Can Tho has a concentrated epidemic, but one that is very
mixed. Among its approximately 5000 people diagnosed with HIV are 109 women
classed as being female sex workers (FSW), 131 men whose risk was being a
client of a female sex worker (MCF), 615 men who have sex with men (MSM), 1446
people who inject drugs (PWID) and 1776 low-risk women (LRW).
Because there are far more low-risk women in the province
(464,000) than ones known to be sex workers (just over 2000) or people who inject drugs (about
3150) and even MSM (about 62,000), HIV prevalence in the groups is very
different from the absolute number of infections. It is currently estimated as
0.4% in low-risk women, 1% in MSM, 5.4% in sex workers, 8.8% in their clients,
and no less than 46% in people who inject drugs.
Using surveillance data, Williams and colleagues were able to
map out a complex network of exactly how common HIV transmission was between
different groups of people – who was infecting whom – and calculate the R0
number of each group. He divided MSM and FSWs into ones who were, and
were not, injecting drugs.
The overall R0 within the HIV-positive population
was 22. This is very high: by comparison, the R0 in sub-Saharan
Africa is about 5. This implies that if ART was given uniformly, 96% or more of
the HIV-positive population would have to be on ART and virally suppressed for
treatment as prevention alone to end the epidemic.
There are, however, vast disparities in the R0 of
different groups. Williams estimated that in terms of infections occurring within
the epidemic at any one time, 41% of infections involved a person who injects drugs either as a
transmitter or recipient and 30% a female sex worker. People who belonged to more than one
group bore an even greater burden of infections/transmissions: 43% of
infections involved an MSM who injected drugs, either as receiver or
The most striking disparities became clear when
the direction of infection was ascertained: whether particular groups were more
likely to be receivers or transmitters, and who they primarily passed HIV on to or acquired HIV from. This varied by a factor of over 100,000 according to which group a person belonged to.
Because infection in low-risk women was rare, the number
of infections each individual transmitted to them was itself low. The R0 number
of male clients of sex workers towards
low-risk women, for instance, was only 0.01: this means that only one in 100
male clients of sex workers would transmit HIV to a low-risk woman in their
lifetime. The same was true of people who inject drugs who might also be partners of low-risk women, and for
bisexual MSM, only one in 1000 would transmit HIV to a low-risk woman. Infection
by low-risk women was so low as to be
impossible to estimate. This means that a generalised epidemic involving the
heterosexual population is unlikely to happen in Vietnam.
The R0s of transmission to and from other
groups were much higher, and all over 1, implying a concentrated epidemic that
will be self-sustaining without more intervention.
The model found that the average number of HIV
infections transmitted by one MSM in Can Tho in his lifetime was 6.1: 4.1 to non-drug-injecting
MSM and 2 to MSM/people who inject drugs. MSM who inject drugs transmitted more: their total R0 was 27.4, of which 19 infections would be transmitted to other people who inject drugs, 4 to other MSM who did not inject drugs, 3.3 to other MSM who also injected
drugs, and 1 to a female sex worker who injected drugs.
Again, however, this is the average number transmitted
by individuals in specific group: because there are far fewer MSM who inject drugs than MSM, the absolute number of infections transmitted by MSM will be
bigger than the number transmitted by MSM who inject drugs.
People who inject drugs had an R0 per individual of 21.5: 19 to
other people who inject drugs who were not MSM or FSWs, 2 to other people who inject drugs who were MSM and 0.5 to
other people who inject drugs who were FSWs.
Female sex workers had a larger R0 still.
Because there were not many sex workers, they were only the source for 12% of
transmissions of HIV: 10% to male clients and 2% to people who inject drugs. However, the
infections were almost entirely one way – from sex workers to their clients. A
male client only had a 1-in-17 chance of passing HIV on to a female sex
worker during his lifetime. On the other hand, the R0 of a female
sex worker was a startling 77: because female sex workers had so many clients, each one was passing HIV on to an average of 77 other people during her lifetime, and female sex workers who were
also injecting drugs even more, a total of 99: 77 infections to male clients, 19 to people who inject drugs, two
to people who inject drugs who are also MSM and one to another female sex worker who injects drugs.
So, according to the model, the average R0, the number of individuals
each person with HIV will infect during their lifetime, is as follows:
- Female sex workers who also inject drugs: 99
- Female sex workers: 77
- MSM who also inject drugs: 27.4
- Other people who inject drugs: 21.5
- MSM who do not inject drugs: 6.1
- Clients of female sex workers: 0.061
- Low-risk women: negligible