Why Is This Year's Flu Season Off To A Slow Start?

Researchers say it might be because of the more temperate weather, or the nature of the flu strains now in circulation. It's been around for a year or two so a lot of people have already been exposed, and have built up a natural immunity.

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It's been a weird winter in many traditionally cold areas of the country - warm when it should be icy, mud where there should be snow. Flowers are blooming way ahead of schedule. Wildlife seems confused. And there's something else strange about this winter: The flu season seems to be largely missing in action. NPR's Rob Stein reports.

ROB STEIN, BYLINE: If this was a typical year, you'd look around and see lots of people feeling pretty miserable. Usually by around now, says Lyn Finelli of the Centers for Disease Control and Prevention, it's peak flu season.

LYN FINELLI: Flu season usually kicks off in a big way in late December, early January. It peaks at the end of February or the beginning of March, and we usually have influenza around until May.

STEIN: But not this year, not by a long shot. It's only now that things look like they might be getting going. And experts say they don't think they've ever seen anything quite like this.

FINELLI: In 29 years, this is the slowest start to a flu season.

STEIN: In lots of places around the country, the flu is simply nowhere to be found. Take Nashville, Tennessee, for example. Flu specialist William Schaffner at Vanderbilt University has been hunting around.

WILLIAM SCHAFFNER: We've been looking hard for influenza, and it's as though it were July. We can't find any.

STEIN: So what's going on? Well, the weirdly warm weather could be playing a part, according to the CDC's Finelli.

FINELLI: We know that influenza virus survives best in cool and dry conditions.

STEIN: And the better the flu virus survives, the better the chances it can infect someone. Plus, when it's cold outside, people tend to spend more time inside, where they're more likely to come close to someone who's infected.

FINELLI: We have large groups of people congregating in one place, and transmission may take place a little bit more effectively in that situation.

STEIN: But Finelli and Schaffner think two other factors may be playing a bigger role. One is that the main flu viruses out there have been around for a year or two. So a lot of people have already been exposed to them and have built up natural immunity. At the same time, lots of people have gotten their flu shots. By November, more than 36 percent of Americans had gotten vaccinated, up from less than 33 percent a year earlier.

SCHAFFNER: And the combination of natural infections - which give you some immunity - as well as widespread vaccination, I think we have a very well-protected population at the moment.

STEIN: That doesn't mean no one's getting sick this winter. Flu seems to be really starting to pick up in some parts of the country, such as California. And there are still lots of colds and other bugs circulating, but a lot less flu than usual.

SCHAFFNER: Fewer people have been sick, fewer doctor visits. Fewer people have had to be in the hospital, fewer deaths due to influenza. So we'll take that. That's all a great post-Christmas present for us.

STEIN: But the real question is: What does this all mean for the future? The simple answer is: No one knows. The flu season could just be a dud. Or it could suddenly take off.

SCHAFFNER: It could be that influenza will now start to rev up, and we could have a big March of influenza, which would be rather late going into April.

STEIN: Whatever happens this year, there's always next year. The flu viruses could mutate, making a lot more people sick. Or a really nasty flu virus could suddenly emerge, causing lots of suffering. Or it could just be another dud.

Rob Stein, NPR News.

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