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Table 2 Attack rates and TIV efficacy for culture confirmed influenza in adults (total 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

Type of culture confirmed

Trivalent inactivated split virus influenza vaccine

N = 4137

Placebo

N = 2066

Vaccine efficacy (1-RR)

 

Number of cases

% Attack rate

Number of cases

% Attack rate

%

95% CI†

P value †

      

LL

UL

 

Any

28

0.7

18

0.9*

22.3

-49.1

58.5

0.433

Influenza A

6

0.1

4

0.2

25.1

-260.9

82.2

0.740

Influenza B

22

0.5

14

0.7

21.5

-65.9

61.6

0.481

  1. * Sample size calculation was based on expected attack rate of at least 4% in the placebo group
  2. † Two-sided Fisher Exact test, the 95% confidence intervals for vaccine efficacy was calculated using a conditional exact method.