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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor=Gonzalo Romero
|QuestionAuthor=Gonzalo Romero (Reviewed by Will Gibson and Yazan Daaboul)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biostatistics/ Epidemiology
|MainCategory=Biostatistics/Epidemiology
|SubCategory=Infectious Disease
|SubCategory=General Principles
|MainCategory=Biostatistics/ Epidemiology
|Prompt=A new vaccine is being developed to prevent a 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 epidemiological effect of this intervention . Which of the following will be the most appropriate statement regarding this new vaccine's effect?
|SubCategory=Infectious Disease
|Explanation=This question is testing basic epidemiologic concepts. [[Incidence]] is defined as 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 stated that the vaccine will be 96% effective in preventing new cases.  Therefore, the incidence will decrease.  Because [[prevalence]] is equal to [[incidence]] multiplied by the average duration of disease, the prevalence of the disease will decrease as well.
|MainCategory=Biostatistics/ Epidemiology
|SubCategory=Infectious Disease
|MainCategory=Biostatistics/ Epidemiology
|MainCategory=Biostatistics/ Epidemiology
|SubCategory=Infectious Disease
|MainCategory=Biostatistics/ Epidemiology
|SubCategory=Infectious Disease
|MainCategory=Biostatistics/ Epidemiology
|SubCategory=Infectious Disease
|MainCategory=Biostatistics/ Epidemiology
|SubCategory=Infectious Disease
|MainCategory=Biostatistics/ Epidemiology
|MainCategory=Biostatistics/ Epidemiology
|SubCategory=Infectious Disease
|Prompt=A new vaccine is being developed to prevent the new strand of Influenza H7N9 or bird flu recently causing an outbreak in China. A Clinical Trial concludes that this vaccine could be used in the general population to prevent this “bird flu”; the vaccine is effective in developing an immune response.  A Committee of practicing physicians in China is trying to understand the effect of such intervention in some epidemiologic measures. What would be the most appropriate statement of the following about this new vaccine's effect?


|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.
[[Prevalence]] = [[Incidence]] x Average duration of disease
 
|AnswerA=Prevalence will decrease and incidence will remain unchanged
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.
|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 will cause people to recover less quickly from [[influenza]] infection.
 
|AnswerB=Both incidence and prevalence will decrease
|AnswerA=Prevalence decreases and Incidence  remains unchanged
|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. Prevalence is proportional to the incidence. Accordingly, prevalence will also decrease.
|AnswerAExp=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
|AnswerC=Both incidence and prevalence will increase
 
|AnswerCExp=Because the [[vaccine]] will decrease the [[incidence]] of the disease, the [[prevalence]] will also decrease.
 
|AnswerD=No effect on either prevalence or incidence will be observed
|AnswerB=Incidence decreases
|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]], the [[incidence]] is expected to decrease.  Because [[incidence]] is proportional to prevalence, prevalence will also decrease.
|AnswerBExp=Correct due to the explained above
|AnswerE=Prevalence will decrease and incidence will remain unchanged
|AnswerC=Incidence will increase and concomitantly prevalence will increase
|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, it is expected that the [[vaccine]] will decrease both the [[prevalence]] and [[incidence]] of this strain of [[influenza]].
|AnswerCExp= This is not logical, because incidence decreases.
|EducationalObjectives=[[Prevalence]] is defined as the number of current cases of a particular disease.  [[Incidence]] is defined as the number of new cases of a particular disease in a given time period. The relationship between [[prevalence]] and [[incidence]] can be summarized in the equation below:
 
|AnswerD=No effect will be seen
|AnswerDExp=The vaccine was proved to be effective, so an effect will be seen
|AnswerE=Prevalence will decrease and Incidence will remain unchanged
 
|AnswerEExp=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.


[[Prevalence]] = [[Incidence]] x Average duration of disease
|References=First Aid 2014 page 52
|RightAnswer=B
|RightAnswer=B
|WBRKeyword=Biostatistics, Epidemiology, Incidence, Prevalence, Vaccine, Risk
|Approved=Yes
|Approved=Yes
}}
}}

Latest revision as of 23:08, 27 October 2020

 
Author PageAuthor::Gonzalo Romero (Reviewed by Will Gibson and Yazan Daaboul)
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biostatistics/Epidemiology
Sub Category SubCategory::General Principles
Prompt [[Prompt::A new vaccine is being developed to prevent a 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 epidemiological effect of this intervention . Which of the following will 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 will cause people to recover less quickly from influenza infection.]]
Answer B AnswerB::Both incidence and prevalence 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. Prevalence is proportional to the incidence. Accordingly, prevalence will also decrease.]]
Answer C AnswerC::Both incidence and prevalence will increase
Answer C Explanation [[AnswerCExp::Because the vaccine will decrease the incidence of the disease, the prevalence will also decrease.]]
Answer D AnswerD::No effect on either prevalence or incidence will be observed
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, the incidence is expected to decrease. Because incidence is proportional to prevalence, prevalence will 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, it is expected that the vaccine will 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 as 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 stated that the vaccine will be 96% effective in preventing new cases. Therefore, the incidence will decrease. Because prevalence is equal to incidence multiplied by the average duration of disease, the prevalence of the disease will decrease as well.

Prevalence = Incidence x Average duration of disease
Educational Objective: Prevalence is defined as the number of current cases of a particular disease. Incidence is defined as the number of new cases of a particular disease in a given time period. The relationship between prevalence and incidence can be summarized in the equation below:

Prevalence = Incidence x Average duration of disease
References: First Aid 2014 page 52]]

Approved Approved::Yes
Keyword WBRKeyword::Biostatistics, WBRKeyword::Epidemiology, WBRKeyword::Incidence, WBRKeyword::Prevalence, WBRKeyword::Vaccine, WBRKeyword::Risk
Linked Question Linked::
Order in Linked Questions LinkedOrder::