US officials say swine flu activity is widespread in 46 states
US President Barack Obama has declared swine flu a national emergency.
The White House said the president signed the proclamation concerning the 2009 H1N1 outbreak on Friday evening.
It increases the ability of treatment facilities to handle a surge in H1N1 patients by easing the implementation of emergency plans.
Last week US officials said swine flu activity was widespread in 46 states. More than 1,000 US deaths have been linked to the virus.
Health officials say the infections are already comparable to peak season flu levels.
US officials said the president's declaration was similar to ones issued before hurricanes make landfall.
SWINE FLU SYMPTOMS
Typical symptoms: sudden fever (38C or above) and sudden cough
1. Other symptoms include: Tiredness and chills
2. Headache, sore throat, runny nose and sneezing
3. Stomach upset, loss of appetite, diarrhoea
4. Aching muscles, limb or joint pain
Source: UK NHS
It allows authorities to bypass certain federal requirements in order to deal more effectively with emergencies.
The aim of the directive is to remove bureaucratic hurdles, allowing sick patients to receive treatment more quickly and giving health-care providers more flexibility in providing it.
Paperwork on patients can be reduced and additional health centres set up outside hospitals to care for the sick.
In his proclamation statement, Mr Obama says the 2009 H1N1 pandemic "continues to evolve".
"The rates of illness continue to rise rapidly within many communities across the nation, and the potential exists for the pandemic to overburden health care resources in some localities."
He said the US had already taken "proactive steps" by implementing public health measures and developing an effective swine flu vaccine.
However, the government has admitted there are delays in the delivery of vaccines.
It had hoped to roll out 120 million doses by mid-October.
It now hopes for about 50 million by mid-November and 150 million in December.
Dr Thomas Frieden, of the US Centers for Disease Control and Prevention (CDC), said on Friday: "We are nowhere near where we thought we'd be by now."
Given the shortfall, New York State on Friday stayed a directive ordering health care staff to be inoculated or risk losing their jobs.
The CDC says widespread influenza activity in 46 states is "unprecedented during seasonal flu".
It said the hospitalisation rates for laboratory-confirmed swine flu were still climbing.
Although figures are hard to verify, it is thought H1N1 has hospitalised about 20,000 people in the US.
Visits to the doctor for influenza-like illnesses were also much higher than expected for the time of year, the CDC said.
The seasonal flu peak is usually between late November and early March.
Children and young adults have been among the hardest hit by H1N1. Almost 100 of the deaths have been children.
HOW SWINE FLU OUTBREAK EMERGED
Flu viruses mutate over time causing small changes to proteins on their
surface called antigens. If the immune system has met a particular
strain of the virus before, it is likely to have some immunity; but if
the antigens are new to the immune system, it will be weakened.
The influenza A virus can mutate in two different ways; antigenic drift,
in which existing antigens are subtly altered, and antigenic shift, in
which two or more strains combine. Antigenic drift causes slight flu
mutations year on year, from which humans have partial, but not
complete, immunity. By contrast, the new strain of H1N1 appears to have
originated via antigenic shift in Mexican pigs
The name "swine flu" is a slight misnomer as it is believed pigs acted
as a mixing pot for several flu strains, containing genetic material
from pigs, birds and humans. Most humans have never been exposed to
some of the antigens involved in the new strain of flu, giving it the
potential to cause a pandemic.
The new virus has made the jump from pigs to humans and has
demonstrated it can also pass from human to human. This is why it is
demanding so much attention from health authorities. The virus passes
from human to human like other types of flu, either through coughing,
sneezing, or by touching infected surfaces, although little is known
about how the virus acts on humans.
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