Fauci said this year's influenza strain, which had seen thousands become ill across the country, was 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 said.

The epidemic, which broke out early last month, has caused some 2,200 to be admitted to hospitals across the US.

Particularly hard hit has been the northeastern city of Boston. City officials said there had 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," said Thomas Menino, Boston's mayor, declaring a public health emergency.

"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 Centres for Disease Control and Prevention's (CDC) influenza division, said officials could not predict how much worse this year's outbreak would 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 hospital admissions," he said.

Dozens of US states, particularly in the northeast of the country, have seen a sharp increase in emergency room visits from patients reporting flu-like symptoms, according to the federal CDC in Atlanta. In Allentown, in the state of 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 was a good match for the strain of influenza circulating around the nation, and offers about 60 per cent to 65 per cent protection against the illness. "You might get the flu, but it will likely to be less severe if you are vaccinated," Fauci said.

The CDC recommends that every person older than six months get vaccinated, particularly those who are at risk of serious complications, such as babies, senior citizens, pregnant women and those with chronic health issues including asthma. People older than 65 account for about 90 per cent of the 36,000 annual flu deaths across the United States.

Henry Schein, the nation's largest distributor of flu vaccines to doctors' clinics, said it had a vaccine available from drug firm Novartis for its customers immunising patients aged four and older. However, it said, "We do not have the paediatric vaccine for children aged six months to four years due to its unavailability from the manufacturer, Sanofi."

Roche Holding has a shortage of the liquid form of Tamiflu, given to children who already have the flu to slow or stop symptoms. Roche told wholesalers and distributors in recent weeks that temporary delays in shipments were imminent.