US officials have warned of delays in the delivery of swine flu vaccines just as deaths from the H1N1 virus climb above epidemic level in some states, BBC reported.
Only 28-30 million doses would be available by the end of the month, said Anne Schuchat of the US Centers for Disease Control and Prevention (CDC).
That was down from an earlier estimate of 40 million.
Swine flu has had an especially strong impact on children, 86 of whom have died this year, Ms Schuchat said.
The number of children who have died so far this year from the H1N1 virus was greater than the number that normally die in an entire flu season, Ms Schuchat said.
Forty-three children are reported to have died from flu since 30 August, with 38 of those confirmed to have been caused by the H1N1 virus.
Half of those deaths were in children aged between 12 and 17.
"These are very sobering statistics," Ms Schuchat said. "Some of these children have been totally healthy."
Ms Schuchat said swine flu activity was widespread in 41 states, and deaths had reached the epidemic threshold in some states and cities.
"Influenza is widespread in the country and illnesses, hospitalisations and deaths continue to increase," she said.
"It's unprecedented for this time of year to have the whole country seeing such high levels of activity."
Testing the vaccines for strength and purity was cited as one reason for a delay in their delivery.
"We are not cutting any corners in the safety of the production of this vaccine or the testing and oversight of the vaccine," Ms Schuchat said.
Some 11.4 million vaccines have so far been made available in the US.
At a meeting on swine flu in Washington, officials from the World Health Organisation said the H1N1 strain was killing unusually quickly.
"In severe cases, patients generally begin to deteriorate around three to five days after symptom onset," said the WHO's Nikki Shindo.
"Deterioration is rapid, with many patients progressing to respiratory failure within 24 hours, requiring immediate admission to an intensive care unit."