A study assessed the association between a new vaccine and traveler's diarrhea (TD). Researchers selected a random sample of people who intended to travel to regions where they were at increased risk for TD and who had received the new vaccine and another independent random sample of people who intended to travel to the same regions and who had not received the new vaccine. These 2 samples of travelers were assessed for the occurrence of TD during the trip and for 7 days after returning home. Which of the following measures of association are the investigators most likely to report?

Question

A study assessed the association between a new vaccine and traveler's diarrhea (TD).  Researchers selected a random sample of people who intended to travel to regions where they were at increased risk for TD and who had received the new vaccine and another independent random sample of people who intended to travel to the same regions and who had not received the new vaccine.  These 2 samples of travelers were assessed for the occurrence of TD during the trip and for 7 days after returning home.  Which of the following measures of association are the investigators most likely to report?

Quizplus · Accepted Answer

This case describes a cohort study in which participants are initially selected based on their exposure status to a risk modifier: exposed (eg, vaccinated) or nonexposed (eg, not vaccinated). Once participants are categorized based on exposure status, the occurrence (ie, incidence) of the outcome/disease of interest (ie, traveler's diarrhea [TD]) within a specific period is determined in each group. The risk of disease is finally compared between exposed and nonexposed groups to estimate the association between exposure to a risk modifier and disease occurrence. A common measure of association in cohort studies is the relative risk (RR). The RR is calculated as follows: RR = (risk of disease in exposed) / (risk of disease in nonexposed) The exposed group could also have been defined as the unvaccinated group and the nonexposed group as the vaccinated group; in this case, the RR would be inversed (interpretation of RR depends on what is chosen to represent the exposure). (Choices A and D) Incidence (ie, new occurrences of a disease in a population over a period) and prevalence (ie, existing cases of a disease in a population at a point in time) are measures of disease burden. They do not directly convey information regarding the relationship between exposure and disease. (Choice B) Median survival time is calculated in cohort studies or clinical trials and is used to compare the median time to an event of interest (eg, TD) in ≥2 groups of individuals (eg, those not vaccinated and those vaccinated). The purpose of this study is to assess the association between exposure to a risk modifier (eg, vaccination) and disease occurrence (eg, TD), not to compare the median time to develop the disease (eg, TD) between exposed and nonexposed groups. (Choice C) Odds ratio is calculated in case-control studies, where 2 groups of individuals are initially identified as "cases" or "controls" according to an outcome (eg, disease of interest), and then the frequency of exposure to the risk factors is compared between cases and controls to estimate the association between the risk factors and the outcomes. Educational objective: A cohort study is used to compare incidence of disease between exposed and nonexposed individuals. A common measure of association in cohort studies is the relative risk.

A Study Assessed the Association Between a New Vaccine and Traveler's