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Public Health Information Network (PHIN) Series II
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Public Health Information Network (PHIN) Series II

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  1. Public Health Information Network (PHIN) Series II Outbreak Investigation Methods: From Mystery to Mastery

  2. Series IISession III “Study Design”

  3. Series II Sessions

  4. Access Series Files Online • Session slides • Session activities (when applicable) • Session evaluation forms • Speaker biographies Alternate Web site:

  5. Site Sign-in Sheet Please submit your site sign-in sheet and session evaluation forms to: Suzi Silverstein Director, Education and Training Emergency Preparedness & Response Programs FAX: (804) 225 - 3888

  6. Session Overview • Session I summary and review: How we arrived at the research hypothesis • Analytic Epidemiology • Developing and testing hypotheses • Study Designs: • selection, implementation, and data analysis

  7. Today’s Presenters Amy Nelson, PhD, MPH Epidemiologist NC Center for Public Health Preparedness Sarah Pfau, MPH Moderator

  8. “Study Design” Learning Objectives • Understand what an analytic study contributes to an epidemiological outbreak investigation • Understand the differences in methodology between cohort, case-control, clinical trial, and cross-sectional study designs • Be able to describe the advantages and disadvantages of alternative study designs

  9. “Study Design” Learning Objectives (cont’d.) • Know how to assess which study design to apply during an outbreak investigation • Understand how to select cases and controls in a case-control study design • Know how to interpret odds ratios from case-control study data and risk ratios from cohort study data

  10. Session I Review

  11. Session I Review • Detected an outbreak of gastroenteritis at a university in Texas, based on a March 11 complaint from a student • Conducted: • active case finding • chart reviews • hypothesis generating interviews • 75 cases detected by March 12

  12. Session I Review (cont’d.) • Collected stool specimens • All negative for bacterial pathogens • Assume viral pathogen • Earliest date of exposure was March 5 • Conducted interviews with 7 of the earliest cases (all students) • Only commonality was 6 of 7 reported eating at deli bar of main campus cafeteria

  13. Session I Review (cont’d.) • Developed a leading hypothesis with the main campus cafeteria as the suspect • No source food identified • Interviewed 30 of 31 cafeteria staff members • Staff member who declined interview worked at deli bar* * Today’s session includes a follow up investigation with this staff member

  14. Session I Review (cont’d.) • Investigators closed deli bar on March 12th based on: • Link between 6 out of 7 interviewed cases and a common food source • Several unsanitary food handling practices identified via an environmental health investigation

  15. Basic Steps of An Outbreak Investigation • Verify the diagnosis and confirm the outbreak • Define a case and conduct case finding • Tabulate and orient data: time, place, person • Take immediate control measures • Formulate and test hypothesis • Plan and execute additional studies • Implement and evaluate control measures • Communicate findings

  16. Analytic Epidemiology Hypotheses

  17. Descriptive vs. Analytic Epidemiology • Descriptive Epidemiology deals with the questions: Who, What, When, and Where • Analytic Epidemiology deals with the remaining questions: Why and How

  18. Analytic Epidemiology • Used to help identify the cause of disease • Typically involves designing a study to test hypotheses developed using descriptive epidemiology

  19. Exposure and Outcome A study considers two main factors: exposure and outcome • Exposure refers to factors that might influence one’s risk of disease • Outcome refers to case definitions

  20. Case Definition • A set of standard diagnostic criteria that must be fulfilled in order to identify a person as a case of a particular disease • Ensures that all persons who are counted as cases actually have the same disease • Typically includes clinical criteria (lab results, symptoms, signs) and sometimes restrictions on time, place, and person

  21. Developing Hypotheses • A hypothesis is an educated guess about an association that is testable in a scientific investigation • Descriptive data provide information to develop hypotheses • Hypotheses tend to be broad initially and are then refined to have a narrower focus

  22. Example • Hypothesis: People who ate at the church picnic were more likely to become ill • Exposure is eating at the church picnic • Outcome is illness – this would need to be defined, for example, ill persons are those who have diarrhea and fever • Hypothesis: People who ate the egg salad at the church picnic were more likely to have laboratory-confirmed Salmonella • Exposure is eating egg salad at the church picnic • Outcome is laboratory confirmation of Salmonella

  23. TX Case Study Hypothesis and Study Design

  24. What do we know? • We know… • 75 cases reported by March 12 – all were students who lived on campus • All of cases sampled (n=7) reported eating at the main campus cafeteria • 6 of 7 at the deli bar • No illness among off-campus students, faculty, or community members

  25. What do we suspect? • We suspect the main cafeteria is the source of infection • We also suspect the deli bar, in particular, may be the outbreak source

  26. What can we “prove”? • Can we “prove” it? • Need to conduct an analytic study • Need a testable hypothesis

  27. What hypothesis will be tested? • Main cafeteria vs. other dining halls? • Within the main cafeteria: • Deli bar vs. grill? • Salad bar vs. hot entrée? • Food item vs. food handler at deli bar?

  28. Our Hypothesis A viral infection was spread by food(s) or beverage(s) served at the university’s main cafeteria between March 5 and 10.

  29. Source Population Town residents (39,000) Students (12,000) 2400 on campus 2000 meal plan

  30. Which Study Design?

  31. Types of Analytic Studies Two main categories: • Experimental • Observational • Experimental – exposure status is assigned • Observational – exposure status is not assigned

  32. Experimental Studies • Can involve individuals or communities • Assignment of exposure status can be random or non-random • The non-exposed group can be untreated (placebo) or given a standard treatment • Most common is a randomized clinical trial

  33. Experimental Study Examples • Randomized clinical trial to determine if giving magnesium sulfate to pregnant women in preterm labor decreases the risk of their babies developing cerebral palsy • Randomized community trial to determine if fluoridation of the public water supply decreases dental cavities

  34. Observational Studies Three main types: • Cross-sectional study • Cohort study • Case-control study

  35. Cross-Sectional Studies • Exposure and outcome status are determined at the same time • Examples include: • Behavioral Risk Factor Surveillance System (BRFSS) - • National Health and Nutrition Surveys (NHANES) - • Also include most opinion and political polls

  36. Cohort Study

  37. Definition of a Cohort In Epidemiology, “Any designated group of individuals who are followed or traced over a period of time.” Last, JM. A Dictionary of Epidemiology, 3rd ed. New York: Oxford University Press, 1995

  38. Cohort Studies Study Population Exposure is self selected Non-exposed Exposed Follow through time Disease No Disease Disease No Disease

  39. Cohort Studies:Prospective vs. Retrospective

  40. Cohort: Forward Directionality Exposure Outcome Cohort Timeline Ill? Not Ill? Exposed Research Timeline

  41. Cohort Study Steps in a Cohort Study: • Identify Cohort • Administer Questionnaire • Analyze Data

  42. Cohort Study Step 1 – Identify cohort • Entire student body (n=12,000) • On-campus students (n=2400) • On campus students with meal plan who mainly ate at cafeteria (n=2000) • Sick students (n=75) • Do not select cohort so that either everyone is exposed or everyone is diseased

  43. Cohort Study Step 2 – Administer questionnaire • Step 2a – Determine exposure status • Main cafeteria • Deli bar • Food item • Step 2b – Determine disease status • Importance of well-defined case definition

  44. Cohort Study Example Dartmouth University: 698 (13.8%) of 5060 students had conjunctivitis in spring 2002 • To identify risk factors... • web-based questionnaire set up • E-mail sent to 3682 undergraduates • No data entry-rapid analysis • 1832 (50%) responded An outbreak of conjunctivitis due to atypical Streptococcus pneumoniae. N Engl J Med. 2003;348 (12):1112-21.

  45. Cohort Study Step 3 – Analyze data

  46. Interpreting a Risk Ratio The risk ratio is the ratio of the risk of disease in exposed individuals to the risk of disease in unexposed individuals • RR=1.0 = no association between exposure and disease • RR>1.0 = positive association • RR<1.0 = negative association

  47. Interpreting a Risk Ratio Example: Outbreak of conjunctivitis at Dartmouth College • RR = 2.5 • Disease = Conjunctivitis • Exposure = Attended a fraternity / sorority party “Students who attended a fraternity or sorority party had 2.5 times the risk of having conjunctivitis compared to students who did not attend a fraternity or sorority party”

  48. Cohort Study Example • Recent norovirus outbreaks on cruise ships • Attempt to interview all passengers • Collect food history information MMWR: December 13, 2002 / 51(49);1112-1115

  49. Cohort Study Examples • Shigellosis among swimmers in a Georgia park • Used park registry to identify park visitors • Whirlpools and Methicillin-Resistant Staphylococcus aureus • Occurred on a college football team