Table 1 Calculation of magnitude of Transmission Pathways

The total expected number of transmission acquired HIV cases is

= BLOOD x T x (A + B + C + D)

Where A is the number of infected donors, who donate despite knowing they are HIV+

A = POP x PREV x INFDON x (1-(1-((1-S_NAT) x (1-S_EIA))))

Where B is the number of infected donors during the windows period where neither the Nucleic Acid Test nor ELISA test can detect HIV.

B = PSC x W_NAT/(365.25 x YRS_PS)

PSC = POP x INC x YRS_PS x DON

Where C is the number of infected donors, who were asymptomatic and were not detected due to false-negative screening results resulting from the period where only the Nucleic Acid Test (but not the ELISA) can detect HIV (C₁) and the period where both NAT and ELISA can detect HIV (C₂).

C = C₁ + C₂

C₁ = PSC x (1-S_NAT) *(W_NAT-W_EIA)/(365.25 x YRS_PS))

C₂ = PSC x (YRS_PS-W_EIA/365.25)/YRS_PS x (1-S_NAT) x (1-S_EIA)

Where D is the number of infected donors who are correctly detected but whose blood nevertheless is allowed to be donated due to laboratory misclassification errors.

D = (PSC + (POP x PREV x INFDON)- 2 (A + B + C)) x LAB

BLOOD = the average number of persons who become infected via transfusions from a single infected donor

T = Probability that recipient of infected blood donation will become infected (%)

POP = numbers aged 18–69 in group.

DON = Percentage of people in group who donate blood (%)

INFDON = Percentage of people who know they are HIV+ but nevertheless donate.

PREV = Prevalence rate

INC = Incidence rate per annum

YRS_PS = Pre-symptomatic period: average length of time from infection to symptoms (years) based on weighted average of first-time to repeat donors over the 10 year period

PSC = Number of pre-symptomatic donors

B₁ = number of infected donors during the window period of the Nucleic Acid Test.

B₂ = number of infected donors during period when the Nucleic Acid Test is working but EIS is still in window period

S_NAT = Sensitivity of the Nucleic Acid Test

W_NAT = Duration of the Nucleic Acid Test window period (days)

S_EIA = Sensitivity of the anti-HIV test

W_EIA = Duration of the anti-HIV window period (days)

LAB = Laboratory misclassification error rate (%)