Question: Dr. Claust suspects that this was a “vehicle-borne outbreak, with food as the vehicle.” Explain what this statement means.

Background

On May 12, 2018, the local health officer in the rural town of Dayson, Australia, reported the occurrence of an outbreak of acute gastrointestinal illness to the District Health Officer. Dr. A. R. Claust, epidemiologist-in-training, was assigned to conduct an investigation.

When Dr. Claust arrived in the field, he learned from the health officer that all persons known to be ill had attended a community supper held on the previous evening, May 11. Family members who did not attend the commnity supper did not become ill. Accordingly, Dr. Claust focused the investigation on the supper. He completed Interviews with 40 of the 47 persons known to have attended, collecting information about the occurrence and time of onset of symptoms, and foods consumed. Of the 40 persons interviewed, 26 persons reported gastrointestinal illness.

Clinical Description

The onset of illness in all cases was acute, characterized chiefly by nausea, vomiting, diarrhea, and abdominal pain. None of the ill persons reported having an elevated temperature; all recovered within 24 to 30 hours. Approximately 20% of the ill persons visited physicians. No fecal specimens were obtained for bacteriologic examination.

Description of the Supper

The supper was held in the basement of the village community center. Foods were contributed by numerous members of the community. The supper began at 5:00 p.m. and continued until 10:00 p.m. Food was spread out on a table and consumed over a period of several hours. Data regarding onset of illness and food eaten or water drunk by each of the 40 persons interviewed are provided in the attached table provided on the next page (and can also be found in the included data file). The approximate time of eating supper was collected for only about one third of the persons who had gastrointestinal illness.

Task I Questions

a.       Would you describe this as an epidemic? Explain your answer. [1 mark]

b.       Dr. Claust suspects that this was a “vehicle-borne outbreak, with food as the vehicle.” Explain what this statement means. [1 mark]

c.       Using the data provided in Table I (and also under the data file named of ‘Assignment Part A_Question 1.xlsx’ in Canvas), are there any cases for which the times of onset are inconsistent with the general expereince? How might they be explained? [4 marks]

d.       Using the data file of ‘Assignment Part A_Question 1.xlsx’ provided in Canvas, constuct a single table which includes the attack rates for each food item served for: [4 marks]

·        People who ate the specific food

·        People who did not eat the specific food

e.       Which food is the most likely vehicle of infection? Explain your answer. [2 marks]

ID

AGE

SEX

TIME OF MEAL

ILL?

DATE ONSET

TIME ONSET

Background

During the early months of 2018, the San Altaz Lab released an emergency warning message to the CDC that the experimental virus that had been working on (Pax-08) had become unstable and was at risk of being released into the atmosphere. Swiftly, the CDC quarantined the San Altaz Lab and all 12 of the lab’s researchers.

The date of onset for each researcher seemed to vary between February 20 and April 20. The CDC identified that Pax-08 lasts for 15 days and confers no immunity. Fortunately, there were no deaths to Pax-08 and no loss to follow up. The data provided in table II represents the dates of onset for the 12 Pax-08 episodes:

Table II: Dates of onset

Task II Questions

a.       Constuct a persons-time chart for this scenario. [4 marks]

b.       Determine the point prevalence of Pax-08 on March 31^st. Express your answer up to two decimal places. [2 marks]

c.       For the entirety of March, what was the period prevalence for Pax-08? Express your answer up to two decimal places. [2 marks]

d.       For the entirety of March, what was the incidence rate for Pax-08? Express your answer up to two decimal places. [2 marks]

e.       Calculate the total person-days for the period March 1^st to April 30^th. [2 marks]

f.        How many days within the period March 1^st to April 30^th are the persons not at risk because they are already ill? Show your working. [4 marks]

g.       How many person-days at risk (total) are there within the period March 1^st to April 30^th? [2 marks]

h.       What is the averge incidence density of Pax-08 for the period March 1^st to April 30^th, inclusive? Express your answer up to three decimal places. [2 marks]

Background

Task III Questions

a.       Using information provided in Table III(a) below, calculate the age-specific mortality rate (per 100,000) for the Ischaemic heart disease among men aged 40-69 years in 1984.  Express your answer up to three decimal places. [2 marks]

Table III(a): Age-specific mortality rate for the Ischaemic heart disease among men aged 40-69 years in 1984

b.       Using information from part (a), explain how the crude mortality rate of 323.5 per 100,000 population for 1984 was calculated. Show your working. [2 marks]

c.       Using information provided in Table III(b) below, calculate the age-specific mortality rate (per 100,000) for the Ischaemic heart disease among men aged 40-69 years in 1994. [2 marks]

Table III(b): Age-specific mortality rate for the Ischaemic heart disease among men aged 40-69 years in 1994

d.       Using information from part (c), explain how the crude mortality rate of 212.7 per 100,000 population for 1994 was calculated. Show your working. [2 marks]

e.       Using information provided in Table III(c) below, calculate the expected number of deaths due to Ischaemic heart disease among men aged 40-69 years for both 1984 and 1994. Express your answer up to three decimal places. [6 marks]

Table III(c): The expected number of deaths due to Ischaemic heart disease among men aged 40-69 years in both 1984 and 1994 using the reference population

f.        Using information from part (e), calculate the age-adjusted mortality rate (per 100,000) for both 1984 and 1994 using the direct method. Show your working and express your answer up to one decimal place. [2 marks]

g.       Is the observed dropped in the mortality rates between 1984 and 1994 simply due to a change in the age structures of the respective populations? Explain your answer. [3 marks]

Background

A physical examination and a videometric test were given to 1800 persons with suspected vision disorders. The prevalence of vision disorders within this population is known to be 15%. The sensitivity of the physical examination is 80% and its specificity is 70%. The sensitivity of the videometric test is 90% and the specificity is 90%.

Task IV Questions

a.       Complete the following table and explain how many people would be labelled as “positive” on the physical examination? [6 marks]

b.       Complete the following table and explain how many people would be labelled as “negative” on the videometric test? [6 marks]

c.       Which test has the larger Positive Predictive Value (PPV)? Report PPV value for both physical examination and videometric test (up to two decimal places) to justify your answer. [3 marks]

d.       What is the net sensitivity for this scenario? Express your answer up to two decimal places. [2 marks]

e.       What is the net specificity for this scenario? Express your answer up to two decimal places. [2 marks]