For more than 30 years, scientists have been making trivalent (3-strain) flu vaccines to protect against the three strains of flu expected to circulate during a given flu season. Four strains of the flu virus are currently circulating, and in six of the last 11 flu seasons the three strains in the vaccine did not include one of the more commonly circulating strains. To counter this trend, scientists have developed broader protection with quadrivalent (4-strain) flu vaccines, which are available for the first time this flu season, and offer the opportunity for even broader protection than the 3-strain vaccine.