The invention relates to methods of producing, and compositions comprising, an isolated alpha (2→8) or (2→9) oligosialic acid derivative bearing a non-reducing end enriched for one or more de-N-acetyl residues and resistant to degradation by exoneuraminidase. A representative production method involves: (i) treating an alpha (2→8) or (2→9) oligosialic acid precursor having a reducing end and a non-reducing end with sodium borohydride under conditions for de-N-acetylating the non-reducing end; and (ii) isolating alpha (2→8) or (2→9) oligosialic acid derivative having one or more de-N-acetylated residues and a non-reducing end that is resistant to degradation by exoneuraminidase. Isolated alpha (2→8) or (2→9) oligosialic acid derivatives that comprise a non-reducing end de-N-acetyl residue are provided, as well as antibodies specific for the derivatives, compositions comprising the derivatives, kits, and methods of use including protection against and detection of E. coli K1 and N. meningitidis bacterial infection, and in diagnosing and treating cancer.