WBR0039

Author	PageAuthor::Gonzalo Romero
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 effect of such 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 decreases and Incidence remains unchanged
Answer A Explanation	[[AnswerAExp::Incorrect. Prevalence could decrease if for example; the mortalitiy or recovery increases even though Incidence or new cases remain unchanged. This is not the case presented in the vignette]]
Answer B	AnswerB::Incidence decreases
Answer B Explanation	[[AnswerBExp::Correct. Correct due to the explained above]]
Answer C	AnswerC::Incidence will increase and concomitantly prevalence will increase
Answer C Explanation	[[AnswerCExp::Incorrect. This is not logical, because incidence decreases.]]
Answer D	AnswerD::No effect will be seen
Answer D Explanation	[[AnswerDExp::Incorrect. The vaccine was proved to be effective, so an effect will be seen]]
Answer E	AnswerE::Prevalence will decrease and Incidence will remain unchanged
Answer E Explanation	[[AnswerEExp::Incorrect. Prevalence depends also in Incidence, Mortality and Recovery. It makes sense in the case of chronic conditions that a decrease in incidence will reduce the prevalence. With this vaccination Incidence will decrease.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::This question is testing basic epidemiologic concepts. A vaccine for an acute condition will decrease the incidence or appearance of new cases. Prevalence = Incidence X Duration, therefore the Incidence equals Prevalence in the case of Acute cases such as Influenza. If hypothetically Influenza would cause chronic cases, this will create a prevalente pot or poll. Creating a vaccine which decreases incidence will eventually decrease the prevalence. For example vaccination against Poliomyelitis decreases Incidence of acute polio, therefore the prevalence of Polio consequences. Educational Objective: Educational Objective: References: ]]
Approved	Approved::Yes
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