SARS Outbreak Study 1

Learning Objectives

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Introduction to Study

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Student Role

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Data Collection

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Data Analysis

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Plan of Action

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Discussion Questions

Step 5: Data Analysis

Back at the DOH:

You suspect that what you have on your hands is an outbreak of SARS in Epiville. An outbreak is defined as the occurrence of more cases of disease than expected in a given area among a specific group of people over a particular period of time. Before going any further with your investigation you need to get guidance from your supervisor. Dr. Zapp outlines the following steps of our outbreak investigation:

1. Establish diagnosis.
2. Define what constitutes a case.
3. Identify cases and obtain information.
4. Collect and analyse descriptive data.
5. Develop hypothesis.
6. Conduct analytical studies to test hypotheses.
7. Conduct special studies.
8. Communicate your findings, including an outbreak report.

You return to your modest cubicle at the DOH and eagerly embark upon the first steps of an outbreak investigation that involve descriptive analysis. Descriptive analysis is concerned with describing the general characteristics of the distribution of a disease. Descriptive studies often provide the first important clues about possible determinants of a disease and are primarily useful for the formulation of hypotheses that can be tested subsequently.

The epidemic should be characterized using the criteria in the case definition. The case definition is the list of specific criteria used to decide whether or not a person has the disease of concern. It is especially important to establish a case definition if the disease is unknown; as is the case with this outbreak. Data collection should not start until the case definition has been established because the case definition determines the data needed in order to classify cases as affected or unaffected.

The case definition is based on:

1. clinical criteria (signs and symptoms found upon physical examination of cases)
2. association of cases with Person, Place, and Time (PPT)

PPT associations are an important element of determining if someone is a case of the suspect disease or not. Key questions in determining PPT are:

1. Person: Was the person connected to other suspected cases of the disease?
2. Place: Was the person connected by place to where the other cases came from?
3. Time: Were the symptoms temporarily related to other known cases of the disease?

This means that in addition to having the same clinical signs and symptoms, the affected cases also needed to be connected by PPT.

Now that you have collected symptoms associated with the outbreak of the unknown respiratory illness (suspected to be SARS), your next step is to prepare the case definition.

Intellectually curious? Learn more about the case definition.

1.	Based on what you just have learned about the case definition how would you go about preparing a case definition?
		a.	compile a table of symptoms in suspected SARS cases and in patients with other diagnosed respiratory illnesses from the infectious disease ward and provide descriptive epidemiology in terms of PPT
		b.	compile a table of symptoms of all patients who were admitted to the hospital during the same time as the first SARS patient was admitted to the Epiville General Hospital
		c.	compile a table of patients who were admitted to the Epiville General Hospital over the last year who also lived in the Amoy Apartment Complex

With your case definition at hand, you go back to the Epiville General Hospital to review the charts of the 70 persons who reside at the Amoy Apartment Complex. Based on your review of the charts, you confirm that 53 out of the 70 suspected cases in the apartment complex are most likely to be SARS cases. Out of the 17 patients who did not meet the case definition 5 had similar symptoms as SARS cases but their chest X-rays were clear, others (6) recovered within a week (this was an indication that they just had flu), and the rest (6) had pneumonia of known etiology. Note that absence of fever is not necessary to rule out SARS diagnosis because often high fever is self-treated with medicines such as Tylenol. All of these 53 individuals are alive and battling the disease.

The case definition you came up with is remarkably similar to the case definition of SARS developed by the World Health Organization (WHO) on the height of the epidemic in the winter- spring of 2000. You decide to use this definition hereafter.

Most of the surveillance that is done on a routine basis is called passive surveillance. In passive surveillance, physicians, laboratories, and hospitals are required to report diseases from the list of reportable diseases (usually a list of 70-80 diseases); they are given the appropriate mailing forms and instructions, with the expectation that they will report all of the cases of reportable disease that come to their attention. Active surveillance requires periodic telephone calls or personal visits to the reporting individuals/hospitals/laboratories to obtain the required data.

3.	What type of surveillance do you think is more difficult to carry out and why?
		a.	Passive surveillance
		b.	Active surveillance

You cannot wait until next morning to show your case definition and report on your visits to the Amoy Apartment Complex to your supervisor. Dr.Morissa Zapp compliments you on your good work and recommends that you contact two other hospitals in the city to see if they have cases of this unknown illness which were not reported through passive surveillance. Your advisor's suspicions are confirmed. There are 3 suspicious deaths in the Star hospital, which should have been reported to the DOH, but were delayed. You set out to do your detective work there.

When you arrive, you notice that the guard is wearing a mask. He tells you that you cannot proceed further without putting on a mask first. You put on your mask and ask where you can find the hospital's administrator. The administrator tells you that they had a 70 year old man admitted two weeks ago with respiratory symptoms, and that a doctor and nurse who were taking care of him got sick within a week of his admission. Both of them had very close contact with the patient and might have come in close contact with the patient's bodily fluids.

When the administration realized how contagious the disease was, the patient was put into isolation and so were the doctor and nurse. Unfortunately, by the time extraordinary precaution measures were instituted to halt the further spread of infection, a large number of staff had fallen ill. However, during that time, 20 more staff members got infected including 5 medical residents. All of them had visited the patient's room while attending the weekly rounds but did not have close physical contact with the patient. The hospital administration notified the DOH by a phone call, but failed to follow-up with proper documents. This small hospital, which serves primarily the community of retirees nearby, was overwhelmed by a shortage of medical staff.

Your intuition tells you to review the chart of the elderly man who was submitted to the hospital and you find out that he too lives in Amoy Apartment Complex but for some reason got admitted to the Star hospital unlike his neighbors. Several days before he got ill, on August 1st 2003, he attended an annual luau party held in the garden of the Amoy Apartment Complex where he resided. Approximately 300 other tenants also attended.

Now that you have gathered information on all the suspect SARS cases you can proceed with characterizing this epidemic. One of the important steps in accurately characterizing an epidemic involves defining a geographic location of cases. Geographic clustering of cases may give important clues to what is going on. A spot map showing where each affected person lives or works is also helpful in examining an epidemic.

4.	Given the place of residence of each SARS affected case: Map distribution of cases, look for clustering [ Open Map ] Do you see any clustering and where?
		a.	clustering around the hospital
		b.	clustering around the Amoy Apartment Complex
		c.	no clustering

The incidence rate is the frequency (number) of events that occur in a defined time period. It is calculated as the number of new cases over a defined study period, divided by population at risk over that period. An incident rate is usually expressed per 100, per 1,000, per 10,000, or per 100,000.

Now that you have mapped out all the cases according to the place of residence, you are ready to do some calculations. Let's calculate incidence for 5 residential areas of Epiville.

^a The Amoy Apartment Complex is located in Area B, the Star Hospital is located in Area C, the staff of 110 persons who works at the Star Hospital resides all over Epiville and Epiville suburbs.

^b 65 cases were hospitalized at the Epiville General Hospital and 1 case was hospitalized at the Star Hospital.

^c 22 cases from the Star hospital include only cases which developed among the staff.

6.	In which area do you observe the highest incidence of SARS? Why?
		a.	Area D because the largest number of Star hospital employees resides here
		b.	Area B because it has the largest population
		c.	Area B because this is where the Amoy Apartment Complex is located and where the largest number of new cases live.

Modes of disease transmission can either be direct or indirect (Gordis, p. 15). A disease can be transmitted from person to person by means of direct contact (this is what typically happens during the flu season). Indirect transmission usually occurs through a common vehicle such as contaminated water supply or air or by a vector such as mosquito (for instance, mosquitoes transmit West Nile encephalitis).

Intellectually curious? Learn more about transmission.

7.	Based on the cases from the Amoy Apartment Complex, which mode of transmission is most probable? (Refer to Gordis, p.15 for more information to help you answer this question.)
		a.	common source: food or water.
		b.	airborne
		c.	vector-borne: cockroaches
		d.	direct: person-to-person

8.	Based on the cases from the Star hospital which mode of transmission is most probable?
		a.	Only via bodily-fluids (hint: this is one type of direct transmission)
		b.	airborne
		c.	Direct: person-to-person

Learning Objectives

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Introduction to Study

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Student Role

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Data Collection

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Data Analysis

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Plan of Action

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Discussion Questions