L14 Basic epidemiology and concepts of infection

Question 1

Definition of incidence?

Answer 1

Number of new cases occurred during a specified time frame in a specified population

Question 2

Definition of prevalence?

Answer 2

Total number of cases (old + new cases) present in a specified time

Question 3

Which specifies a time point? (rather than a time interval)

A. Incidence
B. Prevalence

Answer 3

B

Question 4

Which specifies a time interval? (rather than a time point)
A. Incidence
B. Prevalence

Answer 4

A

Question 5

Which uses time and population as denominator?
A. Incidence
B. Prevalence

Answer 5

A

Question 6

Which uses population only as denominator?

Answer 6

B

Question 7

Which of the following is incorrect?
A. Population means at risk individuals, those with vaccination does not count

B. Consistency means only data based on same definitions of numerator (case) and denominator (population+/-time) can be compared

C. Prevalence and incidence go in the same direction

Answer 7

C

• They may go in different direction
  e. g. HIV
• when preventive programmes appear, the incidence reduced
• with better survival of the patients, prevalence increases

Question 8

Which of the following are correct?
A. Continuous reporting of number of COVID-19 cases is an example of passive surveillance
B. Investigating an outbreak is an example of active case detection
C. Detecting IgG is a type of serological survey, but cannot distinguish vaccination vs natural infection

Answer 8

All of the above

A: Continuous reporting by HCP e.g. 50 notifiable infectious diseases

Question 9

What are the definitions of endemic, epidemic and pandemic?

Answer 9

1. Endemic: infection regularly found among a particular population even without external input
2. Epidemic: rapid spread of infection due to changes in environmental/microbial factors
3. Pandemic: epidemic of infection across a large region

Question 10

What is common source outbreak VS propagated outbreak?

Answer 10

Common source outbreak: all affected individual exposed to a common agent
(point source outbreak - single exposure)

Propagated outbreak: person-to-person spread (e.g. Norovirus)

Question 11

We can compare between populations to determine efficacy of intervention.

What is a cohort study? (VS case-control study) (3)
What are the disadvantages? (2)

Answer 11

1. Prospective time nature
2. Intervention VS control groups
3. Measure the Relative risk

Disadv

• Large number of subjects
• Long follow up

e.g. smokers vs non-smokers > follow over time > compare outcomes

Question 12

What is a case-control study? (3) (vs cohort)

Answer 12

1. Retrospective time nature
2. Diseased VS control group
3. Measure the Odds ratio

e.g. cancer patients vs non-patients > take history > compare

Question 13

What is case infection ratio?

Answer 13

Number of clinical (symptomatic) cases per 100 infections

Question 14

What is case fatality ratio?

Answer 14

Number of deaths attributed to an infection per 100 cases

Question 15

Which of the following is incorrect?
A. Incubation period = Latent period
B. Infectious period is after the latent period
C. Patients can be asymptomatic in the infectious period
D. Latent period is the generation time from acquisition to transmission
E. Incubation period determines the duration of quarantine of exposed individuals

Answer 15

A

Latent period can be longer/shorter than the incubation period
Latent period = time of disease acquisition to transmission
Incubation period = time of disease acquisition to illness onset (become symptomatic)

Incubation period > latent period = 隱形患者

Question 16

What components determine incidence? (3) 
SIR model
- Susceptible
- Infectious
- Recovered

Answer 16

1. Contact rate / Infectious and susceptibility
   - mode of transmission
   - network and distribution
2. Proportion susceptible
   - past history of infection
   - vaccination coverage
3. Proportion infectious
   - population density
   - Transmission efficiency

Question 17

List examples of human-to-human transmission.

Answer 17

1. Measles
2. HAV
3. HIV

Question 18

Basic reproductive rate (R0) is the average number of new infections initiated by a single infectious individual in a complete susceptible population.

What is effective reproductive number?

Answer 18

Actual number of secondary cases （=R0 x proportion of susceptive individuals in a population)

Basic reproductive rate (R0)
e.g. 1>3 1>3 1>3 一個傳三個
then R0 =3

• determined by pathogen characteristic
• Contact patterns within the community

Measles: 12-18 (airborne!)
Poliovirus: 5-7
Influenza: 1.5-2

Question 19

Categories of emerging infections: give examples for each
A. Reappearance of a known disease after a decline incidence
B. Extension of geographic range by newer methods
C. Recognition of an existing disease by newer methods
D. Introduction of new variant
E. Introduction of novel agent to human

Answer 19

A. rubella, measles, rabies
B. Dengue
C. HHV6,7,8, HIV, HCV
D. Influenza, hantavirus
E. SARS, COVID-19

Question 20

Which of the following are correct?

A. Acute short-term infection requires highly efficient modes of transmission

B. Acute short-term infection has short infectious period and excrete high titres of organisms

C. Examples of Acute short-term infection include influenza and measles

D. Persistent infection does not require highly efficiency modes of transmission

E. Examples of persistent infection include HIV and HepB

Answer 20

All of the above

Persistent infection: excreted continuously or intermittently over long period

Question 21

What is epidemic curve? (Exam question)

Answer 21

Graphical display of incident cases over time

= number of new case NOT accumulated