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Table 1 Input values for the mathematical model

From: Needle and syringe programs in Yunnan, China yield health and financial return

Parameter

Description

Values

References

Biological transmission parameters

β

Probability of HIV transmission per injection with a contaminated syringe

0.001 - 0.005

[50–57]

η

Rate of disease progression from chronic infection to treatment-eligible stage

11-15%

[58]

μ c

Death rate for HIV-infected people in chronic stage

0.06-0.11%

[59, 60]

μ a

Death rate for HIV-infected people in treatment-eligible stage

3.00-7.86%

[59, 60]

μ t

Death rate for people on ART

0.06-0.11%

[59, 60]

Τ

Rate of diagnosed people on AIDS stage initiating treatment

0-10%

estimated a

σ

Rate of people on ART stopping treatment

15-20%

[61, 62]

Epidemiology and NSP parameters

p(t)

Prevalence among IDUs in Yunnan province

20-30%

[9]

N

Population size of IDUs in Yunnan province

SF = 2.5

95,000-125,000

estimated b

  

SF = 4.0

150,000-200,000

 

Ï€

Rate of new entrants into the IDU population

6000

estimated c

P n

Total number of syringes distributed through NSP per year (2002-2008)

8.75 × 106

personal communication d

R

Percentage of syringes obtained through NSPs

5-25%

[12, 13]

ω

Percentage of syringes distributed that are not used

0.5-1%

Assumption

Behavioral parameters

n

Average frequency of injecting per IDU per year (weighted average of daily and non-daily injectors)

300-800

[12, 31, 63–65]

s

Proportion of IDUs who share syringes

40-90%

[12, 14, 31, 66–69]

q

Proportion of injections that are shared for IDUs that share syringes

29.4%

[66]

δ t

Average number of times each syringe is used before disposal

2-4

[28–31]

δ p

Average number of times each non-shared syringe is used before disposal

1-3

estimated e

δ s

Average number of times each shared syringe is used before disposal

3-15

estimated e

Syringe cleaning parameters

P c

Proportion of syringes used multiple times by multiple people that are cleaned before re-use

20-40%

[12, 70]

ε c

Effectiveness of syringe cleaning

70-80%

[71, 72]

  1. a. This number is estimated by dividing the number of HIV-infected IDUs initiating ART each year by the total number of IDUs who live with HIV. It is important to note that ART is only initiated after 2004, hence Ï„ = 0 prior to 2004. We estimated that approximately 10% IDUs are on ART in 2008 and assumed a linear growth of percentage between 2004 and 2008.
  2. b. In Yunnan, official figures indicate that the number of registered drug users varies between 50,000 to 70,000 in the last decade [73–75]. Among these registered drug users, 55% are intravenous drug users period [14, 24, 25], which corresponds to an increase of ~30,000 to ~40,000 registered IDUs in the last decade. It is widely accepted that in China behind every registered IDUs there is about 2.5-4.0 implicit IDUs that are unregistered [26, 27]. The total number of IDUs in Yunnan is estimated to lie between 95,000 and 125,000 if the scaling factor equals to 2.5, whereas the population size is between 150,000 and 200,000 if scaling factor equals to 4.
  3. c. The entrance rate of IDUs is calculated from the variation of population size, whose minimum and maximum bounds are estimated to be 30,000 and 50,000 over the period 2002-2008 based on the above scenarios. Therefore, an average entrance rate is estimated to be 6000.
  4. d. The cumulative number of syringes distributed in Yunnan during the period 2002-2008 is approximately 875,000. This number is obtained through personal communication with stakeholders from China CDC.
  5. e. Given that the average usage of a syringe (δ t ) among Yunnan IDUs is about 3 [28–31], the average usage of a non-shared syringe (δ P ) is assumed to be less than 3 and greater than 1. Therefore, the average usage of a shared syringe (δ S ) can then be estimated to be 3-15 by equation δ t = s·q·δ s + (1 - sq)·δ P .
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BMC Public Health

ISSN: 1471-2458

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