Which of the following vaccines contains a synthetic adjuvant?

The vaccine that contains a synthetic adjuvant is Heplisav. This vaccine is specifically designed for the prevention of hepatitis B and incorporates a novel adjuvant that enhances the immune response. The adjuvant used in Heplisav is a toll-like receptor (TLR) 9 agonist, which is synthetic and helps stimulate a stronger and more effective immune response compared to traditional adjuvants.

Other vaccines mentioned in the options are different in their formulations. Engerix and Recombivax are both hepatitis B vaccines that do not use synthetic adjuvants; instead, they rely on standard aluminum-based adjuvants. Havrix, a hepatitis A vaccine, also does not contain synthetic adjuvants, instead employing standard adjuvants typical for its class. Thus, Heplisav stands out due to its inclusion of a synthetic adjuvant that improves the overall immunogenicity of the vaccine.