VOLUME 3, ISSUE 1 Cohort Studies for Outbreak Investigations In a previous issue of FOCUS, we To be certain that an exposure caused introduced cohort studies and talked the disease, the unexposed group CONTRIBUTORS about how to decide whether a co- must be similar to the exposed group hort study is the best design for your in all respects except the exposure. Author: situation. Remember that cohort Therefore they should have similar Amy Nelson, PhD, MPH studies are useful when there is a demographic characteristics such as defined population at risk for devel- age, gender, race, and income. If you Kim Brunette, MPH oping the disease of interest (such compare an “unexposed” elementary FOCUS Workgroup* as all members of the Glee Club) and school to an “exposed” elementary when it is possible to interview all school, both schools should be lo- Reviewers: members or a representative sample cated in neighborhoods with the same FOCUS Workgroup* of the cohort. types of economic and other opportu- nities, similar socio-economic status Production Editors: In outbreak investigations, cohort (SES), similar parent involvement, Tara P. Rybka, MPH studies are usually retrospective similar school resources, and similar Lorraine Alexander, DrPH because in an outbreak an exposure has already occurred, and enough student populations. Gloria C. Mejia, DDS, MPH cases to signal an outbreak have Using groups that have other differ- Editor in chief: also occurred. The aim is to deter- ences may lead to “confounding.” If Pia D.M. MacDonald, PhD, MPH mine what exposures occurred in the you compare a low-income public past to cause these cases of dis- school to a private boarding school, ease. you will find many differences be- tween the two, and you will not know In this issue we cover the basics of whether the difference in disease out- * All members of the FOCUS Work- conducting a cohort study and dis- come is due to an exposure, or group are named on the last page of cuss how to calculate measures of whether it is due to the other differ- this issue. disease (prevalence, risks) and dis- ences. ease association (relative risk). Another way to establish a cohort is to identify a particular population group Establishing the Cohort and then determine whether or not they were exposed. There are two ways to establish a cohort. One way is to choose cohort • People who happened to be at members based on characteristics the same place at the same time that assume exposure has occurred. (for example, all customers at a This is often done in occupational deli during a certain period) might studies. For example, an office build- be considered a population group. ing, school, or neighborhood may be You could characterize them by considered exposed if it is close to whether they ate the ham, potato contamination sources of interest, salad, or any other food that you The North Carolina Center for Public Health Prepared- such as landfills, nuclear plants, or ness is funded by Grant/Cooperative Agreement Num- want to evaluate as an exposure. ber U90/CCU424255 from the Centers for Disease factories; it can be considered unex- posed if it is located far away from • A population group can also be Control and Prevention. The contents of this publication defined by attendance at an are solely the responsibility of the authors and do not these sources of contamination. event, such as a summer concert. necessarily represent the views of the CDC. North Carolina Center for Public Health Preparedness—The North Carolina Institute for Public Health FOCUS ON FIELD EPIDEMIOLOGY Page 2 These people can be characterized by whether or not Prevalence they ate the beef on a stick, played in the duck pond, The simplest descriptive measure of disease is preva- etc. lence. Prevalence is the number of ill people divided by the • Membership in a particular group, such as Rotarians, total population at risk (i.e., the cohort) at a particular the PTA, or the triathlon club, can also define a popu- point in time. Prevalence is often expressed as a percent. lation. For example, athletes from all over the world • In 1993, the prevalence of chronic fatigue syndrome attended Eco-Challenge-Sabah 2000, a multi-sport among patients attending a primary care physician race in Borneo, Malaysia. Reports of disease from health departments across the United States triggered was 0.3%. (2) an investigation, and exposure to the Segama River Risk was implicated. (1) Another measure of disease is risk, which reflects the People who all belong to the same group or attend the probability of acquiring disease. Risk is the number of same event are likely to be very similar to each other. cases divided by the total number of people in the popula- Therefore, confounding may not be a major issue in these tion (including cases and non-cases). cohorts. Risk = # ill people # people at risk Conducting the Investigation The exposure that is causing an outbreak is not always known at the beginning of the investigation, so investiga- • The risk of acquiring HIV from a blood transfusion in tors attempt to measure a number of plausible exposures the U.S. is approximately 0.0002%. (3) and evaluate each one. In an outbreak setting, we like to know the risk of disease For example, investigators might classify exposures as among those exposed to various risk factors. Knowing the people who did or did not eat at a particular restaurant, risk helps us determine how people are getting sick and people who did or did not use recreational swimming facili- how to prevent more cases from occurring. ties, people who did or did not get ice from the hotel ice You can calculate risk in a cohort study, because you know machine, or people who did or did not eat the potato salad the number of people who are at risk of developing the at the church picnic. disease—they are all in your cohort. You cannot calculate Next, investigators will develop questionnaires and inter- risk in a case-control study because only a sample of peo- view members of the cohort to gather demographic infor- ple who are at risk of developing disease are included in mation and exposure to any potential risk factors for dis- the study. You may not even know the total number of peo- ease. (Information on questionnaire development and in- ple at risk in a case-control study. terviewing techniques is available in past issues of FO- In infectious disease epidemiology, risk is also called the CUS.) Investigators will also determine which cohort mem- attack rate, which is the percentage of people acquiring a bers meet the case definition, and then analyze this infor- disease among a group. mation to determine whether there is a relationship be- tween exposure and disease. • An influenza epidemic in a nursing home had an at- tack rate of 65% (43 of 66 residents became ill). (4) After you swoop down and gather the data with perfect technique and FOCUS flair, you can present your data to Risk can be calculated separately for those who are mem- the world and save lives… But first, you must analyze the bers of the group, and for those who are not members of data to determine what caused the outbreak, how to pre- the group. The ratio of these two numbers is the risk ratio vent further illness, and how to prevent future outbreaks. (RR), or the relative risk (the risk of one group relative to the risk of another group). Analyzing the Data Risk ratio = risk in exposed group Oh, no! Not math! Never fear, the basic analyses in a co- risk in non-exposed group hort study are simpler than those for any other study de- sign. North Carolina Center for Public Health Preparedness—The North Carolina Institute for Public Health VOLUME 3, ISSUE 1 Page 3 Risk and Risk Ratios: 2x2 table Table 1. Selected exposures from children attending Daycare X Zoo Day Risk Ill Not Ill Total (Attack Rate) Exposure Ill (n=27) Not Ill (n=34) Exposed a b a+b a/(a+b) Turkey sandwich 21 14 Not Fruit salad 10 30 c d c+d c/(c+d) Exposed Chips 13 17 Risk Ratio = [a/(a+b)] ÷[c/(c+d)] Petting zoo 17 15 To interpret the risk ratio, we compare the value of the RR to 1. If risks in both groups are the same, the ratio of the risks will be 1, indicating that there is no association be- Since we can see that many of the sick children ate the tween the exposure being evaluated and the risk of dis- turkey sandwich, let’s focus on that exposure. ease. • 35 of the 61 children reported eating at least part of If RR = 1, exposure has no association with disease. their turkey sandwich. This is the EXPOSED group. If RR > 1, exposure may be positively related to disease. • The other 26 children reported NOT eating any of the If RR < 1, exposure may be inversely related to disease. sandwich. This group is the UNEXPOSED group. • In an outbreak of histoplasmosis in a high school, the • 21 of 35 exposed children became ill, and 6 of the risk ratio for students in classrooms near the courtyard 26 unexposed children became ill. This kind of infor- during rototilling was 1.3, meaning that the risk of ill- mation is often gathered into a table, such as Table ness for students near the courtyard was 1.3 times the 2, to show the ill and the exposed. risk of illness for students not near the courtyard.