group selected for study was a subgroup of the US Multicentre AIDS Cohort Study
(MACS), which is described in this recent report on life expectancy.
the purposes of this study, the researchers took a look at members of the study
who were HIV negative and recruited between 2001 and 2003, and included every
man who had been seen between January 2003 and September 2004 and had made at
least one other study visit up to September 2011. The length of “trajectory
time” over which men were studied ranged from six years, three months to eight
years, nine months.
each visit, men answered a questionnaire about their sexual risk behaviour
during the previous six months and their behaviour was then given a "sexual risk score"
according to the most HIV-risky sexual activity the person had taken part in
during that time. These ranged from zero points for no anal sex at all to six
points for unprotected receptive
anal intercourse with at least one HIV-positive or unknown-status partner. The degree of risk was broadly
in accordance with a previous study on how risky specific activities were in
gay men (see
this report). Partners' viral load, if known, was not a category captured in this study.
were 419 men in the study group, which was racially diverse (38% white, 42%
black) and with a median age of 38 (20% under 30). On average, 11% of the group
reported having had the highest-risk category of sex since their previous study visit.
feature of this study group is that a much higher proportion were reporting not
having anal sex at all than in many other studies: 43% reported they had not
had anal sex in the last six months on their initial visit, and this rose to
56% at the last visit. This may be due to the ageing of the cohort but may also
be due to higher-risk men dropping out of the study (people who were young, on
low income or black were more likely to drop out).
noted above, the participants fell into three risk groups according to their average sexual risk score, and how consistent it was over time: 63% low risk, 23% moderate risk and 14% high risk. People
generally remained in the same risk group. The likelihood of someone in the
low-risk group having an episode of high-risk behaviour over the following six
months was only 0.9%. In comparison, 29% of the moderate-risk
group were likely to have a high-risk episode in the following six months (this likelihood declined to 17% over the course of the study) and 71% of the high-risk group. This means that in the high-risk group, if they had high-risk sex in one six-month period, only 29% did not have high-risk sex in the following six-month period. There was no-one in the lowest-risk
group who had high-risk behaviour for two consecutive six-month periods:
conversely, 48% of men in the moderate-risk and 93% of men in the high-risk
group had high-risk behaviour for at least two consecutive six-month intervals
over the study.
average length of time over which people stayed in the high-risk category was
one year in the moderate-risk group
and two years in the high-risk group; in other words, even members of the
high-risk group did not stay high risk all the time; they went in and out of
being at risk of HIV as their relationship status changed or as they adopted
different sexual practices.
is best illustrated by a selection of 15 different members of the cohort, five
from each of three risk categories, where periods of high-risk sex are red,
periods of low risk or no sex are blue, and missing visits are blank:
The factor most strongly associated with being in the high-risk group was
having an annual income over US$20,000 – people in the higher-income category
were nearly five times more likely to be in the highest risk group than others
– higher income was also associated with being in the moderate-risk group, though not as strongly. The factor most strongly associated
with being in either the moderate- or the high-risk group (i.e. not in the low-risk group) was being white as
opposed to being black or Hispanic. White men were 3.9 times more likely to be
in the moderate-risk and 3.7 times more likely to be in the high-risk group.
was also strongly associated with being in the moderate- or high-risk groups,
with a 6% decline in the risk of being in the moderate group for every year older, and an 8% decline in the likelihood of being in the high-risk group.
and substance use were associated with being in the high-risk group (2.4 and
2.0 times more likely respectively), but not with being in the moderate-risk
biggest limitation of this study is that members of the MACS study are likely
to be highly motivated gay men who may take more account of sexual risk than
others: as already noted, an untypically large proportion did not have anal
sex. This may mean that in more typical gay populations, high-risk behaviour is
this is the first study ever to document gay men’s “risk careers” over a long
period of time and in such detail. Its findings, as the researchers say, “could
enable clinicians to efficiently screen and identify [gay men] who exhibit
‘seasons of risk’ for potential PrEP use”.