GO:0003828:   alpha-N-acetylneuraminate alpha-2,8-sialyltransferase activity
Catalysis of the reaction: CMP-N-acetylneuraminate + alpha-N-acetylneuraminyl-(2->3)-beta-D-galactosyl-R = CMP + alpha-N-acetylneuraminyl-(2->8)-alpha-N-acetylneuraminyl-(2->3)-beta-D-galactosyl-R.