How do current quadrivalent influenza vaccines differ from trivalent vaccines?

Quadrivalent influenza vaccines are designed to provide broader protection against circulating influenza virus strains compared to trivalent vaccines. The key difference lies in the composition of the strains included in each type of vaccine.

Trivalent vaccines contain three strains: two influenza A strains (H1N1 and H3N2) and one influenza B strain. In contrast, quadrivalent vaccines include an additional influenza B strain, resulting in four total strains: two A strains and two B strains. This extra B strain helps enhance the vaccine's effectiveness, especially during seasons when a different B strain circulates, thereby addressing more variations of the virus.

This addition of the second B strain accounts for the increased coverage offered by quadrivalent vaccine formulations compared to their trivalent counterparts. Thus, the correct answer highlights the distinguishing factor of quadrivalent vaccines in their composition and the added benefit of improved immunization against influenza B viruses.