Citation: Tsaneva-Damyanova DT, Ivanova LI, Pavlova SN, Todorova SB, Popova TK. Evaluation of anti-HBs antibody immune response against hepatitis B virus in vaccinated people in North-Eastern Bulgaria region. Folia Med (Plovdiv) 2019;61(4):572-78. doi: 10.3897/ folmed.61.e47760. AbstractIntroduction: Hepatitis B virus (HBV) is one of the most significant human pathogens responsible for a huge number of acute and chronic liver infectious diseases worldwide.Aim: To find the duration of post-vaccination immune response in individuals allocated to five age groups from 6 months to 20 years.Materials and methods: All tested subjects were born between 1999 and 2018 and therefore covered by the compulsory vaccination program against hepatitis B. For the serological marker anti-HBs Ab we investigated 449 serum samples taken from ambulatory people and patients of St Marina University Hospital in Varna. Results:A positive antibody response (anti-HBs Ab > 10 mIU/ml) was reported in 79.7% (n = 51) of the group of subjects up to one year old, in 70.0% (n = 196) of the subjects in the age range 1 year/1 month to 15 years, and in 39.3% (n = 33) of the subjects 15 years/1 month to 20 years old. Female sex had a better post-vaccination response than male sex with statistically significant relationship between sex and anti-HBs Ab titer (χ 2 = 24.76, p <0.01). Conclusions:Regardless of the mass immunization against HBV in Bulgaria, the relative share of chronic HBV infections does not show a downward trend. Therefore, it is very important to study the duration of the post-vaccination immune response by demonstrating the anti-HBs antibodies and to apply a booster dose from the vaccine if needed.