Infectious Disease Epidemiology Homework 2 Dr. Selassie Alexis Charles

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1 Oswego: An Outbreak of Gastrointestinal Illness Following A Church Supper Objectives: After completing this case studym the student should be able to: 1. Define the terms cluster, outbreak, and epidemic. 2. List the steps in the investigation of an outbreak 3. Draw, interpret, and describe the value of an epidemic curve 4. Calculate and compare food-specific attack rates to identify possible vehicles. 5. List reasons for investigating an outbreak that has apparently ended. Part 1 Background On April 19 th, 1940, the local health office in the village of Lycoming, Oswego County, New York reported the occurrence of an outbreak of acute gastrointestinal illness too the District Health Officer in Syracuse. Dr. A. M. Rubin, epidemiologist-intraining, was assigned to conduct an investigation. When Dr. Rubin arrived in the field he learned from the health officer that all persons known to be ill had attended a church supper held on the previous evening, April 18 th. Family members who did not attend the church supper did not become ill. Accordingly, the investigation was centered about the circumstances surrounding the supper. Interviews regarding the occurrence of symptoms, the date and hour of onset, and the food partaken at the church supper were completed on 75 of the 80 persons know to have been present. A total of 46 persons with gastrointestinal illness were identified. Question 1: Would you call this an epidemic? No, I would consider this to be a disease outbreak because it affected one community (those that attended the church supper). Furthermore, it affected 46 people not thousands. If it were an epidemic, the illness would have had to affect family members not at the church supper. Question 2: Review the steps of an outbreak investigation The first step of an outbreak investigation is the preliminary assessment in order to confirm the existence of the outbreak. Next, define the cases and the population at risk of illness. After, collect and analyze the data in order to create and epidemic curve and generate a hypothesis for the cause and control. The fourth step is to analyze the test your hypothesis using cases and controls to find the odds ratio. Next, verify the hypothesis. After verification, the next step intervention into removing the source of illness, or protecting those at risk, or preventing reoccurrence of illness. Finally, communicate during and after the outbreak to the public and other professionals. 1

2 Part 2 Description of the Supper The supper was held in the basement of the village church. Foods were contributed by numerous members of the congregation. The supper began at 6p and continued until 11p. Food was spread out upon a table and consumed over a period of several hours. Data regarding onset of illness and food was eaten or water was drunk by each of the 75 individuals interviewed are provided in the attached table, called a line listing. For only about half of the persons who had gastrointestinal illness was the approximate time of eating supper obtained. Question 3: What is the value of an epidemic curve? An epidemic curve is valuable in order to graphically display the number of incidence cases of an outbreak or epidemic over time. This allows for a hypothesis to be made regarding the nature of disease and its mode of transmission. Question 4: Graph the cases by time of onset of illness. What does this graph tell you? This graph tells me that most people got sick between 9p and 1a after eating dinner at the church. 2

3 Question 5: Are there any cases for which the times of onset are inconsistent with the general experience? How might they be explained? Yes, there is a case that got sick at 3:00pm, before the dinner. This could be explained if the case was cooking the meal or tasting the meal beforehand. Question 6: How could the data in the line listing be better presented? The data could be better presented based on cases stratified by food and time of sickness. This would allow the investigators to better see which food is the culprit for the gastrointestinal illness. 3

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6 Part 1 Infectious Disease Epidemiology On Tuesday, November 3, 1970, the Texas State Health Department s weekly telegram to the CDC reported 319 cases of measles in all of Texas for the previous week, after 4 weeks of 26 cases per week. On follow-up telephone calls to State health officials, an epidemic of 295 cases of measles in Texarakan was uncovered, 25 of which were allege to be in previously immunized children. An invitation to investigate the situation was extended to the CDC on November 4, An ETS officer departed for Texarkana early November 5 th. Background Texarkana, population 50,481 (1969 census), is a city which straddles the Texas- Arkansas state line. Texarkana, Texas (Bowie County) had a population of 29,393 in the 1960 census; the population has been stable during the 1960 s. Texarkana, Arkansas (Miller County), had a population of 21,088. Although Texarkana is divided by the state line, it is a single town economically and socially. There are many opportunities for contact among persons of all ages on both sides of town. Churches, physicians, offices, movie theatres, and stores draw people from both the Arkansas and Texas sides of town. People cross the state line to attend social functions such as football games and school dances. Many families have relatives who visit back and forth on both sides of town. Private nurseries and kindergartens receive children from both sides of town. The two sides of Texarkana, however, do have separate public school systems and separate public health departments. Question 7: What are the reasons you might investigate this outbreak? This outbreak is worth investigating because there is an alarmingly increased number of measles in this population. This is especially worrisome because measles is highly contagious and deadly. Question 8: What would be the objectives of the initial phase of your investigation? In the initial phase of the investigation, it would be important to pinpoint the areas of outbreak, the population affected, and the population at risk. 6

7 Part 2 The Investigation In this outbreak, names of cases were obtained from the health departments, from physicians, from school and nursery records, from door-to-door survey, and by asking families of cases for names of other cases. Methods of case findings and of epidemiologic investigation were similar on both the Arkansas and Texas sides of town. Clinical Picture The illness was clinically compatible with measles. Typically, the patients had a 4 to 5 prodrome with high fever, coryza, cough, and conjunctivitis followed by the appearance of a bright maculopapular rash. The temperature usually returned to normal 2 to 3 days after appearance of the rash, while the rash persisted for 5-7 days, Question 9: How might you define a case for purposes of this investigation? For the purposes of this investigation, I would define a case as a patient with fever and rash persisting for 2 or more days. Question 10: What is the difference between a sensitive case definition and a specific case definition? What are the advantages and disadvantages of each? Provide an example of a situation where each would be helpful. A sensitive case definition allows for epidemiologists to have a larger number of cases in order to avoid missing cases. A specific case definition, however, narrows the number of cases in order to increase the probability of identifying mode of transmission or risk factors of disease. Question 11: In this investigation the investigators defined a case as an illness which is clinically compatible with measles. Discuss whether you would use this as your case definition. I would use this as my case definition because it would allow for a large number of cases to be identified. This is advantageous because it would ensure that no cases are missed and, hopefully, stop the spread to other people at risk. The Outbreak Six hundred thirty-three cases of measles were reported from Texarkana from June 1970 through Januaray Dates of onset were accurately determined for 535 cases. The epidemic curve (Figure 1) is shown below. 7

8 Question 12: Discuss the key features of the epidemic, which you can learn from this epidemic curve. From this epidemic curve, we can see that the spread of measles in preschool age children increases as school opened. Once it spread in preschool age children, it increases in school age children. Once vaccination campaigns were introduced, the measles cases greatly decreased ETS Summer Preparation Course: Texarkana Though infants, adolescents, and adults were involved in the epidemic, the majority of cases occurred in children 1 to 9 years of age. Measles cases were not evenly distributed within the two counties. Table 2 displays the number of measles cases and population by age group for Bowie County, Texas and in Miller County, Arkansas. 8

9 Attack Rates 19 per per per per per per per per 1000 Question 13: Calculate the attack rates indicated in Table 2. See above. Question 14: Compare the attack rates fro the Texas and Arkansas counties, for rural versus urban children, and for preschool versus school-age children. The attack rates for Texas is much higher than Arkansas counties in both age groups 1-4 and 1-9. The attack rates for urban children are much higher than rural children in Texas. And the attack rates are higher in preschool children aged 1-4 than for school age children

10 Part 3 Infectious Disease Epidemiology Measles in Previously Vaccinated Children Before this outbreak, the proportion of children vaccinated against measles in Miller County, Arkansas was significantly higher than the proportion vaccinated in Bowie County, Texas. In Texarkana, Texas, there had never been a community or school vaccination campaign for measles. In contrast there had been mass community programs against measles for school. In contrast, there had been mass community programs against measles for school and preschool children in 1968 and 1969 in Texarkana, Arkansas. Based on health departments and physician records, it was estimated that over 99% of children aged 1-9 years in Miller County, Arkansas had received measles vaccine prior to the outbreak. The overall vaccination level in Bowie County, Texas was estimated be 57%. In this outbreak, 27 of the measles in Bowie County and all 25 of the measles cases in Miller County gave a history of prior vaccination with live, attenuated, measlesvirus vaccine. Parental history of vaccination was corroborated for all the cases by clinic or physician records. Local health authorities in both counties were very concerned that children who had previously receive measles vaccine got the disease. Question 15: Calculate the attack rates among the vaccinated populations in both counties and comment on your findings. The attack rate for vaccinated children in Miller County was 1000 per 1000 while the attack rates for Bowie County was 55 per In other words, 100% of those who had the measles were vaccinated in Miller County, while 5.5% of those who had the measles were vaccinated in Bowie County. This may lead investigators to believe that vaccination protocols were not followed in Miller County. Vaccine Efficacy The ability of a vaccine to prevent disease effectively depends on its potency and proper administration to an individual capable of responding. The success of vaccination performed under field conditions may be assessed by measuring protection against clinical disease epidemiologic means. This epidemiologic approach has the merit of not requiring laboratory support. It can be very useful in field investigations, particularly when the occurrence of disease in vaccinated individuals leads to doubts about the effectiveness of the vaccination program. Vaccine efficacy is measured by calculating the cumulative incidence rates (attack rates) of disease among vaccinated and unvaccinated persons and determining the percentage reduction in the incidence rate of disease among vaccinated persons relative to unvaccinated persons. The greater the percentage reduction of illness in 10

11 the vaccinated group, the greater the vaccine efficacy. The basic formula is written as: VE = ((ARU-ARV)/ARV) * 100 Where VE = vaccine efficacy; ARU = attack rate in the unvaccinated population; and ARV = attack rate in the vaccinated population. Question 16: Using the basic formula, calculate the vaccine efficacy for Bowie County, TX. VE = (( )/28.8) * 100 = 92.9% ARU: Unvaccinated population: (29293 * 0.45) = vaccinated with measles 27/ = 2.05 per 1000 ARV: Vaccinated population: (29293 * 0.55) = = 466 unvaccinated with measles 466/ = x 1000 = 28.8 per 1000 Question 17: Was inadequate vaccine efficacy primarily responsible for this outbreak? Yes, the inadequate vaccine efficacy could be a primary reason for this outbreak because it allowed for a population at risk to spread it to other populations at risk. Question 18: What are the possible causes for the failure of the vaccine to protect vaccinated children from acquiring disease? Possible causes for failure of vaccine to protect vaccinated children from acquiring disease include poor handling of vaccine before immunization, not vaccinating at the recommended time point, and not receiving the correct number of rounds. Part 4 In previously vaccinated children aged 1-9 years in Bowie County, the measles attack rate in this outbreak was 4.2 per 1000 (table 4); the comparable rate in unvaccinated children was 96.9 per From these data, a vaccine efficacy of 95.7 per This is a minimum figure since it has been assumed that all 27 children were correctly vaccinated and that all of the cases therefore represent vaccine failure. In actuality some of these patients did not receive vaccine under ideal conditions. Eight of the 28 previously vaccinated patients had been vaccinated by nurses from the Texarkana/Bowie County Health Unit at a day nursery. The vaccine for these eight children had been carried back and forth to the nursery from the Health Unit 11

12 in a cooler in a car on three separate days in June and July Although a lapse in technique which allowed warming of the vaccine cannot be documented here, it is a possible explanation. An additional seven patients had been vaccinated under the age of 1 year. These children were vaccinated in the years when it was recommended that measles vaccine be given at age 9 months. It has since been learned that vaccine failure rate as high as 20 percent may accompany vaccination at 9 months in the United States. Question 19: What is the WHO recommended age for measles vaccination in developing countries? What are the factors that account for different recommendations in different countries? The WHO recommends the first measles vaccination be given at 9 months for countries with ongoing transmission and the second does between 15 and 18 months. In countries with low measles transmission, the first dose may be given at 12 months and second dose between 15 and 18 months. The factors that account for different recommendations included transmission rates of measles and average birth weights of children. 12