HSS4303B – Intro to Epidemiology

1. HSS4303B – Intro to Epidemiology Jan 25, 2010 – Natural History of Disease

2. International Culture & Development Week • http://www.scdi-icdw.uottawa.ca/ • Today: • 2:30pm: Launch with Allan Rock, Tabaret Chapel • 4pm: “Casino capitalism”, UCU205 chaired by ME!

3. The Midterm

4. The Midterm Which will it be?!!!!

5. The Abstract • Due on Thursday at midnight • Follow instructions carefully (including how to submit it!!!) • Any issues thus far?

6. Poster Assignment • I’ll be posting details soon • Seek out partners • I’ll be asking for names of teams soon • If you don’t know anyone in the class, let me know and I’ll see what I can do

7. Tutorial • Erin will be available on Thursday • I will upload more exercises tonight (or tomorrow) for you to try before seeing her (If you do a Google image search for “tutorial” these are the first two hits:)

8. Review • Mortality Rate (MR) • #deaths/# at risk • Case Fatality Rate (CFR) • #deaths/#diagnosed • Cause-specific mortality rate • #deaths from specific cause / # at risk • Years of potential life lost (YPLL) • Expected lifespan – observed lifespan • Disability assisted life year (DALY) • (years of life lost) +(years of productive life lost) • Disability Adjusted Life Expectancy (DALES) • Proportionate Mortality Ratio (PMR) • #deaths due to a cause / #deaths total • Quality Adjusted Life Years (QALYs) • #years lived X quality index (0->1)

9. Review • Survival Rate (SR) • (# initial subjects - # subjects dead or censored) / (#initial subjects) • Relative Survival Rate (RSR) • SR among subjects/ SR among total population • Cause-Specific Survival Rate (CSS) • (#initial subjects - #subjects dead from specific cause) / (#initial subjects)

10. Review • Age-specific mortality rate • The mortality rate of a specific population within a specific age stratum • Age-adjusted mortality rate • Total mortality rate for a population, after its age distribution has been adjusted to resemble a standard (reference) population • Crude Mortality (Death) Rate • Un-adjusted total mortality rate

11. Review • Standardized Mortality Ratio (SMR) • (#observed deaths per year) / (#expected deaths per year) • Direct Standardization • Computes age-adjusted mortality rate by multiplying the age-specific rates from the test population by the age-specific populations from the reference • Indirect Standardization • Computes age-adjusted mortality rate by multipling the age-specific rates from the reference population by the age-specific populations from the test population • SMR x (crude death rate in standard population)

12. Artefact • (Artifact is the American spelling; both are acceptable) • a spurious finding, such as one based on either a faulty choice of variables or an overextension of the computed relationship

13. Interpreting observed changes in mortality • Changes in mortality • Artifactual • Problems with the numerator • Problems with the denominator • Real • Identify possible explanations • Develop a hypothesis

14. Artifactual trends in mortality

15. Cohort From Latin “cohors”, it was the basic unit of the Roman Legion.

16. Cohort Refers to a bunch of people who move together.

17. Cohort Refers to a bunch of people who move through time together.

18. Cohort • A group of people who share a particular experience or characteristic(s) over a period of time • Irish women born in 1950 • Engineers who smoked between the ages of 25-30 • HSS students in 3rd year

19. Now…. An example • Pertussis • Whooping cough • Highly contagious bacterial infection • Effective, well tolerate vaccine that lasts several years • One of the leading causes of vaccine-preventable deaths in the world

20. Source: Wikipedia Pertussis DALYs

21. Facts • Beginning in 1990 Canada experienced a resurgence of pertussis. • The mean annual incidence before 1990 was 3.8 cases per 100 000 population which increased to 37.2 thereafter. • The mean annual hospitalization rates increased from 2.7 per 100 000 before 1990 to 5.2 afterward. • The proportion of cases in 0- to 4-year-old children decreased, whereas it increased steadily in all other age groups • Between 1990 and 1998 the median age of cases shifted from 4.4 to 7.8 years. The Pediatric Infectious Disease Journal: January 2003 - Volume 22 - Issue 1 - pp 22-27

22. So What’s Happening? • “The sudden increase in pertussis incidence in Canada can be largely attributed to a cohort effect resulting from a poorly protective pertussis vaccine used between 1985 and 1998.” –NTEZAYABO et al, 2003 • In other words, something that happened in the 80s to infants did not manifest till the 90s in older children, as the cohort moved through time

23. Factors Around Cohort Effect • Smoking behaviours differ between generations • Income differs between generations • Geopolitical circumstances (e.g. war) differ • Health system issues may differ (e.g. infant health care) • etc

24. Example • In the UK, politicians often speak of the “cohort effect” in terms of a certain generation: • Brits born between 1925 and 1945 (centred around 1931) experienced more rapid improvements in mortality than generations born on either side (i.e., younger and older) WHY?

25. Cohort effect • Cross sectional view • Identifies peculiarities and key messages from the data • Which age group has the highest rates of tuberculosis • Cohort effect • Identifies groups with the trait or disease incidence • Group is followed over time to see if the trait develops or disease manifests

26. Cross sectional view • Identifies peculiarities and key messages from the data • Which age group has the highest rates of tuberculosis • Cohort view • Identifies groups with the trait or disease incidence • Group is followed over time to see if the trait develops or disease manifests

27. Cohort vs Cross-Sectional View (1900) Peak mortality occurred for the 30-39 years age group (Cross sectional view)

28. Cohort effect Follow the cohort and the peak mortality occurs for the 20-29 years old group

29. The History of Disease

30. The History of Disease Abdel Omran, 1971…. • Age of Pestilence and Famine • Age of Receding Pandemics • Age of Degenerative and Manmade Diseases In very very very broad terms, historians consider the history of human disease to have occurred in 3 phases: http://www.who.int/bulletin/archives/79%282%29159.pdf

31. Age of Pestilence and Famine • High mortality rates • Wide swings in mortality rates • Little population growth • Very low life expectancy

32. Age of Receding Pandemics • Less frequent epidemics • Less incident infectious disease • A slow rise in degenerative disease

33. Age of Degenerative and Manmade Diseases • Cancers • Obesity • Cardiovascular disease • Diseases associated with high SES and relatively bountiful food • Most countries are here now

34. Omran defined: The Epidemiologic Transition • a human phase of development witnessed by a sudden and stark increase in population growth rates brought about by medical innovation in disease or sickness therapy and treatment, followed by a re-leveling of population growth from subsequent declines in procreation rates • Wikipedia

35. Cf. Demographic Transition • stage one, pre-industrial society, death rates and birth rates are high and roughly in balance • stage two, that of a developing country, the death rates drop rapidly due to improvements in food supply and sanitation, which increase life spans and reduce disease • stage three, birth rates fall due to access to contraception, increases in wages, urbanization, etc. • stage four: there are both low birth rates and low death rates. Birth rates may drop to well below replacement level as has happened in countries like Germany, Italy, and Japan • Stage five: sub-replacement fertility

36. Cf. Demographic Transition

40. Perfectly correlated to per capita alcohol consumption in these countries.

41. Epidemiologic transition from 1990 to 2020

42. Natural History of Disease refers to a description of the uninterrupted progression of a disease in an individual from the moment of exposure to causal agents until recovery or death

43. Natural history of a disease in a patient

44. Natural history of a disease in a patient Death Survival

45. An idealized depiction of the natural history of disease.

46. Natural history of coronary heart disease.

49. Natural History of Disease • …is not the same as the changing patterns of disease in a population • E.g., the distribution of CHD over SES groups may change over time as a society changes…. • But the natural history of CHD will not change

50. “Pyramid” or “Iceberg” of Disease