Booster doses can make the flu vaccine more effective, but the benefit is limited to a few specific groups, reports the New York Times.

Children are one group that may benefit from receiving two doses of influenza vaccine during the same flu season. In a multistate study, boosting increased vaccine effectiveness by nearly twofold in children 6 months to 8 years of age. The benefit was greatest among infants receiving their first influenza vaccinations and was still evident in subsequent flu seasons. Other studies have yielded similar results.

Because of this, the Centers for Disease Control and Prevention recommends that “children 6 months through 8 years getting vaccinated for the first time, and those who have only previously gotten one dose of vaccine, should get two doses of vaccine this season.”