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U.S. flu vaccines are likely to move from quadrivalent to trivalent due to a change in circulating influenza viruses, says a University of Michigan researcher.

Currently, all influenza vaccines in the United States are quadrivalent, meaning that they protect against four different flu viruses.

When it comes to the flu vaccine, there are three types that are recommended for older people, all with long, complicated names: HD-IIV4, or quadrivalent high-dose inactivated influenza vaccine; RIV4, or quadrivalent recombinant influenza vaccine; and aIIV4, or quadrivalent adjuvanted inactivated influenza vaccine.

Vaccines with “quadrivalent” in their names provide protection against four different strains of influenza.

Fluad quadrivalent: This vaccine contains the same amount of flu antigen as the standard shot but also contains something called an adjuvant.