The estimated mean age at infection was 35.6 among people
classed as definite ‘sources’ and 36.9 among ‘recipients’ – already challenging
the assumption that recipients would probably be younger. In 57% of cases the
recipient was older than the source, with an average age difference of 2.16
years. However, this situation was reversed in men under 30, where on average
the recipient was nearly nine years younger.
There was a clear and steady increase in the likelihood of the
source being the younger partner as both partners got older. It was
calculated that for every year older a recipient was, the average age of the
source of their infection only grew by 0.28 years (3.36 months).
When the recipient was under 25, their source partner was on
average ten years older; when 25-29, eight years older; and when 30-34, five
years older. However, the average age difference between pairs aged 35-39 was
zero, and after that source partners became younger than recipients; two years
in men aged 40-44, and 11 years among men over 45. (Of note, there was a long
‘tail’ in this study of older recipient men; several were in their late 60s.)
To put this another way: in recipients under 25, 92% of their
source partners were older than them; in 25-29 year olds, 51%; in 30-34 year
olds, 47%. But in 35-39 year olds, 35% were younger; in 40-44 year olds, 52% were
younger; and in men over 45, 71% were younger. The age at which the age
difference “crossed” was 38.4.
It is obvious why young men are more likely to be infected by
older men; there are simply more older HIV-positive men around. But why were older men more likely to be infected by younger? The researchers suggest three
possible explanations. Older men may simply be less sexually active; or they
may be better at maintaining condom use. Neither of these explanations seem to
be borne out by other studies of condom use in gay men, however; if anything,
younger gay men use condoms more.
The most likely explanation would seem to be that older gay
men with HIV are more likely to be diagnosed, virally suppressed on treatment
and therefore non-infectious.
The findings have implications for prevention; quick diagnosis and treatment may be the most relatively efficient way of preventing HIV transmission in men in their 30s, while men younger and older than this may, as groups, derive more benefit from pre-exposure prophylaxis (PrEP).