Catalyzes the O-acetylation of capsular polymeric sialic acid consisting of polymers of (2->6)-alpha-D-glucosyl-(1->4)-N-acetyl-alpha-D-neuraminosyl residues. Shows high substrate specificity toward polymers of sialic acid that contains a large number of residues. [(2->6)-alpha-D-glucosyl-(1->4)-N-acetyl-alpha-D-neuraminosyl](n) + n acetyl-CoA = [(2->6)-alpha-D-glucosyl-(1->4)-N,7-O-diacetyl-alpha-D-neuraminosyl](n) + n CoA [(2->6)-alpha-D-glucosyl-(1->4)-N-acetyl-alpha-D-neuraminosyl](n) + n acetyl-CoA = [(2->6)-alpha-D-glucosyl-(1->4)-N,O(9)-diacetyl-alpha-D-neuraminosyl](n) + n CoA kcat is 1.3 sec(-1) with acetyl-CoA as substrate. kcat is 2.1 sec(-1) with polymer of sialic acid as substrate. Homotrimer. 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. Belongs to the transferase hexapeptide repeat family.