Question

In Community X (population 20,000), an epidemiologist conducted a prevalence survey in January of 2012 and reported an HIV prevalence of 2.2%. Over the next 12 months, the department of health reported an additional 50 new HIV cases between February 2012 and January 2013. The total population stayed constant at 20,000. Part 1 How many people had HIV in January 2012? Present or describe the formula you used to arrive at your answer. Calculate the incidence rate assuming no HIV-related deaths over the 12-month period. Present or describe the formula you used to arrive at your answer. Be sure to clearly indicate the numerator and denominator used in your calculation and include an appropriate label for the rate. In a summary of 200-250 words, interpret the results and discuss the relationship between incidence and prevalence. Discuss whether or not the epidemiologist should be concerned about these new HIV infections, assuming a previous incidence rate of 0.5 per 1,000 person-years prior to this updated risk assessment.

Answer 1

Based on the results of the study shown below, the epidemiologist has reason to worry because the incidence rate increased over four times the previous level as shown below:

• number of HIV cases = 440 persons
• new incidence rate = 2.25 per 1000 persons

What is prevalence and incidence rate?

Prevalence rate is the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.

• Prevalence rate = number of cases/total population × 100%

Incidence rate is the proportion of new cases of a condition observed within a given period of time.

• Incidence rate = number of new cases/population at risk × 1000
• population at risk = total population - previous cases

The number of people who had HIV in January 2012 is calculated as follows:

• number of HIV cases = prevalence rate × total population

number of HIV cases = 2.2% × 20000

number of HIV cases = 440 persons

Incidence rate between February 2012 and January 2013 is calculated as follows:

Incidence rate = 50/(20000 - 440) × 1000

Incidence rate = 2.25 per 1000 persons.

Based on the new incidence rate, the epidemiologist has reason to worry because the incidence rate has increased over four times the previous level.

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