Titres in individual birds determined at 18, 28, 38, 48, 58 and 68 weeks of age and grouped by flock/farm (n = 25 per farm) and vaccination regimes. Regime A–(blue) birds vaccinated two (SL.4 and KP.1) or three times (SL.1, SL.2, SL.3, SL.7, Cha and Spu) before 18 weeks of age; regime B (green)—birds vaccinated two times before 18 weeks of age, with a single follow-up vaccination at either 47 or 40 weeks (Sta and SL.6); regime C (red)–birds vaccinated two, three or four times before 18 weeks of age, with two or three follow-up vaccinations between 19 and 48 weeks of age. The timing of highly pathogenic avian influenza vaccination events for each farm are shown as dashed vertical lines.