Mayor Thomas M. Menino declared a public health emergency Wednesday morning because of the expanding flu outbreak. Health care centers across the city will be offering free vaccines to anyone who hasn’t yet been immunized. The city has 700 confirmed cases of flu so and four flu-related deaths. Last year Boston had only 70 confirmed cases.

Massachusetts has had 18 flu-related deaths so far this season, according to the state Department of Public Health. Officials emphasized it’s not too late to get a shot. “The best thing you can do to protect yourself and your family is to get the flu shot,” Menino said during a press briefing. “I’ve had my flu shot. I’m asking you to get yours too.” Here’s where to find a free shot.

The city’s declaration of a health emergency was “very unusual,” said Dr. Anita Barry, director of Boston’s Infectious Disease Bureau. She said it was designed to raise awareness about the severity of the flu season and to make public resources available to improve vaccinations.

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Officials from the federal Centers for Disease Control and Prevention said they can’t explain why the flu season has hit early this year. “The timing of the season is unpredictable” said Dr. Joseph Bresee, chief of the CDC’s influenza division. “But this particular strain circulating leads to more severe disease with more deaths and hospitalizations.”

The good news is that the strains of flu virus in circulation this year were anticipated, and 91 percent of the viruses found in testing this year are well-matched to the vaccine, said Kevin Cranston, director of the DPH Bureau of Infectious Disease. There are ample supplies of the shot, he added.

Confronting an earlier and a harsher than usual flu season, hospitals and other health care facilities are limiting visitors who may potentially infect sick or elderly patients with the virus—precautions not seen since the 2009 H1N1 “swine flu” pandemic.

UMass Memorial Medical Center this week decided to ban visits from children age 14 and under, since kids frequently contract the virus in school, and only allow patients to have two visitors at a time. “This is something we do during any type of outbreak,” said Dr. Robert Klugman, the hospital’s chief quality officer. “We had these restrictions when H1N1 was circulating.”

He added that he’s had no complaints from patients. “No one wants to be responsible for making someone ill.”

Most hospitals in the area have been keeping a close eye on visitors walking through their doors, stopping anyone who’s coughing, sneezing, or looking feverish—even if they haven’t put in place formal visitor restrictions. “We will be monitoring closely and ramping up as needed,” said Dr. Thomas Sandora, an infectious disease specialist at Boston Children’s Hospital.

Massachusetts General Hospital said its capacity has been “strained to its limits” with an additional 40 to 80 patients coming in daily to its outpatient clinics and emergency room with flu-like illness. The hospital has restricted visitors to its obstetrics department filled with vulnerable, unvaccinated newborns and has urged all staff to stay home if they have a fever along with other flu symptoms such as a runny nose, sore throat, cough or body aches.

Beth Israel Deaconess Medical Center had to open a previously closed wing to accommodate the influx of patients sick with the flu or the intestinal illness norovirus, which is also circulating.

Based on the rate of hospitalizations and outpatient visits, far more state residents have come down with the flu, and far earlier in the season, than last year. The percentage of doctor visits for flu-like illness—fever, body aches, coughing, runny nose, and fatigue—is now above 4 percent and is still climbing, according to the latest data from the Department of Public Health. Last year, barely more than 1 percent of doctor visits were due to the flu at the season’s peak.

About 3 in 10,000 Americans over age 65 have been hospitalized with the flu so far this season compared with 1 in 100,000 at the same time last flu season.

Since seniors face a higher likelihood of developing pneumonia and other life-threatening flu complications, one nursing home in the area has taken the drastic step of asking family members not to visit until flu rates subside.

“What we’re telling people is that we’re strongly discouraging visitors at this time,” said Novyl Igo, assistant administrator of Marian Manor in South Boston. “If family members really want to visit, we can make accommodations, but we’re asking if they had a flu shot.”

The facility offered flu immunizations in the fall to its staff, and most have been vaccinated, Igo said. Unlike several Boston hospitals, however, Marian Manor doesn’t require its workers to be immunized to stay on the job.

Cranston, of the Department of Public Health, said the flu season has been accompanied by a significant number of cases of norovirus, an intestinal illness, so “it’s been a bit of a double whammy, particularly on families that have sick children.”