WBR0660

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian and Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biostatistics/Epidemiology
Sub Category	SubCategory::Gastrointestinal
Prompt	[[Prompt::A new vaccine is being developed for the prevention of Hepatitis E. In a phase 2 clinical trial, 100 individuals with negative Hepatitis E IgM and IgG anti-HEV antibodies are randomized to receive either the vaccine or placebo. Out of the 50 patients that received the vaccine, 1 went on to develop Hepatitis E in the 6 month follow-up after the vaccination. In the placebo group, 7 out of 50 patients developed Hepatitis E. What is the number needed to treat for this newly developed vaccine according to the results of this trial?]]
Answer A	AnswerA::1.14
Answer A Explanation	AnswerAExp::1.14 is not the number needed to treat
Answer B	AnswerB::7.14
Answer B Explanation	AnswerBExp::7.14 is not the number needed to treat
Answer C	AnswerC::8.33
Answer C Explanation	AnswerCExp::The number needed to treat according to this trial is 8.33 patients
Answer D	AnswerD::300
Answer D Explanation	AnswerDExp::300 is not the number needed to treat
Answer E	AnswerE::Data insufficient to calculate
Answer E Explanation	AnswerEExp::Given the equation below, all variables are available to calculate the number needed to treat.
Right Answer	RightAnswer::C
Explanation	[[Explanation::The number needed to treat (NNT) is an epidemiological measure that refers to the number of patients required to receive a certain treatment for 1 patient to benefit from it. It is the the inverse of the absolute risk reduction (ARR), where ARR is the absolute difference in the event rate between the treatment group (vaccine group) and the control group (placebo group). The ARR can be calculated as follows: Shown below is a detailed explanation of the calculation of NNT: Rate of HEV development in the vaccine group: (1/50) x 100 = 2% Rate of HEV development in the placebo group: (7/50) x 100 = 14% Absolute risk reduction (ARR) = 14 - 2 = 12% = 0.12 NNT = 1/0.12 = 8.33 patients Educational Objective: The number needed to treat (NNT) is an epidemiological measure that refers to the number of patients required to receive a certain treatment for 1 patient to benefit from it. NNT is defined as the inverse of the absolute risk reduction (1/ARR). References: First Aid 2014 page 53]]
Approved	Approved::Yes
Keyword	WBRKeyword::Number needed to treat, WBRKeyword::Absolute risk reduction, WBRKeyword::HEV, WBRKeyword::Trial, WBRKeyword::Vaccine, WBRKeyword::Placebo, WBRKeyword::NNT
Linked Question	Linked::
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