"Monovalent" refers to a type of chemical species that has a valence of one, meaning it can form one bond with another atom or molecule. This term is often used in the context of ions, where a monovalent ion carries a single positive or negative charge. Common examples include the sodium ion Na+ and the chloride ion Cl-, both of which play essential roles in various chemical reactions and biological processes. In the field of immunology, "monovalent" can describe vaccines that target a single strain of a virus or bacterium. For instance, a monovalent vaccine for the influenza virus is designed to protect against one specific strain, as opposed to a multivalent vaccine that covers multiple strains. Monovalent vaccines are crucial for focused immunization efforts, especially when a particular strain poses a significant health risk.