Figure 1.

Influenza cycle divided into baseline, prevaccination and postvaccination periods. The baseline extended from latest of diagnosis date or 1st September in index year to 15 days pre-vaccination, and from 90 days post-vaccination to earliest of 31st August of the next year, date of leaving GP surgery, date of death, or latest date of data collection. Exposed period extended from vaccination to 90 days later, and was categorised as illustrated above.