What distinguishes "trivalent" from "quadrivalent" flu vaccines?

The distinction between trivalent and quadrivalent flu vaccines lies in the number of influenza virus strains they are designed to protect against. Trivalent vaccines contain three specific strains: typically, two influenza A strains and one influenza B strain. In contrast, quadrivalent vaccines are formulated to provide protection against four strains, which include two influenza A strains and two influenza B strains.

This addition in the quadrivalent vaccine aims to enhance the breadth of protection offered during flu season, especially since the circulating strains can vary each year. Therefore, the correct answer accurately identifies that the trivalent vaccine targets three flu viruses, while the quadrivalent vaccine targets four flu viruses, thereby increasing coverage against potential influenza infections.