WASHINGTON — Flu season in the United States is having its earliest start in nearly a decade and health officials say this season could be a bad one.
Although flu is always unpredictable, the early nature of the cases and the predominant type circulating this year could make this a severe flu season, said officials at the Centers for Disease Control and Prevention.
But officials said the vaccine formulated for this year is well-matched to the strains of the virus seen so far and urged those who have not been vaccinated to get a flu shot.
In early December, the CDC said it was noticing an uptick in flu activity about a month before authorities normally see it, and the earliest since the 2003-2004 flu season. The primary strain this season is the H3N2, an influenza A virus that has been associated in the past with more severe flu seasons.
Since the beginning of December, "there has been increasing influenza activity in more parts of the United States and more cases of flu," said Joe Bresee, chief of epidemiology at CDC's influenza division, in an interview last week . "We have seen increasing trends that flu is increasing in the last few weeks of the year, and I wouldn't be surprised if we see that continue into the new year."
Influenza often peaks in January, February or even later.
Cases of flu have been reported in 29 states. As of Dec. 15, higher-than-normal reports of flulike illnesses had been reported in 12 states, most of them in the South
Based on data from the past two decades, during the years when H3N2 is the predominant flu strain, "there are more deaths and hospitalizations," Bresee said.
It is not completely clear why. One factor may be that the elderly, who are at high risk for flu complications, tend to become sicker with the H3N2 strain than the other two common flu strains, he said.
But this flu season offers two bits of good news, as well.
Of the flu strains that are spreading this year, about 80 percent are the influenza A type, and almost all of those are the H3N2 strain, Bresee said. That matches well with this year's flu vaccine, which includes the H3N2 strain.
Flu vaccines are designed to protect against three influenza viruses that experts predict will be the most common during the upcoming season. The three kinds of influenza viruses that commonly circulate among people today are influenza B, influenza A (H1N1) and influenza A (H3N2). Each year, one flu virus of each kind is used to produce seasonal influenza vaccine.
Also, the Food and Drug Administration on Dec. 21 expanded the approved use of Tamiflu to treat children as young as 2 weeks old who have shown symptoms of flu. The drug was previously approved to treat adults and children 1 year and older. The dose for children under 1 year old must be based on their exact weight.
Flu is extremely unpredictable. "You never know when the peak will occur or how big the peak will be," Bresee said. "If you've seen one flu year, it's just one flu year."
Last winter's flu season, for example, set a record for the lowest and shortest peak of influenza-like illness. The season began late and was mild compared with most previous seasons.
The last time a regular flu season started this early was the winter of 2003-04, which proved to be one of the deadliest seasons in the past 35 years, with more than 48,000 deaths. The dominant type of flu back then was the same one seen this year.
But experts said there is a critical difference between then and now: In 2003-04, the vaccine was poorly matched to the predominant flu strain. Also, health officials said more vaccine is available now, and vaccination rates have risen for the general public and for key groups such as children, pregnant women and health-care workers.
About 112 million Americans had been vaccinated by the end of November, the CDC said. Manufacturers were expecting to produce about 135 million total doses this year. Flu vaccinations are recommended for everyone 6 months or older.
On average, about 25,000 Americans die each flu season, according to the CDC.
Flu usually peaks in midwinter. Symptoms can include fever, cough, runny nose, head and body aches and fatigue. Some people also suffer vomiting and diarrhea, and some develop pneumonia or other severe complications.
A strain of swine flu that hit in 2009 caused a wave of cases in the spring and then again in the early fall. But that was considered a unique type of flu, distinct from the conventional strains that circulate every year, experts said.