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Table 1 Meta-analysis results comparing primary outcome (RT-PCR and/or culture) with secondary outcome (RT-PCR and/or culture and/or HAI assay) for all ages

From: Natural attack rate of influenza in unvaccinated children and adults: a meta-regression analysis

Type of influenza # of influenza seasons Sample size (placebo group) # of influenza cases (placebo group) Natural attack rate % (lower 95% CI, upper 95% CI) I 2
Primary outcome (PCR and/or viral culture)
All 37 39115 1774 7.86 (5.48, 10.23) 99.86%
Influenza A 34 38360 1303 5.68 (3.71, 7.65) 99.69%
Influenza B 35 38854 491 1.87 (1.02, 2.72) 99.87%
Secondary outcome (PCR and/or viral culture and/or HAI assay)
All 47 41212 2055 9.84 (7.06, 12.62) 99.77%
Influenza A 41 40074 1505 6.96 (4.49, 9.42) 99.80%
Influenza B 42 40568 536 2.57 (1.45, 3.70) 99.93%
  1. Abbreviations: CI confidence interval, HAI haemagglutination inhibition assay, RT-PCR reverse transcription polymerase chain reaction.