Flu season has arrived in Sonoma County, announced with a chorus of coughs and sniffles and a rush on Vitamin C.

"The flu stuff is flying off the shelf," said Sue Deevy, who works in the health and beauty aids section at Oliver's Markets in Cotati.

"It's on our doorstep," said Karen Holbrook, interim health officer with the county's Department of Health Services.

Instances of the flu have jumped in recent weeks, said Gary Green, chief of infectious diseases at Kaiser Medical Center in Santa Rosa. Kaiser's northern California region, which stretches from San Jose to Santa Rosa, saw the percent of people testing positive for one flu virus, influenza A, leap to 27 percent in the week that ended Sunday. That was up from 7 percent two weeks earlier.

Sonoma and Marin counties are currently a hot spot within the larger region, Green said. Locally, in the week that ended Dec. 29, 34.8 percent of people tested positive for influenza A and an additional 4.3 percent tested positive for influenza B, which tends to be less severe.

"Nationally, Texas and the southeast are the hot spots for the flu right now," he said. "Our numbers in Sonoma and Marin are just as high."

So far, hospital emergency rooms have had few confirmed cases of the flu. Green noted that a young, otherwise healthy adult currently was hospitalized at Kaiser, critically ill with the flu. There also have been two confirmed cases at Santa Rosa Memorial Hospital and four at Petaluma Valley Hospital, said Katy Hillenmeyer, a spokesperson with St. Joseph Health, which runs both hospitals.

But just this week, 52 people showed up at Santa Rosa Memorial's emergency room with flu-like symptoms, she said, "So it is likely that flu activity is picking up as we approach January and February, which for us are typically busier months in terms of treating the flu."

"It's right on schedule," said David Smith, a pediatrician with Annadel Medical Group, adding that high levels are typically first seen on the east coast, which is where the flu has been the most active so far this year. According to the Centers for Disease Control and Prevention, the flu is now considered widespread in 10 states, mainly in the northeast and south. The flu travels west as people get on airplanes for the holidays, Smith said.

The most common strain so far in Sonoma County and around the nation is H1N1, a strain of influenza A. In 2009, a form of H1N1 caused a national pandemic.

Holbrook emphasized that Sonoma County's flu season hasn't peaked and that it's too soon to tell how severe it will be. But Green pointed to an alert issued Christmas Eve by the Centers for Disease Control and Prevention disclosing numerous reports of severe respiratory illness among young and middle-aged adults suffering from H1N1.

"For the 2013-14 season, if pH1N1 virus continues to circulate widely, illness that disproportionately affects young and middle-aged adults may occur," the CDC warned.

The positive news, Green said, is that this year's vaccine is a good match for H1N1 and that it's not too late to be immunized.

Doctors recommend vaccination for everyone over 6 months of age, noting that children under 5 and people over 65, as well as pregnant women and people with diabetes or asthma, are at greater risk of hospitalization due to influenza.

Kaiser Medical Center has given 54,000 doses of the vaccine this season, Green said, and 99 percent of the hospital's physicians have gotten it. Smith said his office has been immunizing heavily ever since the vaccine was available in August.

"It's not too late," Smith said. "Anybody thinking about being around other human beings should get it."

Officials also recommended people wash their hands, especially before touching their face, cover their nose and mouth while sneezing and, if they have the flu, stay home from work and away from public places until 24 hours after the fever is over.

Flu symptoms include high fever, headache, body ache and chills. Those with a high risk of developing complications should contact their doctor if they develop symptoms.