NEW YORK (AP) -- The flu vaccine may not be very effective this winter, according to U.S. health officials who worry this may lead to more serious illnesses and deaths.
Flu season has begun to ramp up, and officials say the vaccine does not protect well against the dominant strain seen most commonly so far this year. That strain tends to cause more deaths and hospitalizations, especially in the elderly.
"Though we cannot predict what will happen the rest of this flu season, it's possible we may have a season that's more severe than most," said Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, at a news conference Thursday.
CDC officials think the vaccine should provide some protection and still are urging people to get vaccinated. But it probably won't be as good as if the vaccine strain was a match.
Flu vaccine effectiveness tends to vary from year to year. Last winter, flu vaccine was 50 to 55 percent effective overall, which experts consider relatively good.
The CDC issued an advisory to doctors about the situation Wednesday evening.
CDC officials said doctors should be on the look-out for patients who may be at higher risk for flu complications, including children younger than 2, adults 65 and older, and people with asthma, heart disease, weakened immune systems or certain other chronic conditions.
Such patients should be seen promptly, and perhaps treated immediately with antiviral medications, the CDC advised. If a patient is very sick or at high risk, a doctor shouldn't wait for a positive flu test result to prescribe the drugs - especially this year, CDC officials said.
The medicines are most effective if taken within two days of the onset of symptoms. They won't immediately cure the illness, but can lessen its severity and shorten suffering by about a day, Frieden said.
He was echoed by Dr. Richard Zimmerman, a University of Pittsburgh flu vaccine researcher. Some doctors may hesitate, reasoning that flu season usually doesn't hit hard until around February. But it appears to have arrived in many parts of the country already and "it's time to use them," Zimmerman said.
Among infectious diseases, flu is considered one of the nation's leading killers. On average, about 24,000 Americans die each flu season, according to the CDC.
Nearly 150 million doses of flu vaccine have been distributed for this winter's flu season.
Current flu vaccines are built to protect against three or four different kinds of flu virus, depending on the product. The ingredients are selected very early in the year, based on predictions of what strains will circulate the following winter.
The ingredients always include a Type A H3N2 flu virus. The most severe flu seasons tend to be dominated by some version of that kind of flu bug. The three most deadly flu seasons of the last 10 years - in the winters of 2003-2004, 2007-2008, and 2012-2013 - were H3N2 seasons.
In March, after the H3N2 vaccine strain was vaccine production was underway, health officials noted the appearance new and different strain of H3N2. "This is not something that's been around before," Frieden said.
But health officials weren't sure if the new strain would become a significant problem in the United States this winter until recently, they said. Lab specimens from patients have shown that the most commonly seen flu bug so far is the new strain of H3N2. Specifically, about 48 percent of the H3N2 samples seen so far were well matched to what's in the vaccine, but 52 percent were not, the CDC said.
An official with one vaccine manufacturer - GlaxoSmithKline - emphasized that about half the samples do match the strain in the vaccine. He also noted flu seasons can sometimes involve a second wave of illnesses caused by a different strain.
"We're at the very beginning of flu season, and it's quite possible different strains will predominate," said Dr. Leonard Friedland, director of scientific affairs for GSK's vaccines business.
Thursday's news follows another problem recently identified by CDC officials, involving the nasal spray version of flu vaccine.
At a scientific meeting at the CDC in October, vaccine experts were told of preliminary results from three studies that found AstraZeneca's FluMist nasal spray had little or no effect in children against the swine flu strain that was the most common bug making people sick last winter.
Because this year's version of FluMist is the same formulation, experts said it's possible the spray vaccine won't work for swine flu this season, either.
However, CDC officials believe H3N2 will be the most common flu bug this winter.
CDC flu Web page: http://www.cdc.gov/flu/index.htm