Question

What was the cumulative incidence of HIV among study participants in January 2003? What was the cumulative incidence of HIV among study participants in January 2003?

In January 2000, 5000 men and women aged 18-30 years were recruited from a semi-urban area near Durban (South Africa) and assessed for human immunodeficiency virus (HIV). Participants who did not test positive for HIV, at baseline in 2000, were followed-up and assessed again 3 years later in January 2003 (only those who previously had tested negative for HIV were tested again at follow-up).

The following results were found:

• In 2000, at baseline: 700 participants were found to have HIV when tested

• In 2003: a further 75 cases of HIV were identified.

• These people were diagnosed at some stage during the 3 years from 2000 to 2003, the exact date of diagnosis was not available.

• Over the 3 years between 2000 and 2003: 25 people who were diagnosed with HIV at baseline in 2000 died

• By January 2003: 500 people without HIV had either died, or could not be found; no dates were known.

Key assumptions:

• HIV was acquired randomly during the 3 year period. That is, those who acquired HIV were at risk for half the time of those who did not acquire HIV or 1.5 years

• LTFU and dead were at risk for half of the 3 years or 1.5 years

Answer 1

Cumulative incidence of HIV among study participants in January 2003 :-

Cumulative incidence = Number of new cases of disease during a given period of time / Total population at risk

Number of new cases in 2003 = 75 new cases of HIV in a 3year period .

Total population at risk = 5000 people at risk

Cumulative incidence = 75 / 5000

= 0.015

15 cases per 1000 men and women during 3years .