The United States was in the grip Thursday of a deadly influenza outbreak that has hit harder and earlier than in previous years, and has claimed the lives of at least 18 children.
"It looks like the worst year we've had since 2003-2004," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
Fauci said this year's influenza strain, which has sickened thousands across the country, is particularly severe.
"The type of flu is one that generally is more serious. It's the H3N2 variety, which is historically more serious than we see with other types of virus," he told AFP.
The epidemic, which broke out in early December, has caused some 2,200 hospitalizations across the United States, federal health officials said.
Particularly hard hit has been the northeastern city of Boston, where officials have declared a public health emergency.
City officials said there have so far been about 700 confirmed cases of the flu, almost 10 times the number from this time last year.
"This is the worst flu season we've seen since 2009, and people should take the threat of flu seriously," Boston Mayor Thomas Menino said in a statement.
"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," he said.
Joe Bresee, chief of the Epidemiology and Prevention Branch of the Centers for Disease Control and Prevention's Influenza division, said officials can't predict how much worse this year's outbreak will get.
"While we can't say for certain how severe this season will be, we can say that a lot of people are getting sick with influenza and we are getting reports of severe illness and hospitalizations," he said.
Flu strikes every year across the United States, bringing chills, fever, coughing and achy misery to millions.
Dozens of US states, particularly in the northeast of the country, have seen a sharp spike in emergency room visits from patients reporting flu-like symptoms, according to the federal CDC in Atlanta.
In Allentown, Pennsylvania, one hospital had to erect a large outdoor tent to admit and treat the large number of flu sufferers.
Health officials said the flu vaccine is a good match for the strain of influenza circulating around the nation, and confers about 60 percent to 65 percent protection against the illness.
"You might get the flu but it will likely be less severe if you are vaccinated," Fauci said.
The CDC recommends that everyone older than six months get vaccinated, particularly those who are at risk for serious complications, such as babies, senior citizens, pregnant women and those with chronic health issues including asthma, diabetes or lung disease.
People older than 65 account for about 90 percent of the 36,000 annual flu deaths across the United States.
In addition to being inoculated against flu, health officials recommend such basic prophylactic procedures as frequent hand washing, and coughing or sneezing into one's sleeve to keep the illness at bay.
The US health agency also has launched a campaign to get all health-care workers vaccinated against the flu, and reported in a recent survey that 86 percent of doctors and 80 percent of nurses complied with that recommendation last year.
Some health facilities even are making flu shots a condition of employment, although one critical-care nurse made headlines recently for refusing the injection—insisting she would rather be fired than take the vaccine.
Carrie Calhoun, a nurse with Alexian Brothers Health System based in Elk Grove Village, Illinois, has said her employer's new rule requiring a flu shot is an infringement of her right to make her own health care choices.
The company insists the measure is needed to protect their patients against the virulent and potentially fatal disease.