You recently completed a year long study among a cohort of young adults living i

Question

You recently completed a year long study among a cohort of young adults living in Puerto Rico (n=3000). You conducted a serosurvey and determined that the prevalence of Zika antibodies was 3%. For those who tested negative for the Zika virus, you followed them for the year to determine if they became infected. You relied on hospital, clinic, and CDC reporting, as well as followed up with another antibody test for exposure at the end of the year.

You wanted to see if those who worked in the outdoors (farming, landscaping, construction, etc) for more than 5 hours a day were at increased risk of becoming infected. 43% of your cohort worked in the outdoors. The incidence of infection among those who worked outdoors was 81 per 10,000 per year. The incidence of those who were not exposed to the outdoors through work was 38 per 10,000 per year.

1. What is the rate ratio?

2. What is the risk ratio (hint: exp (rt)...what is t?

3. What is the odds ratio?

4. What is the AF%?

Suppose that you follow them for 7 years.

5. What would you estimate the risks and risk ratio to be--ceteris paribus?

6. What would be the PAF%?

Explanation / Answer

Solution:

1) Rate Ratio = incidence rate exposed / incidence rate unexposed
= 81/38 = 2.1316
2) Outdoors Non-Outdoors
Infected 0.3483% 0.2166%
Non Infected 42.6517% 56.7834%

Risk ratio = Pevent when exposed/Pevent when not exposed

= 0.3483/(0.3483+42.6517)/ 0.2166/(0.2166=56.7834)

= 2.1315789

3) Odds ratio = (Exposedcases/ Unexposedcases)/(Exposed non cases/ Unexposed non cases)

= (0.3483/0.2166)/42.6517/56.7834)

= 2.1408

4) AF% = [p0.(OR-1)/p0.(OR-1)+1] .100

here p0: exposure proportion in non-outdoors

So p0= 0.2166/0.57
= 0.38
AF% = 0.38 ( 2.1408 - 1) *100 / [0.38 ( 2.1408 - 1) + 1]
=30.24086 %

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