Evidence of influenza* infection in cases and controls, by type of evidence and study year
| 2008 | 2009 | 2010 | Total | |||||
|---|---|---|---|---|---|---|---|---|
| Cases | Controls | Cases | Controls | Cases | Controls | Cases | Controls | |
| Baseline influenza A antibody titre ≥64 | 4 | 0 | 7 | 5 | 12 | 7 | 23 | 12 |
| Baseline influenza B antibody titre ≥64 | 1 | 1 | 2† | 1 | 1† | 0 | 4 | 2 |
| ≥Four fold rise influenza A | 0 | 1 | 5 | 1 | 0 | 1 | 5 | 3 |
| ≥Four fold rise influenza B | 4 | 2 | 1‡ | 0 | 0 | 0 | 5 | 2 |
| NAT positive influenza | 0 | 0 | 1‡ | 0 | 0 | 0 | 1 | 0 |
| Any positive result | 9 | 4 | 13 | 7 | 12 | 8 | 34 | 19 |
*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.