Dr.Noura Abouammoh

Dr. Afnan Younis

Define and calculate measure of frequency of disease (Prevalence, Incidence, Attack rate)

Be able to decide when to use them Be able to interpret the results

5 Ws.

So what? - Prevention and control

Distinguish between case and an episode- Case e.g. Poliomyelitis.- Episode e.g. an asthma attack requiring admission.

If 75 cases of TB in village A and 25 cases in village B, is TB commoner in village A?

The total number of cases of a disease in a given population at a specific time.

Prevalence = No. of cases with the disease at a point in time/Total no. of people in defined pop. at same point in time X 10n

Point vs. period

1000 men were working in factory A were screened for HIV on 1 January 2002 and 50 of them were found to be positive for HIV antibodies. The screening was repeated in the same 1000 men on 1 January 2003 and this time 62 men were positive, including the 50 men who were positive on the first screening.

What is the prevalence of HIV in men working in factory A on 1 January 2002 and 1 Jan 2003?

No. of prevalent cases at 1 January 2002= 50

No. persons in the pop. In January 2002=1000

Therefore, the prevalence at January 2002=50 / 1000 X 1000 = 50/1000

No. of prevalent cases at January 2003= 62No. of persons in the population at 1

January 2003= 1000

Therefore, the prevalence at 1 January 2003= 62/1000 X 1000 = 62/1000

Parents of 700 children in village A were asked if their children have developed diarrhoea during the past 3 weeks. 200 children had a positive history of diarrhoea.

Calculate the prevalence.

No. of cases = 200 No. of population = 700 Prevalence = 200/700 X 1000 =

285/1000

Frequency of new cases of disease in a defined pop. during a specified time period

Incidence= No. of new cases with disease in a specified time period/ No. of disease free people at the start of the time period X 10n

Disease free people = pop. at risk

Prevalence:- Useful in health care settings e.g.

estimate services required.

- Cannot study cases that got better or died.

Incidence:- Useful for investigating causal

relationships and risk factors.

1000 men were working in factory A were screened for HIV on 1 January 2002 and 50 of them were found to be positive for HIV antibodies. The screening was repeated in the same 1000 men on1 January 2003 and this time 62 men were positive, including the 50 men who were positive on the first screening.

What is the incidence of HIV infection in men working in factory A in 2003?

No. of new cases in 2003= 12No. of persons at risk of HIV infection in

January 2003 = 1000 – 50 = 950

The incidence of developing HIV infection = 12/950 X 1000= 12.6/1000

An attack rate is a variant of an incidence

rate, applied to a narrowly defined population

observed for a limited time, such as during an

epidemic.

The attack rate is usually expressed as %

percent.

Of 100 persons who attended a dinner party, 35 subsequently developed gastroenteritis.

Calculate the attack rate of

gastroenteritis

Attendees = 100ILL = 35Attack rate = (35 ÷ 100) X 100 = 35%

In a university student has got H1N1 flu. She attended a class of 80 students, of which 12 students got the flu 5 days later.

Calculate the attack rate of the flu.

Class = 80ILL :• new cases= 12• old case = 1• At risk students= 80 - 1 = 79Attack rate = (12 ÷ 79) X 100 = 15%