EPIDEMIOLOGY

Lecture 3:Incidence

What is incidence?

• The rate at which events occur in a population.

new

Numerator

Denominator

Multiplier

Number of NEW events in a specified periodX 10

n

The numerator strictly refers only to first events of disease.

• Frequency Count• Rate• Proportion

• Average population exposed to risk during the period.– The average size of the population is often the

estimated population size at the midperiod. E.g., mid-year population when calculating annual incidence rates.

The denominator of an incidence - 1

The denominator of an incidence - 2

• A person-time denominator (or other units such as passenger-miles denominator).– For each individual in the population, the time of

observation is the period that the person remains disease-free.

– The denominator used for the calculation of incidence is therefore the sum of all the disease-free person-time periods during the period of observation of the population at risk.

Example 1

0 1 1098765432

Duration (in years)

ABCDE

A

B

D

E

C

8 years

9 years

10 years

4 years5 years

Total p-y = 36

Incidence= (4/36)x1000 = 111.11 per 1000 person-yearsOR 111.11 per 1000 persons per year

Example 1

0 1 1098765432

Duration (in years)

ABCDE

A

B

D

E

C

5 years

6 years

10 years

6 years1 year

Total p-y = 28

Incidence= (4/28)x1000 = 142.86 per 1000 person-years

Example 1

0 54321

Duration (in years)

ABCDE

A

B

D

E

C

5 years

4 years

5 years

1 year3 years

Total p-y = 18

Incidence= (3/18)x1000 = 166.67 per 1000 person-years

• INCIDENCE DENSITY•FORCE OF MORBIDITY

The denominator of an incidence - 3

• Number of people free of the disease in the population at risk at the beginning of the period.

• INCIDENCE PROPORTION

• CUMULATIVE INCIDENCE

Cumulative incidence rate = (274/118539)x1000 = 2.31 cases per 1000 women

When calculating incidence, those having the disease in the population at the start or baseline need to be identified and excluded from the denominator

Attack rate

• The term “attack rate” is often used instead of incidence during a disease outbreak in a narrowly-defined population over a short period of time.

• The attack rate can be calculated as the number of people affected divided by the number exposed.

Secondary Attack Rate

• The number of cases of an infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts; the denominator is restricted to susceptible contacts when these can be determined.

• The secondary attack rate is a measure of contagiousness and is useful in evaluating control measures.

Case fatality

• the proportion of cases with a specified disease or condition who die within a specified time.

• usually expressed as a percentage.

• Case fatality is a measure of disease severity

The relationship between incidence and prevalence

PREVALENCE = INCIDENCE X Average DURATION of disease

If prevalence is low and does not vary significantly with time.

Jan 1, 2013

Dec 31, 2013

Population at risk =200(on 1st July, 2013)

A cohort of 100 people at risk of a disease was followed from 2012 to 2014. Following information is available.Case no Disease start date Disease end date1 Nov 2012 Feb 20132 Mar 2013 Dec 20133 Oct 2013 Feb 20144 Aug 2013 Nov 20135 Sep 2013 Dec 20136 Jan 2013 June 20137 Oct 2013 Mar 20148 Oct 2012 Dec 20129 Feb 2013 Sep 201310 Mar 2013 May 2104

Calculate the incidence rate for the period Jan 2013 to Dec 2013.