Catalyzes the O-acetylation of capsular polymeric sialic acid. Shows high substrate specificity toward polymers of sialic acid that contains a large number of residues. [(2->8)-N-acetyl-alpha-D-neuraminosyl](n) + n acetyl-CoA = [(2->8)-N,O(9)-diacetyl-alpha-D-neuraminosyl](n) + n CoA [(2->8)-N-acetyl-alpha-D-neuraminosyl](n) + n acetyl-CoA = [(2->8)-O(7),N-diacetyl-alpha-D-neuraminosyl](n) + n CoA kcat is 0.51 sec(-1) with acetyl-CoA as substrate. kcat is 1.99 sec(-1) with polymer of sialic acid as substrate. Homotrimer (PubMed:21390252). Hexamer formed by two homotrimers (PubMed:17519228). Polymeric sialic acid-containing capsule provides a means for the bacteria to evade the immune response during infection by mimicking host sialic acid-containing cell surface structures. O-acetylation of the sialic acid residues of capsular polysaccharides alters their immunogenicity and susceptibility to glycosidases. The catalytic efficiency increases linearly with the number of RLKTQDS repeats. Belongs to the transferase hexapeptide repeat family.