question: This trial assessed the efficacy of a commercial essential oil (EO) product on the immune response to vaccination against Newcastle disease (ND) and subsequent challenge with virulent ND virus genotype VII (vNDv genotype VII) by using the following experimental groups of broiler chickens (Each group had 21 birds with 3 replicates in each, n = 7): NC (negative control), PC (positive control), VC (vaccinated), and VTC (vaccinated and treated with EOs). Moreover, in a trial to study the effect of EOs on vNDv genotype VII in vivo as a preventive or therapeutic measure, 2 additional ND-vaccinated groups were used (PRV: medicated 1 D before vNDv challenge for 5 D; and TTT: medicated 2 D after vNDv challenge for 5 D). In addition, the immune-modulatory effect of EOs on the avian influenza (AI), infectious bronchitis (IB), and infectious bursal disease (IBD) vaccines was assessed through the serological response. The use of EOs along with administration of ND vaccines (VTC) revealed a lower mortality rate (42.86%), clinical signs, and postmortem lesion score (11) than ND vaccines alone (VC) (52.28% mortality and score 15), in addition to lower hemagglutination inhibition (P < 0.05) (6.5 ± 0.46) and viral shedding (10 log 2.28 ± 0.24) titres 1 wk after challenge in comparison with VC (8.63 ± 0.65 and 10 log 3.29 ± 0.72, respectively). Nevertheless, the EOs mixture (VTC) (1952 ± 28.82) did not significantly (P > 0.05) improve growth performance compared with the nontreated birds (NC and VC) (1970 ± 19.56 and 1904 ± 38.66). EOs showed an antiviral effect on vNDv in vivo (in chickens) as a preventive measure (PRV) as well as some therapeutic effect (TTT) through decreasing the viral shedding titres (loNC(0)), mortality rate, and severity of clinical signs and postmortem lesions, in addition to serum malondialdhyde level. Regarding the other viruses, the EOs mixture did not improve the immune response to the AI and IB vaccines but significantly (P < 0.05) increased the ELISA antibody titre for IBD virus at the 28th D of age (2,108 ± 341.05). The studied EOs mixture showed an immune-stimulating response to ND and IBD vaccines, antiviral effect against ND virus, especially if administered before the challenge; however, it did not have a growth-promoting effect.