WBR0039

Author	PageAuthor::Gonzalo Romero (Reviewed by Will Gibson)
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biostatistics/ Epidemiology
Sub Category	SubCategory::Infectious Disease
Prompt	[[Prompt::A new vaccine is being developed to prevent the new H7N9 strain of influenza that has recently caused an outbreak in China. A clinical trial concludes that this vaccine provides a relative risk reduction of 96% for influenza infection in the general population. A committee of practicing physicians in China is attempting to understand the potential effect of this intervention on several epidemiologic measures. Which of the following would be the most appropriate statement regarding this new vaccine's effect?]]
Answer A	AnswerA::Prevalence will decrease and incidence will remain unchanged
Answer A Explanation	[[AnswerAExp::Prevalence could decrease if for example; the average duration of disease increases even though incidence remains unchanged. There is no evidence in the prompt that the vaccine would cause people to recover less quickly from influenza infection.]]
Answer B	AnswerB::Incidence will decrease
Answer B Explanation	AnswerBExp::The prompt states that the vaccine will be 96% effective in preventing new cases of influenza. Because the incidence represents the rate of new cases, the vaccine will decrease the incidence.
Answer C	AnswerC::Incidence will increase and prevalence will increase
Answer C Explanation	AnswerCExp::Because the vaccine will decrease the incidence of the disease, prevalence will also decrease.
Answer D	AnswerD::No effect will be seen
Answer D Explanation	[[AnswerDExp::Incidence is an epidemiologic measure representing the number of new cases in a given time period. Because the vaccine is shown to be 96% effective in preventing new cases of influenza, we would expect the incidence to decrease. Because incidence is proportional to prevalence, prevalence would also decrease.]]
Answer E	AnswerE::Prevalence will decrease and incidence will remain unchanged
Answer E Explanation	[[AnswerEExp::Prevalence depends on incidence and the average duration of disease. Prevalence will be much greater than incidence with chronic conditions where the average duration of disease is long. With short-lived conditions such as influenza, the incidence will closely reflect the prevalence. Therefore, one would expect this vaccine to decrease both the prevalence and incidence of this strain of influenza.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::This question is testing basic epidemiologic concepts. Incidence is defined at the number of new cases within a given time period. Prevalence is the number of people affected by a given condition at a single point in time. The prompt has stated that the vaccine will be 96% effective in preventing new cases. Therefore, the incidence will decrease. Because Prevalence = Incidence X average duration of disease, the prevalence of the disease will decrease as well. Educational Objective: Prevalence is defined as the number of current cases of a particular disease. Incidence is the number of new cases of a particular disease in a given time period. References: First Aid 2014 page 52]]
Approved	Approved::Yes
Keyword	WBRKeyword::Epidemiology, WBRKeyword::Biostatistics, WBRKeyword::Biostats, WBRKeyword::Incidence, WBRKeyword::Prevalence, WBRKeyword::Vaccine, WBRKeyword::Risk
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::