Flu season has only just begun but already health officials are warning folks that this year's flu shot may not be effective against the current strain of influenza. 

The flu vaccine usually works by protecting against two to three strains of flu at once. There are always a mix of flu strains going around, and health officials use their resources to guess which ones will be most common each flu season. But despite their best efforts, the flu strain that health experts thought would be making the rounds this year has actually mutated. And the flu shot cannot fully protect against that mutated strain.

The Centers for Disease Control and Prevention (CDC) issued a statement this week to let doctors know that the strain of H3N2 responsible for most cases of the flu so far this season has mutated and only about half of cases match the vaccine. According to the CDC, this year's flu shot could provide some cross-protection that might lessen the severity of some flu symptoms and complications.

"Though reduced, this cross-protection might reduce the likelihood of severe outcomes such as hospitalization and death," said the CDC statement. So they are still encouraging Americans to get the shot. 

Influenza is usually hardest on pregnant woman as well as the very young and the very old. Depending on the season, it is responsible for the deaths of anywhere between 4,000 and 50,000 people each year in the United States.

The CDC hopes that by warning doctors and the public about the lack of effectiveness of this year's flu shot, it will remind people to be vigilant about symptoms of the flu and seek treatment quickly if they occur.

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