Background and objectives:This study aimed to analyze the influence of alloferon on the Epstein-Barr virus (EBV) DNA copy number in saliva samples and the changes of natural killer (NK) cell content, cytotoxic activity of killer cells, and production of interferon alpha and gamma in patients suffering chronic EBV infection (CEBVI) at 6 weeks after therapy completion.Methods: One hundred CEBVI patients (69 females and 31 males were divided into two groups: alloferon (n = 70; nine injections s/c, 1.0 mg every alternate day) and valacyclovir (n = 30; 500 mg two times/day, orally). The EBV DNA quantity in the saliva samples, the number of killer cells in the blood, and the cytotoxic activity of killer cells via spontaneous and induced expression of CD107a, a marker of degranulation, were determined after treatment with alloferon. The dynamics of interferon alpha and gamma production before and after alloferon therapy were also assessed.Results: At 6 weeks after therapy completion, EBV DNA was not found in 38 (54.28%) patients in the alloferon group and in 9 (30.0%) patients in the valacyclovir group (p = 0.001). In addition, a reliable increase of the NK cell content and stimulation of cytotoxic activity of NK cells were detected in the CEBVI patients. Moreover, alloferon treatment did not lead to a reliable increase of interferon alpha or gamma production at 6 weeks aftertherapy completion.Conclusions: Alloferon significantly reduces the EBV DNA copy number in saliva samples and induces the expansion of NK cells and cytotoxic activity of NK cells in CEBV patients. Alloferon also significantly affects the clinical complaints of CEBVI patients.