number of other presentations at the conference looked at transmission
study by the US Centers for Disease Control (CDC) looked at 41,284 viral
genetic sequences in the general US database and found that 31% of them were genetically linked to at least one other virus. They found that 3982 (10%) were
linked to at least four other viruses. This 10% with high linkage were more
likely to be young (50% of people with at least four links in their 20s
compared with 34% of the whole group) and gay (86% with more than four links,
61% of the whole sample). White people were more likely than average to have
more than four links and black people less likely; 41% of people with four or
more links were black but comprised 47% of the sample. This is one more finding that seems
to be hard to reconcile with the much greater incidence of HIV in black people,
but it may make sense if black people take their partners from a much more limited
group with very high HIV prevalence.
transmitted infections (STIs) may be another explanation for higher HIV incidence.
In an analysis from San Francisco, 45% of people diagnosed between 2005 and
2011 were in a transmission cluster, but comprised 57% of those whom incidence assays
showed had been recently infected. Recently infected cluster members were more
likely to be gay men, were younger and were twice as likely to have had a
recently diagnosed STI.
HIV infection was associated with being diagnosed with a STI during the time
interval that overlapped with when HIV transmission most likely occurred,
thereby pointing to intervention opportunities to interrupt HIV transmission,”
the researchers comment.
analysis is a versatile tool. In New York, a large study used it to look at the geographic
spread of the virus within New York City (the illustration shows all the
clusters in the study with more than five members). Fifty per cent of the 39,135 individuals in
the study were found to be phylogenetically connected to at least one person
and 8801 (22.5%) to more than one. (The definition viral similarity here was that they were
no more than 2% genetically different.) Gay men were 2.4 times more likely to be in
a cluster of more than one person than heterosexual people, and people who inject drugs (PWID) half as likely.
New York has surveillance down to district level, the researchers could see how
transmitter-recipient pairs were linked geographically. They found that gay men
on average were furthest separated geographically from the person they acquired HIV from, in terms of where they lived: 10.3km on average, versus 9.0km and 7.8km for PWID and heterosexual people. Thirty-seven per cent of gay men in an infection
cluster had someone who lived outside New York in their cluster compared with
8.2% of heterosexuals, but were no more likely than heterosexual people to include in
their cluster someone from another country (this might be very different in,
say, London). On the other hand, and somewhat unexpectedly, PWID had the
greatest likelihood of having an international connection, with 18% of people in these clusters from outside the US.
study from the CDC looked at who, so to speak, strayed outside their infection
category. What percentage of linkages (this time looking at pairs, rather than
groups) were there between gay men and heterosexual women, heterosexual men and
female PWID, and so on? The majority of gay men (89%) were solely connected to
each other. But because there were so many more gay men in the sample than heterosexual
men, there were actually more heterosexual women in the sample who were
phylogenetically linked to a gay man than to a heterosexual man (1208 were
linked to a gay man and 486 to a heterosexual). This might seem
counterintuitive until one remembers that gay men are something like 50 to 100
times more likely to have HIV than heterosexual men.