Circumstances driving the transition from low-pathogenicity to high-pathogenicity avian influenza viruses (LPAIVs to HPAIVs) remain poorly understood. In July 2015, a UK layer outbreak revealed circulation of an H7N7 LPAIV (H7N7-LP), which subsequently mutated into a high-pathogenicity H7N7 virus (H7N7-HP) at the same farm. The emergent H7N7-HP caused high mortality, though prior H7N7-LP exposure provided partial protection in some chickens. We experimentally assessed the protective effect of prior H7N7-LP exposure upon subsequent H7N7-HP challenge. In the absence of an isolated H7N7-LP direct-precursor to this H7N7-HP outbreak strain, eight chickens were directly-inoculated with an H7N7-LP which was closely genetically related to the outbreak H7N7-HP. At 1-day post-infection (dpi), eight na?ve contact chickens were introduced to assess H7N7-LP transmission. All (100%) directly inoculated chickens became H7N7-LP infected, whereas only 3/8 contacts (37.5%) acquired infection. At 14 dpi, these 16 chickens, together with eight immunologically na?ve controls, were challenged with H7N7-HP. Among the na?ve controls and the previously H7N7-LP contact-exposed chickens, all 16 (100%) shed H7N7-HP, with 62.5% and 50% mortality, respectively. In contrast, none of the chickens previously directly-inoculated with H7N7-LP subsequently shed H7N7-HP, where all remained healthy and survived. Therefore, direct H7N7-LP inoculation induced effective immunity against antigenically related H7N7-HP. However, the more limited H7N7-LP infection acquired via contact exposure resulted in all shedding H7N7-HP, with reduced clinical protection. This sequential infection model highlighted how the route and timing of initial LPAIV exposure, along with acquired immunity, differed in influencing susceptibility to an emergent HPAIV, reflecting outcomes observed during layer outbreaks.