15. During a study of 20 years 5 persons were followed to measure the occurrence of upper respiratory tract infection (URTI). As this infection can occur more than once, all disease events are included in this study. 1 person is lost to follow-up after 1.5 years. 2 persons died respectively after 10 and 15 years from a different cause. 1 person got the first respiratory tract infection after seven years and the second infection after 12 years of follow-up. Both infections take half a year of recovery. This person is followed-up until the end of the study. One person is followed-up the whole period without occurrence of disease. What is the incidence rate in this study?

Question

15. During a study of 20 years 5 persons were followed to measure the occurrence of upper respiratory tract infection (URTI). As this infection can occur more than once, all disease events are included in this study. 1 person is lost to follow-up after 1.5 years. 2 persons died respectively after 10 and 15 years from a different cause. 1 person got the first respiratory tract infection after seven years and the second infection after 12 years of follow-up. Both infections take half a year of recovery. This person is followed-up until the end of the study. One person is followed-up the whole period without occurrence of disease. What is the incidence rate in this study?

Answer 1

To calculate the incidence rate in this study, we need to determine the total number of new cases of upper respiratory tract infection (URTI) and the total person-years of follow-up.

Let's break down the information given:

- Initially, 5 persons were followed for the entire 20-year study period.
- 1 person was lost to follow-up after 1.5 years (meaning we will not have data on them beyond this point).
- 2 persons died from a different cause after 10 and 15 years, respectively (meaning they contributed to the follow-up until their time of death).
- 1 person had their first respiratory tract infection after 7 years and the second infection after 12 years (both lasting half a year).
- 1 person was followed-up for the entire study period without any occurrence of disease.

Now, let's calculate the total person-years of follow-up:

For the 5 persons who were initially followed for the entire 20 years, we have 5 x 20 = 100 person-years.

For the person who was lost to follow-up after 1.5 years, they contributed 1.5 person-years (since they were followed for 1.5 years).

For the 2 persons who died after 10 and 15 years, respectively, they contributed 10 and 15 person-years of follow-up before their death.

For the person who had two infections after 7 and 12 years, each lasting half a year, they contributed 19.5 person-years (20 years minus the 6 months of infection/recovery time).

Finally, for the person who was followed-up for the entire study period without any occurrence of disease, they contributed 20 person-years.

Now, let's calculate the total person-years of follow-up:

100 + 1.5 + 10 + 15 + 19.5 + 20 = 166 person-years.

To calculate the incidence rate, we divide the total number of new cases of URTI by the total person-years of follow-up.

In this case, we have 2 new cases of URTI (the person who had two infections) and 166 person-years of follow-up.

So, the incidence rate in this study is 2/166 = 0.012 (or 1.2%) per person-year.