Last year's flu season was awful. More Americans aged 65 years and older were hospitalized than ever reported since 2005 (when the CDC started counting), and 145 kids died.
The problem was that the dominant strain of flu — called influenza A (H3N2) — rapidly mutated into a particularly nasty type, evading the vaccine's protection.
"The H3N2 virus that spread last season was very different from the H3N2 virus in the vaccine," Lisa Grohskopf, a medical officer in the CDC's Influenza Division, told BuzzFeed News by email. Consequently, last year's vaccine only warded off less common strains, leading to an overall effectiveness of just 23%.
But this year's vaccine is poised to be far better.
"We don't have vaccine effectiveness estimates for this season yet," Grohskoph said, "but the composition of this season's vaccine has been updated to better match circulating viruses." Flu is always unpredictable, but when vaccines are well-matched in this way, they are generally about 50-60% effective.
The flu, which typically causes a sore throat, cough, and fever, can be deadly in people with weak immune systems, such as the elderly.
In the U.S., flu season generally stretches from October to May, and peaks between December and February. Every year in February, the World Health Organization analyzes data gleaned from patients with suspected flu from more than 100 countries to decide which strains should be included in the following year's vaccine.
This year's vaccine protects against at least three strains, including the mutated version of H3N2 that wreaked havoc last year. (Some vaccines protect against a fourth strain as well.)
The CDC recommends that everyone over the age of 6 months get a flu shot, which is available at many doctor's offices, health clinics, and drug stores. It's best to get it as soon as possible, since It takes about two weeks after vaccination for your body to start pumping out antibodies against the flu. Here's a map of where you can get yours.