Elizabeth Hinson ID Homework #2

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1 Question 1: Would you call this an epidemic? I would consider this an epidemic, because an epidemic is the occurrence of cases of a condition in a population in a number greater than expected for a given period of time. With the gastrointestinal illness, the ill had all attended a church supper held the previous evening. 46 of the 75 persons interviewed, who went to the church supper, became ill. Therefore, the number of cases of gastrointestinal illness in the village of Lycoming, Oswego County, New York were greater than expected, making this outbreak an epidemic in this period of time. The county experienced a sudden increase in the number of cases of gastrointestinal illness above what is normally expected in the population of that area. Question 2: Review the steps of an outbreak investigation. In an outbreak investigation one must 1) identify investigative team members and their roles, 2) confirm the existence of an outbreak by comparing the disease rates with background levels, ruling out spurious factors, and verifying diagnosis, 3) select a case definition, which may include time and place of exposure, lab findings, and clinical symptoms, 4) identification of cases through reviewing existing surveillance, surveying hospitals, asking existing cases if they know others who may have been exposed, and 5) identify the population at risk. Determination of study design, collection of data, and then complementary analysis of the data collected are then completed after the investigation. 1 P a g e

2 Question 6a: What is the value of an epidemic curve? The value of an epidemic curve is that it can show the investigators the temporal trend of the infectious disease and therein what type of epidemic they are dealing with: common source, a point epidemic, a propagated epidemic, or a mixed epidemic. This in turn can help the investigators discover the means of transmission of the disease. Question 6b: Graph the cases by time of onset of illness. What does this graph tell you? This graph shows that most of the cases had an onset of illness occur at 12:30am or 1:00am, with everyone except two individuals having their time of illness onset between 9:00pm and 2:30am. There was one individual who experienced an onset of illness before the church supper even occurred (ID #52 at 3:00pm on 4/18) and then one person that had an onset of illness way after everyone else (ID #16 at 10:30am on 4/19). This graph also tells the investigators that this is mostly a common source epidemic in which people became ill from a single source of contamination via a single vehicle, however everyone was not exposed at the 2 P a g e

3 same time, because the time of onset of illnesses differed, so this is not a point epidemic. Some individuals probably ate earlier than others. Question 7: Are there any cases for which the times of onset are inconsistent with the general experience? How might they be explained? There are two cases for which the times of onset are inconsistent with the general experience: a case at 3:00pm on 4/18 and a case at 10:30am on 4/19. The case on 4/18 at 3pm was before the church supper even began (supper was from 6pm to 11pm on 4/18), so maybe this person was sick from something nonrelated to everyone else or maybe this person was the cook and ate some of the contaminated food/water while preparing for the church supper. The case that had a time of onset of illness on 4/19 at 10:30am might have taken the contaminated food/water home to eat later. Maybe they ate their leftovers for breakfast on 4/19 and that s why they didn t get sick until later than everyone else. Question 8: How could the data in the line listing be better presented? The data in the line listing could be better presented if they were sorted by those who became ill and those who did not instead of being sorted by ID #, which doesn t tell us very useful information. Once this has been done, within the data for those who did become ill, it would be better if these data were sorted by date and time of onset of illness. 3 P a g e

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6 Question 1a: What are the reasons that you might investigate this outbreak? I might investigate this outbreak, because: 1) Measles is a highly contagious infectious disease with a reproductive number of 12-18, 2) Immunized children are allegedly contracting the disease, 3)Texarkana is a city that straddles the Texas-Arkansas state line, which puts even more people at risk, 4) families visit relatives back and forth on both sides of town and children go to private nurseries/kindergartens on both sides of town, 5) there are many opportunities for contact among persons of all ages on both sides of town, and 6) this is an epidemic. Question 1b: What would be the objectives of the initial phase of your investigation? The objectives of the initial phase of this investigation would include gaining information about the disease, pathogens, and risk factors to implement urgent preventative measures, while also quarantining the healthy/exposed and isolating the sick. This would be done through the steps conducted in an outbreak investigation which include: 1) identify investigative team members and their roles, 2) confirm the existence of an outbreak by comparing the disease rates with background levels, ruling out spurious factors, and verifying diagnosis, 3) select a case definition, which may include time and place of exposure, lab findings, and clinical symptoms, 4) identification of cases through reviewing existing 6 P a g e

7 surveillance, surveying hospitals, asking existing cases if they know others who may have been exposed, and 5) identify the population at risk. Determination of study design, collection of data, and then complementary analysis of the data collected are then completed after the investigation. Question 2a: How might you define a case for purposes of this investigation? Note: Some information retrieved from For purposes of this investigation, a case might be defined by time and place of exposure, lab findings, and clinical symptoms. For example: MEASLES Clinical description: Measles is an acute, highly contagious illness that can be transferred from person to person via a cough or sneeze; The disease is characterized by a 4-5 day prodrome with high fever, coryza, cough, and conjunctivitis followed by a bright maculopapular rash. Temperatures return to normal 2-3 days after rash appears and the rash will last 5-7 days. Case Classification: Probable- an illness that meets the measles clinical description with No epidemiologic linkage to a laboratory-confirmed measles case; and Noncontributory or no measles laboratory testing. Confirmed an acute febrile rash illness with Isolation of measles virus from a clinical specimen; or Detection of measles-virus specific nucleic acid from a clinical specimen using polymerase chain reaction; or 7 P a g e

8 IgG seroconversion or a significant rise in measles immunoglobulin G antibody using any evaluated and validated method; or A positive serologic test for measles immunoglobulin M antibody; or Direct epidemiologic linkage to a case confirmed by one of the methods above. Question 2b: 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. The difference between a sensitive case definition and a specific case definition is that a sensitive case definition is a wide net that investigators cast to ensure that all true cases are identified, however it will also include individuals that do not actually have the disease. A specific case definition increases the likelihood that the cases identified are cases, but being warier of diagnosing someone as a case can decrease the sensitivity making it likely to miss someone that is actually a case. Therefore, in the initial case definition there is a greater emphasis on sensitivity than specificity, then subsequent case definitions might have greater specificity. In the initial conducting of an outbreak investigation it would be helpful to have a sensitive case definition, while when choosing subjects for a research study it might be more helpful to have a specific case definition to increase the accuracy of the study conclusions like risk factors for transmission. Question 2c: In this investigation, a case was defined by the investigators 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 in the initial conducting of an outbreak investigation it is better to use a sensitive case definition to increase the likelihood of correctly identifying all the measles cases. Isolating those without measles will not hurt people, but leaving the measles infected cases unidentified will lead to an increase in the spread of the disease resulting in more measles cases and more harm to a larger number of individuals in the community of Texarkana and potentially in both the states of Texas and Arkansas. 8 P a g e

9 Question 3: Discuss the key features of the epidemic which you can learn from this epidemic curve. Key features of the measles epidemic in Texarkana that one can infer from the epidemic curve above are that the epidemic is following a periodic trend with seasonal and annual variation. The cases of measles increase when schools open back up and continue to increase until finally vaccine campaigns were held. The epidemic curve also shows that there are more cases within the School Age students than the Preschool Age children. This might be because not all parents choose to send their children to preschool, but all children are required to go to kindergarten. Once the vaccine campaigns are held then the amount of measles cases decreases and it seems that the investigators quite recording data on if the children were Preschool Age or School Age, maybe because it was irrelevant at that point in the study. The disease may have initially started with Preschool Age children and then spread to School Age children at the cases homes, because they are siblings. Either way, once schools opened back up measles was able to spread among large amounts of persons. The 9 P a g e

10 epidemic curve also shows that this epidemic is a propagated epidemic and gives key information about the incubation time of the measles infection. (47/ 2452) *100= 1.92% (178/ 3242) *100= 5.49% (225/ 5694) *100= 3.95% (195/ 2481) *100= 7.86% (73/ 3010) *100= 2.43% (268/ 5491) *100= 4.88% (242/ 4933) *100= 4.91% (251/ 6252) *100= 4.01% (493/ 11185) *100= 4.41% (19/ 2671) *100= 0.71% (6/ 3345) *100= 0.18% (25/ 6016) *100= 0.42% Question 4a: Calculate the attack rates indicated in Table 2. See above for calculations. Question 4b: Compare the attack rates for the Texas and Arkansas counties, for rural versus urban children, and for preschool versus school-age children. Texas county had a total attack rate of 4.41%, which is about ten times higher than the Arkansas county total attack rate of 0.42%. Rural children in Texas county had an attack rate of 3.95%, while urban children in Texas county had an attack rate slightly more of 4.88%. This could be because rural children might be homeschooled so they can work on the farm etc. 10 P a g e

11 In Bowie County, Texas the preschool-age children had a higher attack rate of 4.91%, while the school-age children had an attack rate of 4.01%. In Miller County, Arkansas the preschool-age children also had a higher attack rate than the school-age children, 0.71% and 0.18%. Therefore, in both counties preschool-age children have the higher attack rate. Question 5: Calculate attack rates among the vaccinated populations in both counties and comment on your findings. Measles No Measles Total Vaccinated (0.57* 11185) = 6375 Not Vaccinated Total (total population) Bowie County, Texas: Attack Rate (among vaccinated) = (27/ 6375) *100= 0.42% Measles No Measles Total Vaccinated (0.99* 6016) = 5956 Not Vaccinated Total (total population) Miller County, Arkansas: Attack Rate (among vaccinated) = (25/ 5956) *100= 0.42% Both counties have the same attack rates among the vaccinated populations of 0.42%. Therefore, measles attacks 0.42% of those vaccinated within each of the counties. 11 P a g e

12 Question 6a: Using the basic formula, calculate vaccine efficiency for Bowie County, Texas. Measles No Measles Total Vaccinated (0.57* 11185) = 6375 Not Vaccinated Total (total population) Attack Rate (among vaccinated) = (27/ 6375) *100= 0.42% Attack Rate (among unvaccinated) = (466/ 4810) *100= 9.69% VE= [ (ARU- ARV)/ ARU ] *100= [ ( )/ (0.0969) ] *100= 95.7% Question 6b: Was inadequate vaccine efficiency primarily responsible for this outbreak? Inadequate vaccine efficiency was not primarily responsible for this outbreak, because there is a 95.7% reduction of illness in those who were vaccinated compared to those who were not vaccinated. Perhaps the lack of vaccine coverage was the primary reason for this outbreak, but not inadequate vaccine efficiency. 12 P a g e

13 Question 7: What are the possible causes for the failure of the vaccine to protect vaccinated children from acquiring disease? Possible causes of failure of the vaccine to protect vaccinated children from acquiring disease include ineffective manufacturing of the vaccine virus type and insufficient dose of the antigen, the vaccine could have been administered incorrectly, maternal antibodies that protect neonates may interfere with the introduction of the vaccine, and the vaccine could have been given after the natural infection occurred. Question 8: What is the WHO recommended age for measles vaccination in developing counties? What are the factors that account for different recommendations in different countries? WHO recommends that children in developing countries receive 1 dose of measles vaccine by their first birthday through routine health services. Two doses of the vaccine are recommended to ensure immunity and prevent outbreaks, as about 15% of vaccinated children 13 P a g e

14 fail to develop immunity from the first dose. Factors that lead to different vaccine recommendations in different counties are things like sanitation, income, and health infrastructure. 14 P a g e