Coccidiosis, a prevalent intestinal disease in poultry caused by Eimeria protozoan parasites, poses significant challenges to the industry. The shift towards “Antibiotic-Free” or “No Antibiotics Ever” or “Raised Without Antibiotics” practices, driven by antibiotic resistance and public concerns, necessitates alternative nutritional strategies for coccidiosis control. This work aimed to explore the combined effects of coccidia vaccine, phytase, and 25-hydroxyvitamin D3 (25OHD) supplementation on the gut and skeletal health of broiler chickens under Eimeria infections. Five studies were conducted in this dissertation. The initial two studies assessed the impact of phytase and coccidia vaccine on broilers' growth performance, nutrient digestibility, bone development, and intestinal gene expression. Phytase supplementation at 1,500 FTU/kg enhanced growth performance, bone mineralization, and nutrient digestibility, irrespective of vaccination status. The third and fourth studies examined broilers fed calcium (Ca) and phosphorus (P)-reduced diets supplemented with phytase, or 25OHD, or combination, under Eimeria spp. challenges, focusing on growth performance, body composition, bone health, and intestinal integrity. Results from the third study indicated dietary Ca and P can be moderately reduced with phytase supplementation, especially in Eimeria-infected birds. In the fourth study, the results showed reducing dietary Ca and P levels negatively impacted feed conversion ratio (FCR) and bone development but did not affect intestinal integrity. Supplementing phytase, alone or combine with 25OHD in Eimeria-infected birds could enhance broilers' growth performance and bone health. The final study investigated the effects of coccidia vaccine, phytase, and 25OHD supplementation on growth performance and intestinal health in Eimeria-challenged birds. The coccidia vaccine inhibited feed intake (FI) of broilers during pre-challenge, but improved growth performance, decreased intestinal permeability, enhanced intestinal morphology, and modulated tight junction protein gene expression of broilers during post-challenge period. Reducing dietary Ca and P levels adversely affected growth performance during the recovery phase, but these negative effects were mitigated by supplementing with phytase or 25OHD.