ATLANTA – This year's flu season draws to a close as one of the mildest in recent years, partly because the vaccine was a good match for this winter's most common virus.
Overall flu and pneumonia deaths were below those of a typical flu season, and health officials say fewer than two dozen children's deaths were reported.
The one exception to the overall good news is a nasty outbreak of a different flu virus that hospitalized more than 30 children in Houston.
In about half of the states, reports of flu-like illness are sporadic or virtually nonexistent now, according to the U.S. Centers for Disease Control and Prevention.
"Flu season is pretty much over," said David Engelthaler, Arizona's state epidemiologist.
Flu was widespread in only five states — Connecticut, Delaware, Indiana, New York and Rhode Island — the week of April 9-15, the most recent data available.
The long-lasting season started in December with a rush of cases in the Southwest that swamped hospital emergency rooms. It then rotated to other regions, with widespread activity in some areas as late as this month.
But it hasn't been bad. "It has been a very mild season," said Dr. Roland Levandowski of the National Institute of Allergy and Infectious Diseases.
One indicator is the nasal and throat specimens taken from patients with flu-like symptoms by physicians who are part of a national flu surveillance network.
Between the beginning of October and April 15, 13 percent of specimens tested positive for flu. For roughly the same period a year ago, it was 16 percent and in 2003-04, 20 percent.
Another indicator is deaths. The severe 2003-04 season started in terrible fashion, with flu-related deaths of 93 children between that October and early January. Overall, deaths from pneumonia and flu were considered epidemic for nine consecutive weeks that winter.
This season, the number of deaths are lower than normal.
One possible reason: The vaccine seems to be doing the job.
Flu viruses are categorized by types — Type A, which are found in animals and people, and Type B normally found only in humans.
Last year, health officials — making essentially an educated guess — formulated the vaccine against three flu viruses. They were Type A New Caledonia, Type A California, and Type B Shanghai.
Test results show 84 percent of people with confirmed flu this season had a Type A flu and the rest had a Type B. Of the patients who had Type A viruses, about 80 percent or more had viruses identical or similar to the A bugs in the vaccine.
"The vaccine was a good match to the circulating viruses," said Dr. Jeffrey Engel, North Carolina's state epidemiologist.
Good, but not perfect: The B virus was not a good match.
Nearly 70 percent of the people testing positive for a B virus had Type B Victoria, a version not found in the vaccine. For adults, that wasn't a big deal, because they've been exposed to both lineages of B flu through the years and could muster an immune response.
But for some children never exposed to Type B Victoria, the result was more serious.
Since mid-January, 31 children have been sent to Texas Children's Hospital in Houston for treatment of Type B Victoria flu. Some have suffered a rare complication called myositis, a painful inflammation of the calf muscles that hurts their ability to walk.
"You don't see that every year with an influenza B outbreak," said Dr. W. Paul Glezen, a molecular virologist at the Baylor College of Medicine.
Next year's flu vaccine will include Type B Victoria, health officials said.
Some experts say there was additional bad news in the current flu season: The discovery that one Type A flu virus circulating this season is largely resistant to rimantadine and amantadine, two drugs commonly used to fight influenza.
The resistance of such a common flu bug to the drugs could require significant changes in how doctors treat flu patients in the future, said Dr. Bruce Ribner, epidemiologist for Atlanta's Emory University Hospital.
Some health officials also worry that the mild nature of this flu season will lull the public into forgetting the potential dangers of the common flu bug — even as governments try to prepare for a potential pandemic virus that could develop from the bird flu in Asia and Europe.
"I hope people would not judge what might be coming in the future based on what's happened this year," Levandowski said.