Download Workbook Method - World Bank Group

Modelling the expected distribution
of new HIV infections
by exposure group
Purpose
• To understand the sources of new HIV
infections and the importance of each
• To use this information for program
planning
2007 en
2
Methods
The spreadsheet uses
• the current prevalence of HIV infection
• numbers of individuals with particular exposures, and
• the rates of these exposures
to calculate the expected incidence of HIV
infection over the coming year
2007 en
3
Major modes of transmission of HIV in adults
• Sexual contact
– Commercial sex
– Casual sex
– Marital sex
– Men who have sex with men
• Injecting drug use
• Blood transfusions
• Medical injections
2007 en
4
New infections from sexual transmission
• New infections = f (HIV prevalence in
partners, number of
partners, acts per partner,
transmission probability
per act, STI co-factors,
condom use)
• Protection
2007 en
=
condom use
5
Probability of HIV Transmission (sexual)
I = S [ 1- { pB[1-β´ ]α(1-ν) + p(1-B)[1-β]α(1-ν) + (1-p) }n ]
S = Number susceptible
p = HIV prevalence in partners
B = prevalence of STIs in partners
β
α
ν
n
2007 en
=
=
=
=
probability of transmission (with STI (β´), without STI (β) )
contacts per partner
proportion acts protected by condom use
number of partners
6
Probability of HIV Transmission (IDU)
I = S [ 1- { p [1-β ]α(1-ν) + (1-p) }n ]
p = HIV prevalence in needle sharing partners
β = probability of transmission
α = contacts per partner
ν = use of sterile needles
n = number of partners
2007 en
7
New infections from unsafe medical injections
• New infections = Population * [1 – (1 – Ps * Pt * Pc)n]
Ps
Pt
Pc
n
• Protection
2007 en
=
=
=
=
Prevalence (total population)
Transmission probability
prevalence * % unsafe needles
number of injections per year
= % unused or sterile needles
8
New infections from blood transfusions
• New infections = number of unscreened
units transfused
* HIV prevalence
* (1 – HIV prevalence)
• Protection
2007 en
= percent of units screened
9
Data needs
Population sizes
HIV prevalence
STI prevalence
Number of acts
Number of partners
Percent protection
Transmission probability
2007 en
10
Data needs
2007 en
Population sizes
Surveillance, MOH statistics
HIV prevalence
Surveillance
STI prevalence
Special studies
Number of acts
BSS
Number of partners
DHS, BSS
Percent protection
DHS, BSS, service statistics
Transmission probability
Literature
11
Distribution of infections
Injecting Drug Use (IDU)
Partners IDU
Sex workers
Clients
Partners of Clients
MSM
Female partners of MSM
Casual heterosexual sex
Partners CHS
Low-risk heterosexual sex
No risk
Medical injections
Blood transfusions
2007 en
14
Blood transfusions
Medical injections
No risk
Low-risk heterosexual sex
Partners CHS
Casual heterosexual sex
Risk group Female partners of MSM
MSM
Partners of Clients
Clients
Sex workers
Partners IDU
Injecting Drug Use (IDU)
0
5
10
15
20
25
30
35
Percent
2007 en
15
Priority populations and programs
Route
Population
Intervention
Sex work
SW and clients
Out-reach, condoms, STI Treatment
Casual sex
Youth, military, truckers, etc.
Out-reach, condoms, STI
Low risk
Married couples
Promote testing
IDU
IDU
Risk elimination, harm reduction
MSM
MSM
Out-reach, condoms, STI Treatment
Injection
Patients
Sterile needles
Blood
Transfusion recipients
Screening
Mother-to-child
Pregnant women
PMTCT
2007 en
16