Check Up: So far, very little flu

The U.S. Centers for Disease Control and Prevention has confirmed what you already guessed: This has been a remarkably mild flu season.

The influenza virus likes cold weather, so infections normally occur from October through March. But technically, the flu season doesn't start until labs that test respiratory swabs from sick people find the virus in more than 10 percent of the samples.

This season, that threshold wasn't reached until the week ended Feb. 11, making this the kindest flu spell in 29 years.

Pennsylvania, for example, had only 80 confirmed cases in all of January - barely more than one achy, feverish, nauseated citizen per county.

What's going on?

No one really knows.

"With flu, everything is unpredictable," said immunologist Scott Hensley, a flu expert at the Wistar Institute in Philadelphia. "I don't think we're out of the woods; it could just be a delayed season."

Then again, maybe the flu has been as scarce as snow because snow has been scarce.

"Flu is more easily transmitted in colder temperatures. This has been a mild winter," Hensley said.

Another theory is that the population has high levels of immunity to the influenza strains now circulating, which include the one that caused the 2009 "swine flu" pandemic. Because the strains have been so stable, people have had time to develop antibodies against them. Vaccination has also boosted immunity.

Although Hensley subscribes to this theory, he adds a caveat: "If that's true . . . the virus will start mutating" to evade human defenses. "A novel strain might emerge in the next couple of months."

While there's no room for complacency, let us celebrate the signs, monitored by the CDC, that the flu has given the nation a respite:

One person per 100,000 has been hospitalized with the flu since October. That's a 95 percent drop from last season's rate of 22 people per 100,000.

Only 1 percent to 2 percent of visits to doctors since October have been for flulike illness. The usual rate is 3 percent to 8 percent.

This flu season, there have been three flu-related deaths among children, compared with 122 pediatric deaths last season - and 348 during the 2009 pandemic.

- Marie McCullough