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# Modes of HIV Transmission UNAIDS by mikeholy

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```									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    =     condom use

2007 en                                                       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   =   probability of transmission (with STI (β´), without STI (β) )
α   =   contacts per partner
ν   =   proportion acts protected by condom use
n   =   number of partners

2007 en                                                                              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   =   Prevalence (total population)
Pt   =   Transmission probability
Pc   =   prevalence * % unsafe needles
n    =   number of injections per year

• Protection        = % unused or sterile needles

2007 en                                                 8
New infections from blood transfusions

• New infections = number of unscreened
units transfused
* HIV prevalence
* (1 – HIV prevalence)

• Protection    = percent of units screened

2007 en                                                 9
Data needs
Population sizes
HIV prevalence
STI prevalence
Number of acts
Number of partners
Percent protection
Transmission probability

2007 en                                   10
Data needs
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

2007 en                                                             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

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