SEARO – CSR Training on Outbreak Investigation Cohort and case-control studies Observational studies

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Learning Objectives At the end of the sessions, the participants will be able to: –Define cohort and case-control studies –Identify applications for cohort and case- control studies –Describe two-by-two table –Compute different measures of association

SEARO – CSR Training on Outbreak Investigation Cohort study Observational studies: Cohort and case-control studies

SEARO – CSR Training on Outbreak Investigation Learning Objectives At the end of the session, the participants will be able to: –Define cohort study –Describe prospective and retrospective cohort studies –Compute attack rate and relative risk –Enumerate pros and cons in using cohort studies

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation What is a cohort? Cohort: Latin word for one of the 10 divisions of a Roman legion A group of individuals –Sharing same experience –Followed-up for a specified period of time Examples –Birth cohort –Occupational cohort chemical plant workers –A Rapid Response Team

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Exposure occurrence Study startsDisease occurrence Growth-nutrition studies, Folic acid and NT defects Prospective Cohort Study Time ill exp + - Prospective assessment of exposure and disease Selection of population

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Prospective cohort study Chernobyl, Industrial accidents, Flood victims ill exp + - Disease occurrence Study startsExposure occurrence Prospective assessment of disease Selection based on exposure

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Food borne outbreaks, closed environment outbreaks (school, prisons, etc) Retrospective cohort study Transversal studies Study takes place Disease occurrence Exposure occurrence Retrospective assessment of exposure and disease Selection based on population ill exp Real Time Now

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Presentation of Cohort Data two-by-two table Exposed Unexposed illnot ill a c b d Incidence in exposed (I e+ )= a/(a+b) Incidence in non-exposed (I e- )= c/(c+d)

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Hypothesis –Is the incidence among exposed higher than among unexposed Absolute measures –Risk difference (RD) I e+ - I e- Relative measures –Relative risk (RR) Rate ratio Risk ratio Effect measures in cohort studies I e+ I e- a/(a+b) c/(c+d) =

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Ate ham Did not eat ham ill not ill Risk difference = 0.1 (10%) Relative risk 0.5 / 0.4 = 1.25 Foodborne Outbreak in a Wedding, Dublin Incidence % %

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Does HIV infection increase the risk of developing TB among a population of drug users? Source: Selwyn et al., New York, 1989 Presentation of cohort data Population at risk Drug users (f/u 2 years) 8 HIV - TB Cases HIV Incidence (%) Relative risk 3.7 (8/215) (1/289)

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Advantages and disadvantages of cohort studies Advantages –Can measure incidence and risks –Good for rare exposures –Clear temporal relationship between exposure and outcome –Less subject to selection bias Disadvantages –Requires a large sample size –Latency period –Lost to follow-up –Ethical considerations –Resource intensive High cost Timely

SEARO – CSR Training on Outbreak Investigation Case-Control study Observational studies: Cohort and case-control studies

SEARO – CSR Training on Outbreak Investigation Learning Objectives At the end of the session, the participants will be able to: –Define case-control study –Describe steps in setting up a case-control study –Compute Odds Ratio –Enumerate pros and cons in using case-control studies

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Retrospective assessment of exposure +- ill Disease occurred Exposure occurred Study takes place ill exp Case-Control Study Selection based on disease status Real Time Now

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Setting-up a case-control study Identify group of cases Identify group of controls Question both groups for possible exposure Measure the frequency of exposure occurrence in both groups Compare the frequency of exposure between cases and controls

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Presentation of case-control data two-by-two table Exposed Not exposed casecontrol a c b d % of exposure among cases = a/(a+c) % of exposure among controls = b/(b+d)

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Odds of exposure Exposed Not exposed casescontrols

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Effect measures in case-control studies OR = a/c b/d a*d b*c = OR = 1 – no association OR > 1 – there is an association OR < 1 – the factor is some way protective

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Drink water From well Cholera Control Yes3025 No3260 Total6285 % exposed 48.4% 29.4 % 30 * 60 OR = = 2.25 (95%CI ) 32 * 25 Distribution of Cholera Cases and Controls by Exposure to Shallow Well Water N'Djamena, July 2001

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Multiple Exposure Levels B1B1 High A1A1 D Not exposed C Cases Exposure level B2B2 Medium A2A2 B3B3 Low A3A3 OR 1 OR 2 OR 3 Reference ControlsOR

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Relation of Hepato cellular Adenoma to duration of oral contraceptive use in 79 cases and 220 controls Months of OC useCasesControlsOdds ratio Ref >= Total79220 Source: Rooks & col. 1979

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Advantages and disadvantages of case- control studies Advantages –Suitable for rare diseases –Can explore several exposures –Low cost –Rapid –Can cope with long latency –Small sample size –No ethical problems Disadvantages –Cannot calculate the risk –Not suitable for rare exposures –Temporal relationship difficult to establish –Subject to bias Selection of controls Recall bias …

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation Cohort studies are better but harder to carry out and provide true measure of risk Case-control studies are rapid and easy to carry out, but only provide estimates of risks Prefer cohort to case-control when feasible In field epidemiology, case-control studies are more frequently used Conclusions

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation