Table 2. Observations and model parameters for Monte Carlo simulation OBSERVATIONS DTES OD deaths SIF OD events MODEL PARAMETERS IDU in DTES Non-fatal OD rate 2004 = 2005 2006 2007 2008W ALL 20.1 71 27.8 126 28.5 99 28.7 95 14.4 62 137.7 453 N (4700, 500) Scenario 1: N (0.05, 0.01) Scenario 2: N (0.2, 0.01) Scenario 3: N (0.3, 0.01) = March 1, 2004 to December 31, 2004/WJanuary 1, 2008 to July 1, 2008 doi:10.1371/journal.pone.0003351.t002 the facility. For instance, individuals can receive nurse-delivered education in safer injection practices which may reduce risk behaviours for overdose outside the facility. Over one-third of individuals report receiving this training in a representative cohort of SIF clients [32]. Although this issue requires further study, IDU who experience supervision of a nurse within the SIF may subsequently be more cautious when injecting in environments which are not supervised by trained emergency personnel [33]. OD Deaths Averted in SIF In conclusion, we observed that non-fatal overdose was a common occurrence at Vancouver’s SIF and, using a modelling technique based on evidence-based parameter values, we estimat- ed that the facility prevented between eight and 51deaths over the study period. This is equal to between 37.0 per cent and 6.1 per cent of overdose fatalities in the DTES over the same time or two to 12 averted deaths per annum over the study period. These findings are consistent with analogous evaluations and support the conclusion of the facility’s positive impact on public health in Vancouver’s Downtown Eastside [10]. Materials and Methods In this analysis, we sought to estimate the number of overdose deaths averted by a supervised injection facility using methods described previously [16,31,34,35]. This estimate was calculated by determining how many overdose events in the SIF would have been fatal had they happened outside the facility [16]. Specifically, the number of averted deaths is the product of the number of on- site overdoses multiplied by the ratio of fatal to non-fatal overdoses in the DTES [31], or: f ODAVERTED~nf ODSIF: f ODDTES nf ODDTES For each term in the equation, we used evidence-based estimates Table 3. DTES fatal overdose rate, non-fatal to fatal overdose ratio and SIF averted deaths from model 195.4 (108.7–282.1) 232.1 (127.8–336.5) 232.9 (127.7–338.2) 232.3 (126.4–338.1) 116.7 (63.9–169.6) 1010.7 (557.5–1463.9) 784.7 (605.6–963.8) 939.0 (722.6–1155.3) 937.9 (719.1–1156.7) 937.9 (720.5–1155.3) 470.3 (362.1–578.4) 4068.3 (3129.8–5006.8) 1176.4 (918.7–1434.1) 1408.8 (1099.8–1717.9) 1407 (1097.2–1717.3) 1407.3 (1099.6–1715.1) 705.8 (551.0–860.7) 6108.9 (4770.3–7447.6) Scenario 14 Scenario 25 Scenario 36 Scenario 1 Scenario 2 Scenario 3 Scenario 1 Scenario 2 Scenario 3 Scenario 1 Scenario 2 Scenario 3 Scenario 1 Scenario 2 Scenario 3 DTES fatal OD 5.1 (4.0–6.2) rate3 NON-FATAL TO FATAL OVERDOSE RATIO NON-FATAL OVERDOSES IN DTES SIF AVERTED OVERDOSE DEATHS 20041 9.7 (5.4–14.0) 39.0 (30.1–47.9) 58.5 (45.7–71.3) 7.3 (3.4–11.3) 1.8 (1.4–2.3) 1.2 (0.9–1.5) 36.3 (16.9–56.2) 9.0 (7.0–11.4) 6.0 (4.5–7.5) 2005 5.9 (4.6–7.2) 8.4 (4.6–12.1) 33.8 (26.0–41.6) 50.8 (39.6–61.9) 15.1 (1.9–28.3) 3.7 (2.8–4.6) 2.5 (1.9–3.1) 54.3 (6.8–101.8) 13.3 (10.1–16.5) 9.0 (6.8–11.2) SIF AVERTED OVERDOSE DEATHS (PROPORTION OF DTES OD DEATHS) SIF AVERTED OVERDOSE DEATHS (PER YEAR) 2006 6.1 (4.7–7.4) 8.2 (4.5–11.9) 32.9 (25.2–40.6) 49.4 (38.5–60.3) 12.1 (5.6–18.7) 3.0 (2.3–3.7) 2 (1.5–2.5) 42.5 (19.6–65.6) 10.5 (8.1–13.0) 7.0 (5.3–8.8) 2007 5.3 (4.1–6.4) 9.4 (5.1–13.7) 37.9 (29.1–46.7) 56.9 (44.4–69.3) 10.1 (4.3–15.9) 2.5 (1.9–3.1) 1.7 (1.3–2.1) 35.2 (15.0–55.4) 8.7 (6.6–10.8) 5.9 (4.5–7.3) 20082 5.3 (4.1–6.4) 9.4 (5.2–13.7) 37.9 (29.2–46.6) 56.9 (44.4–69.4) 6.6 (3.0–10.2) 1.6 (1.2–2.0) 1.1 (0.8–1.3) 23.0 (10.4–35.5) 5.6 (4.2–7.0) 3.8 (2.8–4.5) ALL 5.6 (4.3–6.8) 8.9 (4.9–12.9) 35.8 (27.6–44.1) 53.8 (42.0–65.6) 50.9 (23.6–78.1) 12.6 (9.6–15.7) 8.4 (6.5–10.4) 37.0 (17.1–56.7) 9.2 (7.0–11.4) 6.1 (4.7–7.6) 11.7 (5.4–18.0) 2.9 (2.2–3.6) 1.9 (1.5–2.4) 1March 1, 2004 to December 31, 2004 2January 1, 2008 to February 6, 2008 3Expressed as deaths per 1,000 person years 4Scenario 1: Non-fatal overdose incidence is 50 per 1,000 person years or 5% per person per year 5Scenario 2: Non-fatal overdose incidence is 200 per 1,000 person years or 20% per person per year 6Scenario 3: Non-fatal overdose incidence is 300 per 1,000 person years or 30% per person per year doi:10.1371/journal.pone.0003351.t003 PLoS ONE | www.plosone.org 4 October 2008 | Volume 3 | Issue 10 | e3351