BOSTON -- Boston declared a public health emergency yesterday as flu season struck in earnest and Massachusetts reported 18 flu-related deaths so far.

The city is working with health care centers to offer free flu shots and hopes to set up places where people can get vaccinated. The city said there had been four flu-related deaths, all elderly residents, since the unofficial start of the flu season Oct. 1.

The city was experiencing its worst flu season since at least 2009, Mayor Thomas Menino said, with about 700 confirmed cases of the flu, compared with 70 all of last season. "The best thing you can do to protect yourself and your family is to get the flu shot," he said.

Massachusetts was one of 29 states, including New York, reporting high levels of "influenza-like illness," according to the most recent weekly flu advisory issued by the Centers for Disease Control and Prevention.

The CDC said the proportion of people visiting health care providers with flu-like symptoms climbed from 2.8 percent to 5.6 percent in four weeks. By contrast, the rate peaked at only 2.2 percent during the relatively mild 2011-2012 flu season.

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The estimated rate of flu-related hospitalizations in the United States was 8.1 per 100,000 people, high for this time of year, according to Dr. Joe Bresee, chief of the epidemiology and prevention branch of the CDC's influenza division. The agency's next advisory will be issued Friday.

Barbara Ferrer, director of the Boston public health commission, said the 700 confirmed cases represent only those reported to the city and that thousands of other people may be ill.

Hospitals around the state were also taking precautions to protect patients and staff members from exposure to the flu.