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Table 3 Attack rates and TIV efficacy for ILI during the influenza season* (ATP cohort for efficacy)

From: Challenge of conducting a placebo-controlled randomized efficacy study for influenza vaccine in a season with low attack rate and a mismatched vaccine B strain: a concrete example

Trivalent inactivated split virus influenza vaccine
N = 4011
Placebo
N = 2003
Vaccine efficacy
       % 95% CI† p value †
Number of events Number of subjects reporting at least one event % Attack rate Number of events Number of subjects reporting at least one event % Attack rate   LL UL  
254 240 6.0 120 113 5.6 -6.1 -33.8 15.5 0.642
  1. * Influenza season starts the first week with two culture confirmed cases in the area, and closes at the end of the last week with one culture confirmed case in the areas.
  2. † Two-sided Fisher Exact test, the 95% confidence intervals for vaccine efficacy was calculated using a conditional exact method.