Table 2

Evidence of influenza* infection in cases and controls, by type of evidence and study year

Baseline influenza A antibody titre ≥6440751272312
Baseline influenza B antibody titre ≥64112†11†042
≥Four fold rise influenza A01510153
≥Four fold rise influenza B421‡00052
NAT positive influenza001‡00010
Any positive result941371283419
  • *Serological evidence of influenza is either a fourfold rise in titre between baseline and convalescent sera or a baseline titre of ≥64 in an unvaccinated individual. Vaccination status was validated by GP records.

  • †Also had a baseline antibody titre for influenza A ≥64 (only counted once) (1×2009 case and 1×2010 case).

  • ‡Also had a >fourfold rise in influenza A antibody titre (counted once in total).

  • GP, general practitioner; NAT, nucleic acid testing.