THURSDAY, Feb. 23 (HealthDay News) -- It took a long time to get started, but this winter's flu season is finally here, say experts at the U.S. Centers for Disease Control and Prevention.
They report that flu activity picked up in the past couple of weeks, making this the latest start to any influenza season since 1987-1988. Lab-confirmed cases of influenza have now been spotted in all 50 states, but the weekly percentage of lab-tested respiratory specimens did not exceed 10 percent -- the threshold for declaring that a flu season has begun -- until Feb. 4, the CDC report found.
Why this year's flu season is starting so late is most likely the result of a complex set of circumstances that remain unclear, said Dr. Joseph Bresee, chief of the CDC's epidemiology and prevention branch in the CDC's influenza division.
"It's probably related to several things and probably other things we don't understand well," Bresee said. "Mostly, it's related probably to the fact that flu is unpredictable. There are a lot of things about flu we don't understand."
The flu update was published in the Feb. 24 issue of Morbidity and Mortality Weekly Report, a CDC publication.
In most flu seasons, there are some 200,000 hospitalizations and 36,000 deaths from flu complications, according to the CDC.
However, so far hospitalizations for flu have remained below epidemic levels -- just one hospitalization per 100,000 people, compared to nearly 22 per 100,000 people in the 2010-2011 season.
There have been reports of three infant deaths in the United States so far from flu complications, much lower than the total of 122 flu-linked infant deaths seen last season and the 348 infant deaths observed during the 2009-2010 H1N1 pandemic.
Only one state, California, is reporting "widespread" influenza cases, the CDC added.
This year, three flu strains are circulating: Influenza A (H3N2) viruses (which has predominated this season), influenza A H1N1 (the "swine flu" strain) and influenza B.
These strains haven't evolved or mutated and are the same strains that have been circulating for the past few years, Bresee said. They also match the strains included in the flu vaccine over the past two years.
Good vaccination coverage may be playing a role in flu's relative inactivity this year. "We have had very high vaccination rates in the last couple of years, and that probably dampens the amount of flu," Bresee said. "The underlying immunity of the population is probably higher than it usually is to the viruses we are seeing."
"But there are a lot of things that play into it, most of which we don't understand but are thankful for," Bresee added.
And does the late arrival of the flu herald its early departure this year? "It's hard to know if the late start to the flu season means that it will go on longer," Bresee said.
"We are getting a late start, but we don't know when the peak will be, if it will be a lower peak or a normal peak," he said. "We always can predict after the year is over, sadly."
"So, the good news is that because of the late start, folks who haven't been vaccinated still have a chance to do so," Bresee said. "Since we are seeing a late start, most communities have the opportunity to get vaccinated ahead of the flu season."
Everyone aged 6 months and older should get a flu shot, according to the CDC.
For more on the flu, visit the U.S. Centers for Disease Control and Prevention.
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