"This year's flu season started about a month early, prompting federal health officials to warn it could be one of the worst in years. They're urging everyone to get their flu shots."

"But like every flu season, there are lots of reports of people complaining that they got their shot but still got the flu. What's up with that?"

"Well, as Michael Jhung of the Centers for Disease Control and Prevention explains, there are lots of possible reasons."