The researchers calculated a quantity called assortativity.
This measures how likely members of a population were to be directly connected to other members of a
population in a cluster, which implies direct transmission from one person to
another, though it cannot rule out an undiagnosed third person being in between
them. An assortativity figure of zero implies that people aren’t connected any
more than would be expected by random mixing.
This was the case with people whose ‘risk factor’ was that
they injected drugs. The zero assortativity seen in this group means that
infection via needle sharing was in fact probably not how most of them acquired
HIV – they probably got it through sex like everyone else. Los Angeles county
has had syringe and needle exchange since 1994 and this probably shows that HIV
infection via needles is rare, thanks to this.
Assortativity in MSM and in heterosexual men and women (to
each other) was high, which is what you’d expect.
But it was also quite high in TGW, although not as high as in heterosexuals
or MSM. This implies a degree of sexual contact between TGW.
Numerically, TGW were most likely to be connected in a
cluster (not necessarily directly) to MSM, but that was because MSM were the
most numerous group. They were in fact 22% less likely to be connected to MSM
than one would expect through random mixing. In contrast they were 45% more
likely to be connected to cisgender heterosexual men, and 4.5 times more likely
to be connected to other TGW.
Partners of TGW also clustered together strongly. If a
non-TGW was in a cluster which contained a TGW, then they had a ninefold higher
likelihood of also being connected to another TGW.
This implies, as
a similar study from San Francisco found last year, that there is a
distinct population or network of cisgender men – who may define as MSM or
heterosexual – who either sometimes or primarily have TGW as their partners.
Manon Ragonnet-Cronin said that phylogenetic analysis could
help to identify the characteristics of these men so that prevention messages
suitable to TGW and their partners can be devised and interventions made.
Given the epidemiology, TGW who are partners of these men may be either
undiagnosed or at very high risk and could be contact-traced so they can
receive either HIV treatment or pre-exposure prophylaxis (PrEP).