Methods for making an immunogenic conjugate that includes a hapten or an antigen covalently linked to a carrier. The methods include reacting a first agent with a dihydrazide resulting in a hydrazine-modified first agent, wherein the first agent is a hapten, an antigen or a carrier; reacting a second agent with a benzaldehyde compound resulting in a benzaldehyde-modified second agent, wherein the second agent is a hapten, an antigen or a carrier, provided that the first agent or the second agent is a carrier; and reacting the hydrazine-modified first agent with the benzaldehyde-modified second agent resulting in an immunogenic conjugate comprising a hapten or an antigen covalently linked to a carrier via a hydrazone linkage.