This year’s flu vaccine may not be strong enough to fend off the current strain of influenza that’s causing coughs and sore throats this season. On Wednesday, the Centers for Disease Control’s Tom Frieden said that the virus that it was designed to defeat has mutated, rendering flu shots markedly less effective.

Part of the reason this year’s vaccine is less of a flu-fighting champion and more of a disease-succumbing dud is because flu vaccines are created months in advance of flu season. In February, the CDC formulated a strategy for creating this winter’s weapon based off of previous bouts with the seasonal flu. Each vaccine protects against three or four different strains of influenza. As such, a CDC panel chooses which strains to incorporate in a given year’s vaccine based off of the viral intelligence they previously gathered. For this winter, Frieden advised doctors to additionally prescribe their patients drugs such as Tamiflu and Relenza to defend from the unexpected viral onslaught, Reuters reported.

According to the Los Angeles Times, this season’s vaccines thwart two types of influenza A viruses known as H1N1 and H3N2, in addition to a couple strains of influenza B viruses. Between October 1 and November 22 of this year, the CDC performed reconnaissance on their potential viral foes, and sent collected samples back to the lab for testing. But of the 85 influenza virus samples they collected, only 48-percent matched the flu strain that their vaccine is best equipped to combat, according to CNN.

Although it’s common for flu viruses to mutate, it weakens the vaccine's ability to defeat it. That’s why the CDC has recommended early use of anti-viral medicines, as a sort of reinforcements for the vaccine. Despite the setback, the CDC still urges that people get their flu shots.

"We are recommending strongly still that people who haven't been vaccinated get vaccinated," the CDC's Joe Bresee said to NBC. That message goes extra for parents with children. According to the CDC, of the 149 children that died during the 2012-2013 flu season and had flu-like symptoms, 90-percent were unvaccinated. That take-home message, Lisa Thebner, a New York City pediatrician, told CNN is that "the vaccine isn't perfect, but it's the best protection we have for prevention."