SARS Outbreak Management: Epiville Simulation Analysis

Background of the Epiville SARS Outbreak

A case definition is a list of specific criteria used to decide whether a person has the disease under investigation. The case definition for the Epiville SARS outbreak is: a visitor to, or resident of, the Amoy Apartment Complex, or a worker at Star Hospital, who has onset of respiratory flu-like symptoms (high fever, cough, difficulty breathing) between August 10 and August 23, 2003, without other apparent cause. Epiville General Hospital suspects that the disease is severe acute respiratory syndrome (SARS), although diagnostic tests have not yet been conducted to confirm this. A clinical case is defined as an acute respiratory illness lasting two or more weeks, with onset from August 10, 2003, in a person living in the Amoy Apartment Complex, without other apparent cause. A suspected case is a clinical case epidemiologically linked to another clinical case through common household residency. All other clinical cases are considered probable.

The choice of case definition influences the scope of an outbreak investigation. If the case definition is too broad, the epidemiologist will include all other illnesses meeting the stated clinical criteria in their investigation (CDC, 2012). On the other hand, if the case definition is too strict, the investigation will only include people confirmed to have the disease (CDC, 2012). In the Epiville outbreak, the case definition includes both residents of and visitors to the Amoy Apartment Complex, meaning the epidemiologist must focus the outbreak investigation on both groups. If the case definition had only considered residents, non-residents would have been excluded and the investigation would focus solely on the population living within the complex.

The epidemiologist considers several factors when formulating a case definition:

i) Person — demographic characteristics of the affected population, including age, sex, occupation, and race.

ii) Place — the geographical coverage of the suspected outbreak, such as a healthcare facility, street, town, or apartment complex.

iii) Time — representing the onset of illness in days, weeks, months, or years. Delineating the time period helps to exclude similar illnesses that are unrelated to the outbreak under investigation.

iv) Clinical features — the symptoms and clinical characteristics associated with the illness under investigation.

v) Laboratory confirmation — whether or not confirmatory laboratory tests have been carried out, which helps in grouping cases as either confirmed, probable, or suspected.

vi) The aim of the outbreak investigation — for instance, if the aim is to determine whether anyone in a particular area has symptoms of a communicable disease so that planning can begin, the case definition may be broad to identify all possible cases (CDC, 2012). However, if the aim is to determine the causes of an outbreak, a stricter case definition may be adopted to ensure that only people confirmed to have the disease are included (CDC, 2012).

Person: The outbreak disproportionately affects older individuals aged over 50. Descriptive variables include age group, sex, occupation, race, and exclusion criteria of the affected population.

Place: The outbreak is localized within the Amoy Apartment Complex and Star Hospital.

Time: The onset of illness falls between August 10 and August 23, 2003.

In summary, the Epiville SARS outbreak is localized within the Amoy Apartment Complex and Star Hospital, with an onset from August 10 to August 23, 2003, and disproportionately affects older individuals above the age of 50.

i) Define the epidemic by defining the clinical cases, the population at risk of illness, and calculating attack rates.

ii) Examine the distribution of cases by person, place, and time — the epidemiologist does this by reviewing patient charts and creating lists of patients who have died from the disease to identify their profile, including residence and age group.

iii) Look for combinations and interactions of relevant variables.

iv) Develop hypotheses about the cause of the illness based on existing knowledge of the disease and its relationships to diseases whose etiology is known.

v) Test the hypotheses through an appropriately designed study and collect additional data if necessary.

vi) Recommend control and management measures such as quarantine, isolation, and public education on prevention and control.

Surveillance Techniques and Case Ascertainment

Surveillance is the continuous collection and analysis of data on cases that is not necessarily followed by immediate action. Surveillance methods used to ascertain cases in the Epiville outbreak include:

i) Review of case records, including centralized disease registries and hospital discharge notes.

ii) Microbial laboratory monitoring.

iii) Hospital reporting of confirmed cases to the Department of Health.

iv) Physical counts and visits to the Amoy Apartment Complex.

v) Periodic telephone calls to healthcare facilities, individuals, and laboratories.

Suspected cases are cases that demonstrate the clinical features associated with the disease but have not undergone confirmatory testing. There are 53 cases that can be categorized as suspected in the Amoy Apartment Complex, as they show respiratory symptoms without laboratory confirmation. Probable cases are suspected cases for which testing has returned inconclusive results (CDC, 2012). Of the 70 cases showing symptoms, 53 are confirmed cases, while 5 of the remaining 17 had clear chest X-rays despite showing symptoms. Six recovered within a week, suggesting influenza, and 6 had pneumonia. The 5 individuals who show symptoms but have clear chests and no other attributable cause are categorized as probable cases.

The mode of transmission at Amoy Apartment Complex is person-to-person, occurring through the spread of respiratory droplets when an infected person sneezes, speaks, or coughs. Person-to-person transmission is the most likely mode at the complex, as it requires close proximity — droplets fall to the ground within a few feet. Cases occur close together in time, shortly after attending a gathering at Amoy Garden where residents were in close contact for several hours. Transmission at Star Hospital also occurs through direct person-to-person contact — all cases arise close in time following the admission of the index case. The fact that infections occur in close temporal proximity indicates a common source of infection. Infected healthcare workers also reported being in close contact with the index patient during treatment, and infections are likely to have resulted from contaminated droplets produced when the infected person sneezed, coughed, or spoke.

Hypotheses are developed using one of two processes: based on what is already known about the disease, or by deducing insights from comparable diseases whose etiology is known. In the Epiville outbreak, hypothesis development is based on the limited knowledge available about the disease. For instance, drawing on what little is known about the transmitting agent and its incubation period, the epidemiologist was able to infer that the mode of transmission at both Amoy Apartment Complex and Star Hospital was direct person-to-person contact when an infected individual speaks, coughs, or sneezes.

Case-Control Study Design and Epidemic Curve Analysis

A case-control study design would be most effective for the current outbreak at Epiville. A case-control design divides the population into two groups that are otherwise identical but differ by disease status. The aim is to identify potential risk factors for the defined disease. Using data from the disease registry, the epidemiologist could categorize the Amoy Apartment population into an exposed group and a control group. Data would then be collected and analyzed to determine the possible risk factors for individuals in either group. Given that little is still known about the disease under investigation, a study design that provides crucial insights into potential risk factors is necessary to support better prevention and control measures.

Data to be collected in this study design includes: existing comorbidities, current medications, family living conditions, and the severity of illness.

The outbreaks at both Amoy Apartment Complex and Star Hospital are represented by a person-to-person (propagated) epidemic curve. The person-to-person mode of transmission is evidenced by the fact that infections occur in close temporal clusters. Infections span approximately 10 days in the case of Amoy Apartments and 13 days at Star Hospital.

From the curves, the individual infected on the 17th most likely infected the one who began showing symptoms on the 20th, who in turn likely infected the person whose symptoms began on the 23rd at Star Hospital. Similarly, from the Amoy Apartment curve, the individual infected on the 10th likely infected the one who began showing symptoms on the 12th — a span of two to three days. Given that individuals may have been infected some days before they fell ill on the reported dates, the agent appears to have an incubation period of approximately 2 to 20 days. Understanding the incubation period is crucial, as it helps the epidemiologist determine when an infected individual is likely to be asymptomatic and when they are most likely to spread the disease (Giesecke, 2017).

Conclusion

The processes of outbreak investigation and management were carried out effectively in the Epiville simulation. The broad case definition, which includes both residents and non-residents of the Amoy Apartment Complex, increases the scope of the investigation and reduces the likelihood that a positive case in the population would go undetected. Techniques employed throughout the outbreak management process include continuous surveillance and the development of effective working hypotheses to guide the investigation. Ultimately, quarantine and isolation were used to minimize movement in high-risk areas and reduce the risk of direct person-to-person transmission.

The relatively low case fatality rate may indicate that the disease is not highly lethal — or alternatively, that the outbreak investigation and management were effective. A key lesson from the analysis is that the case definition forms the foundation of an outbreak investigation, and the success of the investigation depends heavily upon it. Continuous surveillance is equally crucial; however, to be effective, surveillance must be proactive, with investigators contacting relevant stakeholders to ascertain cases and gather additional information. Finally, communication among key stakeholders is essential to ensure that all parties remain informed and coordinated throughout the response.

CDC. (2012). Lesson 1: Introduction to public health. Centers for Disease Control and Prevention. Retrieved from https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section5.html

Giesecke, J. (2017). Modern infectious disease epidemiology (3rd ed.). CRC Press.