This study investigated risk factors for SARS transmission from intubated patients to healthcare workers during the 2003 SARS outbreak in Toronto, Canada. The study found that healthcare workers who were present in the patient's room during intubation or did not implement proper infection control precautions had higher odds of contracting SARS. Not wearing protective equipment like N95 masks, gowns, and gloves during high-risk procedures increased the risk of transmission. The presence of "super-spreader" patients who infected many healthcare workers also increased the likelihood of SARS spreading to staff.

1. Article Critique
Risk Factors for SARS Transmission
from Patients Requiring Intubation:
A Multicentre Investigation in
Toronto, Canada
Citation: Raboud J, Shigayeva A, McGeer A, Bontovics E, Chapman M, et
al. (2010) Risk Factors for SARS Transmission from Patients Requiring
Intubation: A Multicentre Investigation in Toronto, Canada. PLoS ONE
5(5): e10717. doi:10.1371/journal.pone.0010717
By: Swarali S Tadwalkar
PHC 6002: Infectious Disease Epidemiology [Fall 2014]

2. Summary
• 2003 Toronto SARS outbreak
• Greater transmission in health-care settings
from patients to volunteers, visitors, HCWs
• Controversy regarding procedures and behaviors
resulting in greater transmission
• Activities/behaviors posing greater risk of
transmission among HCWs from patients
• Retrospective hospital study to assess risks and
behaviors associated with SARS-CoV
transmission

3. Major Objectives
• Assessing behaviors posing major risk of
transmission to HCWs (Health-care Workers)
• Assessing risk of SARS-CoV (SARS Coronavirus)
transmission associated with
▫ Adherence to infection control precautions
▫ Performance of high-risk procedures adjusting for
patient-related characteristics

4. Hypothesis/Research Question
• To identify risk factors associated with
transmission of SARS-CoV from patients
requiring intubation to HCWs (Health-care
Workers) involved in their care in 20 hospitals
in Toronto, Canada during the Toronto SARS
Outbreak in 2003

6. Study sample
• Size: 624 HCWs (90% of 697)
• Health-care Workers (HCWs) present in 20
hospitals in Ontario, Canada
• Identified by review of patient charts, work
schedules, assignments, on-call schedules and by
asking individuals being interviewed to recall
which other staff members were present
• Eligibility: HCWs providing care to intubated
patients during treatment and transportation,
entry in patient room and direct patient contact
from 24 hours before to 4 hours after intubation

7. Source and Target population
HCWs
Working in
20 hospitals
in Ontario,
Canada HCWs
providing
care to
intubated
SARS
patients

8. Exposure
• Intubated SARS patients (details obtained through
review of health records)
▫ Who met clinical and epidemiological criteria
▫ Laboratory confirmed cases of SARS
• High risk exposure: Presence in patient
room/involved in transportation either during 24
hours prior to intubation or at any time when
precautions were not implemented
• SARS-CoV
▫ Detected through serum antibodies
▫ Isolated by cell culture by clinical/autopsy specimen
▫ SARS-CoV RNA detected by RT-PCR from two
specimens by two different laboratories

9. Outcome
• Infected HCWs from intubated patients
• SARS Contraction among HCWs during
intubation
▫ Laboratory confirmation
• Assessing other risk factors
▫ Protective equipment usage
▫ Presence in ECG room
▫ Exposure to eye/mucous membranes
▫ P/F ratio

10. Chance
• Includes 95% Confidence Intervals and p-values
• Total sample size = 624 (90% of 697) which gave
enough power to the study
• Diseased cases = 26 (4.2% of Total)
• P-values were less than 0.05 for all Multi-variate
analysis for transmission from patient to HCWs
• Large Confidence Intervals for few statistics
indicating lack of power
• Narrow interval for transmission when HCW
was present during intubation

11. Bias
• Selection Bias
▫ Selection of hospital cases of SARS may differ in:
 Severity
 Transmissibility
 Biologic variation in manifestation of symptoms
• Information Bias
▫ Interviewer bias: HCWs were interviewed through
structured questionnaire
 Variability in face-to-face and telephonic interviews
▫ Recall bias
 Recalling presence of other staff-members
 With respect to information asked in the questionnaire
▫ Variation in the protection measures used
▫ Secondary infection from other sources during the
outbreak

12. Confounding
• Heterogeneity in transmission of SARS-CoV may
have caused some confounding
• Super-spreader effect confounded by P/F ratio since
maximum transmissibility occurred in low P/F ratio
(Over-estimation of super-spreader effect)
• Individual patient characteristics may have differed
(1 patient infecting 7 or 8 versus 1 patient infecting
2)
• Possibility of secondary infections and other sources
of hospital infections or through other infected
HCWs
• Infected HCWs were more likely to be paramedics
with lesser infection control training and hence,
there was variation in the degree of protective
equipment used

13. Causality Criteria
• Strength of Association
▫ Odds Ratio: Statistically significant values since
p<0.05
▫ Confidence Intervals: Significant
• Biologic Plausibility
▫ SARS studied widely specially after a major outbreak
in 2003
▫ Air transmission of SARS-CoV established through
immunologic studies
• Dose-response
▫ Not shown explicitly in the study
▫ Effect of super-spreader widely mentioned with 1
patient infecting more than 5 HCWs and
simultaneously 1 patient infecting only 2 HCWs

14. Causality Criteria (contd.)
• Temporality
▫ Retrospective Cohort study hence, temporality
established clearly
▫ Exposure to SARS-CoV of intubated patients
followed by SARS infection in HCWs involved in
their care
• Consistency
▫ Studies concerning transmission of infection from
patients to HCWs
▫ Lack of use of protection equipment affect
consistency of transmission

15. Results
Figure 1: Classification and regression tree analysis of risk factors for
SARS transmission, allowing patient specific covariates

16. Results (contd.)
• Analysis using Classification and regression trees (CART) and Generalized
Estimating Equation (GEE) to account for variability
• Categorical variables: Chi-square and Fischer’s exact test
• Continuous and ordinal variables: Wilcoxon rank-sum test
• 26 HCWs acquired SARS, all survived and no intubation was required
• All Odds Ratios have significant 95% CI and p-values <0.05
• Large 95% CI indicate lack of statistical power
• HCWs who were infected were more likely to be paramedics with lesser
infection-control training

17. Implications of Study
• Puts light on precautions required in Health-
care settings for infectious diseases
• Strong adherence to barriers for prevention of
infections in HCWs from infected patients
• Last outbreak of SARS was in 2003
• Other forms of Respiratory viral outbreaks seen
like MERS

18. Future Research
• More extensive research on viral respiratory
infections including both clinical and
epidemiological, required to control outbreaks
• Lack of protective measures in health-care
settings is a concern since it often elevates
transmission
• Knowledge of risk factors is essential to
understand etiology and transmission
• Every outbreak provides learning avenues and
research motivation

19. Thank you!!
Questions??