Ferocious outbreaks of flu have overwhelmed hospitals in the United States, forcing some to cancel surgery to free up beds for victims of the virus.

The first substantial numbers of cases were reported in California and Arizona last month. Since then 19 states have been affected.

Cases have been increasing in the last three weeks, according to Dr Keiji Fukuda, at the Centre for Disease Control and Prevention (CDC) in Atlanta.

"It's pretty clear in some local areas that they have gotten hit pretty hard," Dr Fukuda said.

The New York State Health Department has reports of 80 outbreaks, compared with 19 at the same time last year.

At some hospitals, supervisors are trading their business suits for white coats to bolster overtaxed staff.

The director of patient services at one hospital in Indiana said: "This morning our medical-surgical unit was at 150% capacity. I think we are approaching record numbers."

And in Maryland, a health official warned: "If the trend continues the way it is now, within the next month we are going to have a great challenge to find beds to admit anybody in the state."

Epidemic fears

Almost a dozen hospitals in Maryland are on "yellow alert", bracing for the possibility of a genuine epidemic.

In an attempt to free up beds, they are referring patients to other hospitals and cancelling non-emergency surgical procedures.

Some hospital emergency units say they have been overwhelmed by the influx of flu victims, which is making it difficult for them to treat the more seriously ill and injured.

The onset of the current strain is particularly fast, with symptoms appearing as soon as a day after exposure.

Sufferers toughing it out at home are emptying pharmacy shelves of over-the-counter flu remedies.

Is the vaccine working?

The CDC says 99% of cases reported so far are due to the Sydney influenza strain, a type-A virus which also struck last year.

Affected states include Pennsylvania, New York, New Jersey and Virginia.

"It's bad all over the country," said Dr Robert McNamara, chief of emergency medicine at Temple University Hospital in Philadelphia. "The flu shot apparently is not doing its job, and from all reports, it could be one of the worst seasons we've seen in years."

But the CDC, which distributes the flu vaccine, said it was well matched for the viruses in circulation.

Deaths

The CDC says flu causes about 20,000 deaths and more than 110,000 hospitalisations a year.

The young, the aged and those with compromised immune systems, such as people suffering from Aids, are especially susceptible to life-threatening complications due to the virus, including pneumonia and dehydration.

It can also exacerbate chronic conditions such as congestive heart failure and asthma.

Flu is most often spread when infected people sneeze or cough, ejecting the virus into the air.