The nation's annual influenza outbreak, which started last fall east of the Mississippi River, has swept across the country and is just now arriving in Sonoma County, touching down lightly but expected to grow in impact, local health officials said.

The seasonal flu, measured by reports of influenza-like illness, is approaching a "moderately severe" level nationwide and prompting alerts in the East and Midwest.

In Boston, officials declared a public health emergency on Wednesday, citing a sharp rise in flu cases over last year. In Milwaukee, hospital emergency rooms were diverting ambulances because of an influx of older patients with flu-like symptoms.

The flu's impact remained below the pandemic year of 2009-10, officials said, noting that the vaccine designed as the nation's primary flu defense was proving effective against more than 90 percent of the viruses infecting the public this year.

With about three months remain in the flu season, Sonoma County health officials and physicians urged the public — especially preschoolers, people over 65 and pregnant women — to get vaccinated as soon as possible.

Kaiser Medical Center in Santa Rosa has vaccinated nearly 52,000 patients, along with 98 percent of the medical staff, said Dr. Gary Green, chief of infectious diseases.

"Get it now before the season peaks," he said.

"There is still time; we are not in the midst of it (flu season) yet," said Dr. Karen Holbrook, Sonoma County deputy health officer, noting that seasonal influenza is "just starting to arrive" in California.

Laboratory confirmed flu cases are appearing in the county, but the level is currently minimal, she said.

In Kaiser's Northern California region, flu tests were running 3 percent positive in mid-December, but by Monday had bumped to 16 percent, a level Green characterized as "mild influenza activity."

The U.S. Centers for Disease Control and Prevention's latest report said 38 states had high or moderate levels of influenza-like illness, while 10 states — including California — had low or minimal levels.

Nationwide, the proportion of people seeing a health care provider for influenza-like illness moved up sharply over the past four weeks from 2.8 percent to 5.6 percent. Last season, a relatively mild one, the rate peaked at 2.2 percent; during the 2009 flu pandemic the rate hit 7.7 percent.

"Our antennae are up — we're waiting," said Dr. Veronica Jordan, who hasn't seen a flu case yet at the Sebastopol Community Health Center.

There is no cure for the flu, but antiviral medication can reduce the duration and severity of flu symptoms, which include fever, nasal congestion, muscle aches and general malaise that "makes you not want to get out of bed," Jordan said.

The latter symptom is what distinguishes seasonal influenza from the more common cold and flu, which occur at the same time of year, she said.

However, antiviral influenza medication generally must be started within 48 to 72 hours of the disease's onset to be effective.

Doctors recommend vaccination for everyone over six 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.

Those who come down with the flu should do one thing: stay home to avoid spreading it, doctors say.

Deaths among children under 18 is among the measurements for influenza, and to date 18 fatal cases have been reported nationwide this flu season, compared with 34 deaths in 2011-12 and 282 during the 2009-10 pandemic.

No one knows how severe the current flu season will be, Green said, but he expects the infection rate to rise in Sonoma County.

"We haven't seen the peak yet," he said.

(You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.)