Transcript X The HIV Transmission Gradient: Relationship-level Estimates of Risk X X X X X X X X David C. Bell, Isaac D.

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The HIV Transmission Gradient:
Relationship-level Estimates of Risk
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David C. Bell, Isaac D. Montoya
Affiliated Systems Corporation, Houston
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BACKGROUND
RESULTS
Of critical importance in the transmission of HIV are “gatekeepers,” the
HIV-negative partners of persons who are HIV-infected. These are the
persons at risk and they are the persons who can eventually spread the
disease further. And since the highest infectivity comes in the first months
after infection, usually before knowledge of infection, the behavior of these
“gatekeepers” while they are HIV- is critical.
There is a gradient of infection potential by which HIV can diffuse from the
HIV+ population through “gatekeepers” to the rest of the population.
Sex Risk by Gender
.478
[34]
HIV+
.124 [15]
Male
.048 [15]
.542
[1]
.323
[1]
RESEARCH METHODS
Proximal
Partner
Female
.049
[3]
.286 [11]
We collected a representative
sample of 267 persons
Random walk
consisting of 169 quasi-randomly
recruited index persons and 98 of
their sex and drug injection partners
Peer recruitment
from high drug use neighborhoods.
They described 3254 relationships
involving 1538 persons.
Matched nonuser
sample
Female
Random
i
Risk
part.
Coupon
i
Risk
part.
risk indexik
Male
.242 [11]
.464
[4]
Risk
part.
HIV+
120
Estimating Potential Transmission
HIV+
.281 [11]
Index
Neighborhood
advertising
.082 [37]
Distal
Partner
Female
.000 [1]
.000
[8]
.330
[14]
.375
[34]
Informant
i
.141[37]
.305
[205]
Injection Risk by Gender
Risk
part.
Index
i
.202
[205]
.083
[12]
.000
[6]
Risk
part.
Index
i
HIV Risk Index:

cijk 
 1   (1  f j ) 
 j

Distal
Partner
Male
.249 [7]
.355
[84]
.282
[8]
Proximal
Partner
Male
.156 [2]
.215 [10]
0.001
0.0005
0.000025
0.0000125
0.02
0.0087
0.0008
0.00035
0.0067
0.000067
NETWORKS AND RANDOM MIXING
Networks organize the possibility of disease transmission. In this set of seven interlocking
networks, there are three HIV+ persons (in red). They are linked to eight other persons by risk
links (also in red). None of the other persons in the networks is at direct risk.
.356
[84]
Distal
Partner
Female
.307 [10]
Proximal
Partner
Female
Imputed per-act transmission probabilities:
.313
[162]
.325
[34]
.000
[2]
.312
[2]
where
i is respondent
k is partner
j is risk behavior
fj is per-act risk
cijk is number of 30-day risk behaviors
Vaginal sex from male to female without condom
Vaginal sex from female to male without condom
Vaginal sex from male to female with condom
Vaginal sex from female to male with condom
Receptive anal sex without a condom
Insertive anal sex without a condom
Receptive anal sex with a condom
Insertive anal sex with a condom
Shared injection equipment (infected)
Shared cooker, cotton, or water
Proximal
Partner
Female
.314
[8]
Female
Distal Partner
Male
The HIV Transmission Gradient
0.70
0.60
HIV Risk Index
We use a “pipeline”
estimate of HIV risk.
By estimating the
“HIV carrying capacity”
of the joint behaviors
between the partners,
we can estimate the
conditional probability
of HIV transmission
if the partner were HIV+.
Random
i
.397 [11]
.183 [27]
.025
[3]
Proximal
Partner
Male
HIV+
Distal
Partner
Male
.000[23]
0.50
0.40
Injection risk
0.30
Sex risk
0.20
0.10
0.00
Among HIV+
HIV+ -->
Proximal
Partner
Proximal
Partner -->
Distal Partner
Among Distal
Partners
CONCLUSIONS
Relative Prevalence
We projected HIV transmission by using 30-day risk estimates and projecting them over the
next 20 years. Next we examined the effect of network organization by modifying our data.
We took all links from a given person and randomly selected a different partner from the list of
partners and then projecting transmission over these relationships.
Random mixing
projection
1200
1100
1000
900
800
700
600
500
400
300
200
100
Network-based
projection
Years
0
5
10
Years
15
20
• The network organization of sex and injection behaviors
reduces transmission by almost 70% over random mixing.
• In general, the highest levels of risk we observe are among
HIV+ individuals.
• There is an HIV transmission gradient that operates to limit
the spread of HIV.
• In the general population, people are acting approxi-mately
as if their sex partners were HIV+.
• Among drug injectors (at least cocaine injectors), we find
very high levels of risk.