A new conjugate vaccine for cholera has been developed. The invention includes a new method to conjugate the O-specific polysaccharide-core part of the bacterial lipopolysaccharide and protein subcomponents. Conventional technology has entailed chemical treatment of both components to introduce linkers, which made them amenable for covalent linking. The new method simplifies production by utilizing squaric acid chemistry for conjugating the free amine-containing species (e.g. polysaccharides) directly to amine-containing species (e.g. proteins) without prior modification of either component. While demonstrated in this new cholera prototype vaccine, the technology is envisioned as generally applicable, thereby streamlining a complex production process.