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In a case-control study of the association between a new influenza vaccine and viral pneumonia in...

In a case-control study of the association between a new influenza vaccine and viral pneumonia in elderly patients, cases of influenza were selected from the patient records of a family medicine practice. Controls are selected from patient records of the same family practice, who did not have influenza and were of same age group. However the investigators were not masked to the case control status of study subjects

Question: what type of bias would result from this study and why, give the direction of the bias, under or overestimation

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Ans) In conclusion, the 2010-2011 seasonal influenza vaccine was effective for preventing laboratory-confirmed influenza, especially for influenza A virus, in a South Korean adult population. However, evidence of the effectiveness of the influenza vaccine in the elderly or against influenza B virus was not found. For the elderly, in particular, the use of next-generation influenza vaccines such as adjuvanted, intradermal, or high-dose vaccines should be considered if improved effectiveness and cost-effectiveness are proven.

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