A study published in 1990 (Am J Pub Health 80:p. 209-210) investigated the occur

Question

A study published in 1990 (Am J Pub Health 80:p. 209-210) investigated the occurrence of HIV infection among prisoners in Nevada. Of 1105 prison inmates who were tested for HIV upon admission to the prison system, 36 were found to be infected. All uninfected prisoners were followed for a total of 1207 person-years and retested for HIV upon release from prison. Two of the uninfected inmates demonstrated evidence of new HIV infection. Assuming that the two prisoners were infected during their time in prison:

a. Based on the above information, calculate the incidence rate of HIV infection among prisoners in the Nevada prisons.

b. Express the incidence rate calculated in part a in terms of cases per 1000 person-years.

c. Calculate the prevalence of HIV infection among the incoming prisoners in the Nevada prisoners under study.

d. What is the size of the population at risk for HIV at the beginning of the study?

Explanation / Answer

A. The incidence rate of HIV infection among prisoners can be calculated as follows;

Incidence rate = Incidence density = no. of disease onsets divided by Sum of person - time @ risk

Number of disease onsets is 36+2=38

Sum of person-time@risk is 1207

So the incidence rate is approximately 31%.

B. The incidence rate in terms of cases per 1000 person years can be calculated as follows;

Incidence rate = Incidence density = no. of disease onsets divided by Sum of person - time @ risk

Number of disease onsets is 36+2=38

Sum of person-time@risk is 1000

So the incidence rate is approximately 26%.

C. The prevalence of HIV infection among incoming prisoners under study can be calculated as follows;

Prevalence = no. of cases divided by population size

Number of cases - 38

Population size - 1105

So Prevalence rate is approximately 29%.

D. The size of population at risk for HIV at the beginning of the study is (1105-36) 1069.

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