The nation's severe flu season continued to worsen Wednesday, with reports of hospitals overflowing with sick patients and at least one major U.S. city declaring an influenza emergency and urging citizens to get vaccinated before the peak is reached.

In Boston, Mayor Thomas Menino declared a public health emergency because of a sharp rise in cases. That city has seen about 700 confirmed cases of influenza since the season began in October, a 10-fold jump over the 70 cases reported for all of last year, said Nick Martin, a spokesman for the Boston Public Health Commission.

While last year was an unusually mild flu season, according to government health officials, this year's numbers are worrisome, the city's mayor said.

"This is the worst flu season we've seen since 2009, and people should take the threat of flu seriously," Menino said in a news release. "This is not only a health concern, but also an economic concern for families, and I'm urging residents to get vaccinated if they haven't already. It's the best thing you can do to protect yourself and your family.  If you're sick, please stay home from work or school."

Flu cases have accounted for more than 4 percent of Boston emergency room visits recently, and about a quarter of those patients have required hospitalization, said Martin. The city has recorded four deaths since October, Martin said. All of those deaths were in elderly people. Community health clinics will offer free flu shots to citizens this weekend, Martin added.

Nationwide, the flu has spread to more than 80 percent of the U.S., latest figures from the Centers for Disease Control and Prevention indicated. More than 2,200 people had been hospitalized since October, and 18 children have died. Other viruses besides the flu are also circulating -- CDC data shows only about a third of all people with flu-like symptoms actually are testing positive for influenza.

The severe season, the earliest in nearly a decade, is starting to strain hospitals across the country. In Milwaukee, emergency departments have been forced to divert incoming ambulances to other hospitals because they've been flooded with older patients with severe flu-like symptoms, according to the Milwaukee Journal Sentinel newspaper. Several of Milwaukee's 11 hospitals have reported so-called "rolling diversions" because of flu patients, the paper reported.

In Delaware, the Christiana Care Health System has expanded its emergency room into a nearby conference room to accommodate the jump in flu cases, officials told NBC News.

The bad season has been blamed on an especially virulent flu strain, the A H3N2 strain. Another A strain, H1N1, and two influenza B strains are also causing illness. Vaccines prepared for this year are a good match for three of the viruses, although one strain of influenza B is not covered by the vaccine and may be accounting for 8 percent to 10 percent of flu cases, according to infectious disease experts.  

It can take two weeks for the flu shot to provide full protection, and in the interim, people may still get sick, health officials caution. The shots don't provide 100 percent protection, only between 65 percent and 80 percent in a healthy person, but they can lessen the severity and duration of symptoms. The earlier people get shots, the sooner they'll be protected, health officials add.

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