A single immersion vaccination of rainbow trout fry using a Yersinia ruckeri bacterin confers immunity to reinfection but only for a shorter period. A longer protective period is needed in practical trout farming and we have shown that booster vaccination prolongs immunity. Due to economic considerations and management practices it is not possible to immersion vaccinate large trout (20–30 g) with the recommended high bacterin concentration. We here demonstrate that booster vaccination using dilutions of the bacterin (1:100, 1:1000 and 1:2000) with increased exposure time (1 h, 2 h) confers a higher and longer lasting immunity although a short term (30 s) booster in 1:10 bacterin proved superior. This has practical implications for fish farmers because this diluted booster vaccination over longer time can be applied when farmers handle and transport fish between ponds and farms. Therefore such a practice will not challenge farm management and economy. Increased antibody levels were recorded after challenge of vaccinated fish but not after immersion vaccination alone which suggests that immersion induces priming of memory cells.