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Seniors should be vaccinated for the flu before December, according to new vaccination recommendations from the Centers for Disease Control and Prevention.

The influenza season typically peaks between December and February. For adults over the age of 65, the flu vaccine’s effectiveness can decline “significantly” in the months following vaccination, the CDC noted. While delaying vaccination may help older adults have greater immunity later in the flu season, the CDC encourages seniors to get vaccinated before the virus begins to circulate in order to avoid “difficulties in vaccinating a population within a more constrained time period.”

The 2015-2016 recommendations were released through this week’s issue of the CDC’s Mortality and Morbidity Weekly Report.

The push for earlier vaccinations in seniors comes after a particularly nasty flu season, which saw higher-than-usual flu cases in nursing homes and other healthcare facilities. During the first full week of 2015, adults over 65 were hospitalized for the flu at a rate of 91.6 per 100,000, up almost 20 points from the same period in 2013.

Leaders in the House Energy and Commerce Committee sent a letter to Health and Human Services Secretary Sylvia Burwell requesting documents on how the administration plans to prepare for the upcoming flu season in late July.

“The mismatched seasonal influenza vaccine and the high death rate among the elderly and other high-risk populations in the U.S. during the 2014-2015 influenza season highlight the need for an improved response, including making seasonal influenza vaccines more effective and promptly available,” the letter read.