of all partners).
Note that this was all anal sex, protected or not. In the case of unprotected insertive anal sex (UIAI, the behaviour most likely to pass on HIV) it declined from four UIAI partners in the last three months at baseline to one five years after diagnosis, and then very slowly increased to 1.5 at year ten. The proportion of insertive sex that was unprotected became larger than the proportion with negative or unknown-status partners nine years after diagnosis, perhaps indicating that by this time the majority of men were only having UIAI with other men known to have HIV.
The majority of UIAI was with other HIV-positive partners throughout. The number of partners of negative or unknown status with whom the men had UIAI declined from 1.8 in the last three months at baseline to 0.57 after a year and was only 0.14 after five years.
The researchers calculated that even without taking viral suppression into account, this meant that the San Francisco men reduced the risk of their passing on HIV by 71% a year after diagnosis, 87% two years after diagnosis, and 92% five years after. If the proportion with viral loads under 500 was taken into account, then gay men reduced their chance of passing on HIV by 97% after two years and maintained that reduction in risk.
Comparing the two
It’s difficult to compare the two studies directly because but there is not one single measure they use in common. The proportion of men in the Amsterdam study who had unprotected anal sex in the post-HAART era only declined by 20% a year after their HIV diagnosis, whereas the San Francisco men had reduced the amount of insertive unprotective sex they had by nearly 50% at this point. They had reduced UIAI by 75% four years after diagnosis, by which time the UAI levels in Amsterdam men were back almost to baseline.
These figures do not, however, take account of the possibility of “strategic positioning”: the San Francisco men might be having a higher proportion of sex that was unprotected, including with negative or unknown-status partners, if, post-diagnosis, the majority of that sex was as the passive partner. We can’t tell from this study.
It is noticeable that, in the San Francisco study at least, the majority of the reduction in the risk of transmission was due to behaviour change, rather than the reduction in infectiousness due to decreased viral load.
Heijman RLJ et al. Changes in sexual behaviour after HIV diagnosis among MSM who seroconverted before and after the introduction of ART. Eighteenth Conference on Retroviruses and Opportunistic Infections, Boston, abstract 1034, 2011.
Vallabhaneni S et al. Seroadaptive tactics adopted by HIV-positive MSM can contribute to profound and sustained reductions in HIV transmission risk following HIV diagnosis. Eighteenth Conference on Retroviruses and Opportunistic Infections, Boston, abstract 1038, 2011.