One potent strain, Influenza A (H3N2), is causing more people to become severely ill and more people to be hospitalized this season, local health officials and the U.S. Centers for Disease Control (CDC) said.
As of Dec. 29, the strain had not yet made it into Santa Clara County, county public health officials said, but hospitals and health agencies are watching, since the strain has been associated with at least 18 child deaths.
More severe illness is typical with H3N2 viruses, and officials are predicting a "moderately severe" flu season, according to the CDC. But one concern is that the season has started earlier than usual. The flu season typically begins in February, but it started in early January in many states this year, according to the CDC.
The percentage of people nationwide seeing a doctor for flu-like illness is more than double last season's peak of 2.2 percent. In the past four weeks, the percentage has jumped sharply from 2.8 to 5.6 percent, according to the CDC, which tracks the flu's progression nationwide.
Although 29 states have reported high levels of influenza-like illness, with another nine states reporting moderate levels, California has not yet seen many cases, according to Santa Clara County officials.
Dr. Cornelia Dekker, medical director of the Stanford-Lucile Packard Children's Hospital Vaccine Program, said so far only 15 confirmed cases of influenza have been reported by the Stanford lab, which tests suspected cases coming into Stanford's main hospital, clinics, emergency room and children's hospital.
"We're nowhere near what the other states are experiencing, but I'm sure our time is coming," she said.
Dr. Joe Bresee of the CDC's Influenza Division said in a statement that the number of hospitalizations is also high for this time of year.
"While we can't say for certain how severe this season will be, we can say that a lot of people are getting sick with influenza. ... Anyone who has not already been vaccinated should do so now," he said.
Dekker said that flu sets its own pace, and although it does tend to go from east to west, no one can really predict which states will be hardest hit.
"Strains also mutate over time, so we're always dealing with a new set of characters," she said.
This year's flu started with Influenza B infections, but now the predominant strain is the Influenza A (H3N2), she said. The old pandemic H1N1 strain from 2009 is going around this year, but is occurring relatively infrequently, she said.
Dekker said there is still time to get a flu shot. It takes about two weeks to build up antibodies for protection. Three strains of influenza are in the vaccine, which uses only dead viruses. "Fortunately, the vaccine choice was a good match for what we're seeing right now," she said.
Only about 50 percent of children and adults were immunized last year, she said. The more people who are vaccinated, the smaller the potential pool of infected people who can spread the germ, she said.
People who are concerned about vaccines containing preservative can obtain preservative-free flu shots, and now new micro-needles are available for people ages 18 to 64 who fear injections, she said.
Another new influenza vaccine that has four times as much antigen is available for seniors to give them added protection. The alternatives are available through many pharmacies and doctors, she said.