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VOLUME 1, ISSUE 5 Epidemic Curves Ahead An epidemiologist is hardly the first Outbreak pattern of spread person to call in a health emergency. The overall shape of the curve can When someone is having a heart at- reveal the type of outbreak (common CONTRIBUTORS tack, no one cries, “Is there an epide- source, point source or propagated). miologist in the house?” But epidemi- Author: ologists can offer to count that person A common source outbreak is one in Michelle Torok, MPH and tally him in the “Yes” column for which people are exposed intermit- heart attacks. And epidemiologists are Reviewers: tently or continuously to a common really good at counting! harmful source. The period of expo- Amy Nelson, PhD, MPH sure may be brief or long. An inter- With infectious diseases, when we mittent exposure in a common FOCUS Workgroup* count cases in an outbreak, we like to source outbreak often results in an Production Editors: put that tally to good use. We do this epi curve with irregular peaks that by creating an epidemic curve, or epi Lorraine Alexander, DrPH reflect the timing and extent of the curve. While outbreak investigations exposure (2). Figure 1 shows an ex- Gloria C. Mejia, DDS, MPH can throw many unexpected curves, ample of a common source outbreak Editor in chief: the epi curve is one that should be with intermittent exposure. Continu- created in every potential outbreak Pia D.M. MacDonald, PhD, MPH ous exposure will often cause cases situation. to rise gradually (and possibly pla- This issue of FOCUS explains how epi teau, rather than peak) (2). Figure 2 curves are used and describes meth- gives an example of a continuous ods for making an epi curve. exposure. An epi curve with a sharp upward slope and a gradual downward slope What exactly is an epi curve and how typically describes a point source * All members of the FOCUS workgroup are can it help in an outbreak? named on the last page of this issue. outbreak. A point source outbreak is An epi curve is a graphic depiction of a common source outbreak in which the number of outbreak cases by date the exposure period is relatively brief of illness onset. It is useful because it and all cases occur within one incu- can provide information on the out- bation period. Figure 3 illustrates a break’s (1): point source epidemic curve. • Pattern of spread A propagated outbreak is one that is spread from person to person. Be- • Magnitude cause of this, propagated epidemics • Outliers may last longer than common source epidemics and may lead to multiple • Time trend waves of infection if secondary and tertiary cases occur. The classic • Exposure and/or disease incuba- propagated epi curve has a series of tion period progressively taller peaks, each an The Center for Public Health Preparedness is funded by a incubation period apart, but in reality cooperative agreement between the Centers for Disease Each of these aspects of an epi curve Control and Prevention and the Association of Schools of will be discussed in detail. the epi curve may look somewhat Public Health, Project # A1011-21/21 different (2). Figure 4 is an example of a propagated epi curve. North Carolina Center for Public Health Preparedness - The North Carolina Institute for Public Health FOCUS ON FIELD EPIDEMIOLOGY Page 2 Figure 1. Example of an epidemic curve from a com- Figure 2. Example of an epidemic curve from a com- mon intermittent exposure source mon continuous exposure source 8 10 7 9 8 6 7 5 6 5 4 4 3 3 2 2 1 1 Number of Cases 0 Number of Cases 0 123456789101112131415 1 3 5 7 9 11131517192123252729313335373941 Day of Onset Day of Onset Figure 3. Example of a point source epidemic curve Figure 4. Example of a propagated epidemic curve 14 14 12 12 10 10 8 8 6 6 4 4 2 2 Number of Cases Number of Cases 0 0 1357911131517192123252729 1 4 7 101316192225283134374043464952555861646770737679 Day of Onset Day of Onset Magnitude of the outbreak Outbreak outliers An epidemic curve can provide a sense of the magnitude of Cases at the very beginning or end that do not appear to the outbreak as well. For example, there were 73 cases be related to the outbreak are referred to as “outliers.” reported in the point source outbreak shown in Figure 3—a The first thing that should be done when considering out- fairly large outbreak for certain diseases in a small geo- liers is to make sure they are not mistakes due to miscod- graphical area. Additional information about the magni- ing or data entry error. Assuming they are not errors, im- tude of the outbreak within subpopulations can be ob- portant information can be deducted from outliers. For tained by stratifying the epi curve, that is, separating the example, an early case may not be part of the outbreak; it sample into several subsamples according to specific crite- may represent the baseline level of illness. However, it ria, such as gender, age, clinical symptoms or geographic may also represent the source of the outbreak, such as an location. infected food handler, or it may be a case that was ex- posed earlier than the others. A late case may not be part of the outbreak; but alternatively, a late case may repre- Outbreak time trend sent an individual who had a long incubation period, who was exposed later than the other cases, or who was a sec- Again, using the point source outbreak (Figure 3) as an ondary case (acquired the disease from a primary case) example, the epi curve allows us to glean useful informa- (2). tion about the time trend involved. Illness onset for the first case patient was on Day 11, and cases continued to be reported for the rest of the month. The outbreak Period of exposure/incubation period for the outbreak peaked on Day 21 and then began to decline. No new cases were reported after Day 28. Unless there If the timing of the presumed exposure is known, epi has been secondary spread (cases of disease acquired curves can be used to estimate the incubation period of from a primary case), based on the curve, this outbreak the disease, and this may facilitate identification of the appears to be over. causative agent. The period between the known or hy- North Carolina Center for Public Health Preparedness - The North Carolina Institute for Public Health VOLUME 1, ISSUE 5 Page 3 spectively. Three days back from the peak case (the 11th) Figure 5. Epidemic curve from outbreak of norovirus would be the 8th of the month. Counting 1 day back from gastroenteritis in U.S. Army trainees in 1998 the first case would also be the 8th. Thus, the hypothe- sized exposure date is the 8th of the month. Since this technique is not precise, the exposure period should be widened several days on either side, which would give an approximate probable exposure period from the 5th to the 11th. Now potential exposures during this time frame can be investigated in the hope of finding the source of the outbreak. How to make an epi curve As shown in the epi curves above, the structure of an epi curve is straight forward. Simply plot the number of cases pothesized exposure time and the peak of the epi curve of disease reported during an outbreak on the y-axis (the represents the hypothesized median incubation period vertical line) and the time/date of illness onset on the x- (3). Figure 5 was created from a U.S. Army trainee out- axis (the horizontal line). Here are some technical tips: break of norovirus gastroenteritis in 1998 (4). In this • One of the trickier aspects of creating an epi curve is case, the exposure was thought to have occurred August choosing the unit of time for the x-axis. This choice is 26 or 27. Based on the epi curve, the median hypothe- usually based on the incubation period of the illness sized incubation period was very short, 24-36 hours. This and the time interval of the outbreak. In general, a is consistent with noroviruses, which have an average incubation period of 12-48 hours (5). Figure 7. Epidemic curve using one day as time unit on x- In common source outbreaks involving diseases with axis known incubation periods, epi curves can help determine the probable period of exposure (2). This can be done by 10 9 looking up the average incubation period for the organism 8 and counting back from the peak case the amount of time 7 of the average incubation period. To estimate the mini- 6 5 mum incubation period, count back the minimum incuba- 4 tion period time from the earliest case on the epi curve. 3 2 1 Number of Cases Figure 6. Epidemic curve to estimate exposure period 0 2002 2002 2002 2002 2002 2002 1/2002 7/2002 0/ 0/3/ 0/5/ 0/ 0/9/ 11/2002 15/200217/2002 21/2002 25/2002 16 1 1 1 1 1 0/ 0/13/ 0/ 0/ 0/19/ 0/ 0/23/ 0/ 14 1 1 1 1 1 1 1 1 12 10 Date of Ons e t 8 6 4 Figure 8. Epidemic curve using one week as time unit on 2 0 x-axis Number of Cases 8 9 10 11 12 13 14 15 16 17 18 19 20 21 50 Date of Ons e t 40 30 Ideally, the minimum and average outbreak incubation periods should be close, and the time between them will 20 represent the probable period of exposure.
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