An isolated immunogen comprising a variant hepatitis B surface antigen, wherein the variant hepatitis B surface antigen comprises a hepatitis B surface antigen with one or more transmembrane domains of the hepatitis B surface antigen replaced with an gp41 antigenic insert, wherein the gp41 antigenic insert comprises:
- a) an antigenic polypeptide fragment of gp41 comprising the amino acid sequence of SEQ ID NO:
- 1 (NEX1X2LLX3LDKWASLWNWFDITNWLWYIX4) wherein the antigenic polypeptide fragment of gp41 is between 28 and 150 amino acids in length;
- and
- b) a transmembrane membrane region of gp41 comprising the amino acid sequence set forth as SEQ ID NO:
- 25 (X5FIMIVGGLX6GLRIVFTX7LSIV), wherein the transmembrane domain of gp41 is between 22 and 40 amino acids in length and wherein the transmembrane domain of gp41 is C-terminal to the antigenic polypeptide fragment of gp41, wherein X1, X2, X3, and X4 are any amino acid and X5, X6, and X7 are any hydrophobic amino acid.