The present invention relates to the field of vaccines, and in particular vaccines comprising antigens of low isoelectric point at pH 7.0, in particular the capsular polysaccharide or oligosaccharide of H. influenzae B (PRP). Immunogenic compositions and methods of making such compositions are presented in which the PRP is surprisingly protected from immune interference by adding a polyanionic polymer (such as PLG - poly-L-glutamic acid) to the composition.