LOURDES GARCIA-NAVARRO: The flu season has already begun, and this year it could be a particularly bad one. Flu is already at epidemic levels according to the Centers for Disease Control and Prevention. So for a better sense of what this all means, and what it means for you in particular, we turn now to our science correspondent Nell Greenfieldboyce. Welcome to the studio.
NELL GREENFIELDBOYCE, BYLINE: Thanks.
GARCIA-NAVARRO: Flu seasons are often hard to predict. Why do experts think this one could be so severe?
GREENFIELDBOYCE: Well, there's a couple of things that have them worried. For one thing, the most common flu virus out there in the U.S. causing disease right now is what's called H3N2. This particular flu virus has been associated with more severe seasons in the past. We saw one like this a couple of years ago, and that was a moderately severe season. So by severe what they mean is more illness when people get the flu and also more deaths and more hospitalizations. Another thing is that a lot of the H3N2 that's out there is a little bit different from the virus component in the vaccine that's supposed to protect against it. That means the vaccine may not offer as much protection as public health officials had wanted.
GARCIA-NAVARRO: Well, this is something that really interests me. I mean, how do they choose the vaccine? How do they find out what they're going to put in these vaccines so that people can be protected?
GREENFIELDBOYCE: Flu vaccine formulation is a little bit of a guessing game, so they need time to make millions of doses of vaccine. And that means months in advance, experts have to decide what's going to go in it. Basically, they look at what's circulating out there and pick what they think. But the trouble is, between their recommendation and the actual flu season, the viruses can change, and that's what happened here.
GARCIA-NAVARRO: So it's a bit of a guessing game. But how dangerous is this particular strain?
GREENFIELDBOYCE: Well, CDC officials say it's too soon to tell at this point in the season. They'll have to wait until the season is over. But in general, it's good to remember that flu can be a deadly disease. Thousands of people die from flu every year. It's especially dangerous for young people under the age of five, as well as older people over 65, plus pregnant women and anyone with an underlying medical condition like heart disease, asthma, diabetes.
GARCIA-NAVARRO: So people listening to this might think, all right, if the vaccine isn't going to protect me, then I shouldn't get vaccinated. Should they?
GREENFIELDBOYCE: Well, the CDC says you should. And the reason is the vaccine will still offer some protection against the main flu strain. Plus, the vaccine includes other components that offer good protection against other flu strains that are out there causing disease. Basically, the CDC wants everyone over the age of six months to get vaccinated, but in reality, less than half of those people do.
GARCIA-NAVARRO: Beside vaccination, what else can people do though?
GREENFIELDBOYCE: Well, if you're in one of those high-risk groups and you have symptoms of flu - so things like cough, fever, sore throat - call your doctor quickly. Go get evaluated so that you can get some treatment, because the antiviral drugs work best if they're given early, like the first two days after the symptoms start.
GARCIA-NAVARRO: Thank you so much. That's NPR's science correspondent Nell Greenfieldboyce.