Abstract: The present invention relates to a compound comprising a polyelectrolyte and, covalently linked thereto, an immunological adjuvant and/or cell targeting ligand, wherein the covalently linked entity can have both adjuvant and cell targeting characteristics. The compound is used in the preparation of hydrophilic vaccine nanoparticles, which preferably have an antigenic compound or therapeutic agent, or genetic information encoding such compounds or agents entrapped in their matrix, or covalently linked to their surfaces. Vaccine compositions comprising the particles of the invention are advantageous, because a strong and long-lasting immune response is obtained following administration of a single dose. In a preferred embodiment, the polyelectrolyte of the compound is an anionic polymer, and the particle comprises a matrix comprising chitosan.