Inactivated influenza vaccines (IIVs) are the most commonly used influenza vaccines. They are made from inactivated or killed influenza viruses that cannot cause disease. IIVs are available in two formulations. Trivalent and quadrivalent. Trivalent IIVs contain three influenza virus strains, including two strains of influenza A viruses and one strain of influenza B virus. Quadrivalent IIVs contain four influenza virus strains, including two strains of influenza A viruses and two strains of Influenza Vaccines B viruses. The additional strain of influenza B virus in quadrivalent IIVs can provide broader protection against influenza B viruses, which can cause significant morbidity and mortality, especially in children. IIVs are typically administered as a shot in the arm or thigh muscle. They are approved for use in individuals aged 6 months and older, including pregnant women and individuals with chronic medical conditions.
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