The HBV vaccine works by stimulating the production of a particular antibody to HBV, called anti-HBsAg, which targets a surface antigen on the virus. ‘This is a neutralising antibody and plays a central role in protecting individuals against HBV’ explains Dr Haruki Komatsu, an Associate Professor in the Paediatric Department of Toho University Sakura Hospital in Japan. Along with Africa, the western Pacific region has the highest prevalence of hepatitis B in the world. Komatsu and colleagues are involved in the efforts to understand the transmission between mothers and children as well as why the vaccines sometimes fail. Investigations into the latter have shown the virus is mutating, creating ‘vaccine escape mutants’. These mutants have a change in the surface antigen that anti-HBsAg target, making them resistant to the vaccine.