PUBH7600 Introduction to Epidemiology

Tuesday 30 August, 2 pm (QLD time)

QUESTION 1 [TOTAL 10 MARKS]
Background
Norovirus continues to pose a significant burden on cruise ships, causing dozens of major
outbreaks internationally each year. In January 2009, a suspected norovirus outbreak on
Cruise Ship X occurred early during a 15-day cruise and prompted an investigation. CDC
personnel boarded the ship and investigated the cause of the outbreak.
Of the 1842 surveys distributed to passengers, 1533 were returned. Of these, 236 met the
case definition for acute gastroenteritis (AGE), 1269 did not meet the case definitions and
were considered to be from well passengers. Symptom onset for case passengers occurred
during the period 3-14 January. Six case passengers with an onset date of 3 January were
excluded from risk factor analyses because their exposure most likely occurred prior to
boarding. No passengers reported illness onset prior to 3 January.
(a) What type of study design was used to determine the cause of the outbreak and why? [2
marks]
(b) Which type of relative disease measure is the most appropriate for this study? Justify
your answer. [2 marks]
(c) Calculate the attack rate and relative disease measure for each exposure item and fill the
results into table 1. [4 marks]
Table 1: Summary of questionnaire data collected during outbreak investigation
Exposure Exposed Unexposed
No. of ill
passengers
Total no. of
passengers
Attack
rate (%)
No. of ill
passengers
Total no. of
passengers
Attack
rate (%)
Relative
disease
measure
Present in
stateroom when ill
cabin mate was
present
87 305 121 1466
Eating at Main
dining
240 1345 10 41
Eating at Poolside 109 761 98 768
Witness vomiting
during boarding
5 14 232 1424
Witness vomiting
at shoreside
12 61 207 1452
Witness vomiting
onboard
21 136 203 1333
(d) Describe the results of relative disease measure calculated in question c and interpret
the results. [2 marks]
QUESTION 2 [TOTAL 10 MARKS]
A study was conducted to investigate the asthma development among children. The study
started with newborn babies (n=3687), followed annually until the age of 8 years. All
participants attended the first follow-up (at year 1). The table shows age-specific prevalence
and incidence of asthma.
Table 2 -Age-specific prevalence and incidence of asthma
Age years Prevalence Incidence
1 221/3687 221/3687
2 213/3639 80/3551
3 353/3629 201/3480
4 291/3621 79/3331
5 279/3621 56/3263
6 273/3616 39/3213
7 219/3615 34/3124
8 230/3576 25/3071
Data are presented as n/N
(a) What was the point prevalence of asthma among study participants at the beginning and
end of study? [2 marks]
(b) What was the cumulative incidence of asthma among the study participants over the
study period? [4 marks]
(c) How many infants were still at risk of developing asthma at age 5 [1 mark]?
(d) What are the reasons for the differences in the numerators and denominators between
prevalence and incidence? Explain (1 paragraph). [3 marks]
QUESTION 3 [TOTAL 15 MARKS]
Chronic kidney disease is defined as kidney damage or reduced kidney function which lasts
for an extended period. End-stage kidney disease (ESKD) is the more severe form of chronic
kidney disease where kidney replacement therapy such as dialysis or kidney transplant is
necessary to avoid death.
Table 3 presents the number of new cases of end-stage kidney disease (ESKD) in Indigenous
Australians and the total Australian population, by age group.
Note: Three years of data on new cases are presented (2007-2009). Population data comes
from a single year (2008). It is conventional to present incidence rates as annual average
rates.
Table 3
Age Group
(Years)
New cases of ESKD
(Indigenous)
2007-2009
Indigenous
Population
2008
New cases of ESKD
(Australian total)
2007-2009
Total Australian
Population
2008
0-34 61 441,136 612 10,065,970
35-44 143 80,034 650 3,106,209
45-54 222 57,375 1,154 2,957,349
55-64 183 30,883 1,543 2,411,800
65-74 59 13,362 1,726 1,495,010
75+ 7 6,377 1,564 1,337,660
TOTAL 675 629,167 7,249 21,373,998
a) Calculate the crude incidence rate (per 100,000) for end-stage kidney disease (ESKD) in (i)
the Indigenous population and (ii) the total Australian population [4 marks]
b) Calculate the crude incidence rate ratio (expressed as a %) for end-stage kidney disease
(ESKD) in the Indigenous population relative to the total Australian population [1 mark]
c) How many cases of ESKD would you expect in the overall Australian 55-64 year old age
group if they had the same incidence rate as 55-64 year old Indigenous Australians? [2
marks]
d) Calculate the direct age-standardised incidence rate (per 100,000 per year) for ESKD in
the Indigenous population using the Australian population as the standard. [4 marks]
e) What does the difference between the age standardised rate for ESKD in the Indigenous
population and the crude incidence rate in the Australian population tell us about the
burden of ESKD in Indigenous Australians relative to the total Australian population? (1
paragraph) [2 marks]
f) What does the difference between the crude incidence rate of ESKD in the Indigenous
population and the direct age-standardised incidence rate of ESKD in the Indigenous
population tell us about the study population (Indigenous Australians) and the standard
population (the total Australian population)? (1 paragraph) [2 marks]
QUESTION 4 [TOTAL 15 MARKS]
A study was conducted to investigate the association between the consumption of sugar-
sweetened carbonated beverage (SSCB) and obesity in children and adolescents from
Navarra (Spain). Researchers recruited 174 children and adolescents who were obese (BMI
above the age-and sex-specific 97th percentile according to Spanish reference charts).
Researchers also recruited 174 children and adolescents who were not obese. Information
about SSCB consumption was obtained from a questionnaire. Among obese children and
adolescents, 118 consumed more than 1 serving per week of SSCB – the rest reported never
or almost never consuming SSCB. In comparison 68 non-obese children and adolescents
reported never or almost never consuming SSCB. The rest consume SSCB more than 1
serving per week.
(a) What is the study design? Justify your answer (1-2 sentences) [2 marks]
(b) In your own words, what are key advantages of this study design? (2-3 sentences) [2
marks]
(c) Construct a 2x2 table conveying the information above and calculate an appropriate
relative measure of the strength of the association between obesity and SSC
consumption. Interpret your results (1-2 sentences) [4 marks]
(d) Calculate the attributable fraction of obesity that is associated with SSCB consumption [2
marks]
(e) How would you interpret the finding in Q4(d)? [1 mark]
(f) What proportion of obesity in the population is potentially preventable, assuming a
causal association between SSCB consumption and obesity? [4 marks]