A novel hepatitis B virus variant in the sera of immunized children.

A novel hepatitis B virus (HBV) variant was detected in the sera of two children in The Gambia, West Africa. The children had been immunized with plasma-derived vaccine and had developed antibody titres of 1448 international units x 10(-3) (mIU)/ml and 133 mIU/ml respectively against the hepatitis B surface antigen (HBsAg). Despite the protective levels of antibodies, HBV DNA was subsequently detected in both children. The complete surface (S) protein gene sequence demonstrated that this HBV isolate was closely related to the ayw4 subtype. However, five nucleotide changes were identified and two of these were unique to the Gambian isolate. One of these changes was within the region of the S gene coding for the immunodominant a determinant of the S protein. A unique nucleotide change from adenosine to guanosine at nucleotide 421 was found, resulting in an amino acid substitution at residue 141 from lysine to glutamic acid. Previous studies have shown that amino acids 141 to 146 are critical for binding to the protective anti-HBsAg antibodies. The presence of a variant HBV in these children suggests the emergence of a novel strain of HBV which can evade immune recognition. This has potential implications for HBV diagnosis and prophylaxis.