New York's governor has declared a health emergency over a flu epidemic that has hit more than 19,000 people in the state, and in an exceptional measure cleared pharmacists to immunize infants and children.

To try to curb the spread of the potentially lethal virus, Andrew Cuomo said it was critical to suspend, for the next 30 days, a legal restriction under which New York pharmacists can administer flu vaccines only to patients 18 or older.

His statement on Saturday came amid a nationwide epidemic expected to last several weeks.

"We are experiencing the worst flu season since at least 2009, and influenza activity in New York State is widespread, with cases reported in all 57 counties and all five boroughs of New York City," said Cuomo.

"Therefore, I have directed my administration, the state health department and others to marshal all needed resources to address this public health emergency and remove all barriers to ensure that all New Yorkers -- children and adults alike -- have access to critically needed flu vaccines."

Already Wednesday, Boston Mayor Thomas Menino had declared a public-health emergency in the city after local health officials confirmed 700 cases of flu -- 10 times the number for the entire flu season last year.

So far this season, 20 children have died from the flu, according to the Centers for Disease Control and Prevention (CDC).

While there have been at least 28,747 cases reported across the country, the true figure is likely far higher as many who fall ill do not get tested, it added.

In New York state alone, 19,128 cases have been confirmed this season, a jump from the total of 4,404 during the 2011-12 flu season.

The New York Health Department is launching a large-scale campaign to promote immunization, using the media, dedicated websites and social media sites such as Facebook and Twitter.

Cuomo urged non-immunized New Yorkers to schedule a flu shot immediately. "It is not too late to get a vaccination," he stressed.

He said relatives and caregivers who regularly come into contact with young children or people at high risk should get a shot against the flu virus, which spreads through coughing or sneezing.

It poses an increased risk for infants and toddlers under the age of two, people over 50, pregnant women and those with weakened immune systems or chronic medical conditions.

Flu strikes every year across the United States, bringing chills, sore throats, fever, coughing, aches and congestion to millions.

The number of annual deaths has ranged from a low of 3,000 to a high of 49,000 since 1976, according to the CDC.

Most of those deaths were among people aged 65 and older.

Meanwhile, Massachusetts has had 18 flu deaths so far this season, according to the state Department of Public Health.

The hospitals in Boston have been overwhelmed.

Jim Heffernan, chief of primary care at Boston's Beth Israel Deaconess Medical Center said the emergency room at his hospital was "overflowing because there aren't enough places to put people. It just snowballs."

Last Monday, Beth Israel got 400 more calls than normal to its urgent care hotline, hospital officials said.

"We had to open a new unit to accommodate all the patients." hospital spokeswoman Kelly Lawman said.

Influenza was also rampant in New Jersey.

"It's here. Big time," said Doctor Thomas Birch, president of the medical staff at Holy Name Medical Center in Hackensack, New Jersey.

"When the virus comes into a community with susceptible individuals, it will literally spread like those wildfires in California that burn everything in their path."

Specialists and government officials urge Americans to protect themselves with flu shots.

However, the CDC also acknowledged Friday that influenza vaccines, on average, are only about 62 percent effective.

"There is a growing consensus among the public health communities that we need better influenza vaccines," says Michael Osterholm, director of the Center for Infectious Disease Research and Policy. "We're operating largely in the 1950s for our flu technology."