With the nicer weather comes the spring flu virus

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The intrepid sun has finally beaten the frost into reluctant retreat. The croci have gathered enough confidence to venture their tender shoots outside the earth’s blanket. After months of hibernation, you step outside to take a deep breath of warm, sweet, fresh air and .

Hack. Wheeze. Gasp. Shiver. Moan.

Back to bed you go.

Influenza B, the annual spring flu, is on the rise. With its cruel sense of timing, the virus has hit the nation with slightly more virulence than usual.

“Influenza B is theoretically a milder disease than influenza A,” said Neil Fishman, an infectious-disease doctor at the Hospital of the University of Pennsylvania. “Supposedly, it is much closer to the common cold.”

But the patients Fishman has treated at HUP during the past two weeks have had more severe symptoms than he usually sees this time of year.

Normally, as the winter flu season ends and officials track precipitous downward trends in cases of influenza A, there is an accompanying increase in influenza B cases, Fishman said.

For some reason, however, influenza A lingered longer than usual, and influenza B arrived slightly later than expected, he said.

Although they cannot be certain, Fishman said, epidemiologists have speculated that these shifts may be due to the particularly cold weather this winter, which allowed the virus to survive longer, or the fact that most of the influenza A was the H1N1 virus, which tends to be a more long-lived illness.

Flu data for the week ended March 22 show that 60 percent of samples tested were positive for influenza A and nearly 40 percent for influenza B.

In mid-February, influenza B was detected in less than 10 percent of samples.

Fortunately, the vaccines given last year accurately targeted most of the strains of flu, Fishman said. There are two influenza B strains, however, and the less common of them, he noted, was not covered by the vaccine that most people received.

Only people inoculated with the more expensive quadrivalent vaccine or those younger than 50 who were given the inhaled form would have been specifically protected against both types of influenza B.