O-acetyltransferase that catalyzes 9-O-acetylation of sialic acids. Sialic acids are sugars at the reducing end of glycoproteins and glycolipids, and are involved in various processes such as cell-cell interactions, host-pathogen recognition. acetyl-CoA + N-acetylneuraminate = CoA + N-acetyl-9-O-acetylneuraminate acetyl-CoA + N-acetylneuraminate = CoA + N-acetyl-7-O-acetylneuraminate Belongs to the PC-esterase family. CASD1 subfamily. Contaminating sequence. Potential poly-A sequence. Contaminating sequence at C-terminus.