The CDC says the supply of flu vaccine in the US is increasing
Swine flu has killed nearly 4,000 people in the US, including 540 children, officials said after devising a new counting method.
The Centers for Disease Control and Prevention (CDC) said the new system is based on more precise figures provided by 10 states.
The previous estimated death toll from the H1N1 virus in the US was 672.
Latest figures show about 22 million Americans contracted the virus in six months with some 98,000 hospitalised.
"This is just the first six months and I am expecting all of these numbers, unfortunately, to continue to rise," said Dr Anne Schuchat of the CDC.
She said that, although still imprecise, the new statistics provide "a bigger picture of what has been going on in the first six months of the pandemic".
The CDC now estimates that 3,900 people in the US have died from the virus in the past six months.
Four times higher
Dr Schuchat said that in children under 18, an estimated eight million have had swine flu, with 36,000 hospitalised and 540 deaths.
The new estimated death toll for children is four times higher than the previous estimate.
"We will be updating the toll that the pandemic has taken... about every three to four weeks," she said.
Dr Schuchat added that 41.6 million more doses of swine flu vaccine had been made available on Thursday for distribution around the country.
However, delivery remained far below initial estimates and expectations, she said.
The global death toll from the flu pandemic passed the 6,000 mark last week according to figures from the World Health Organization.
The virus emerged in Mexico in April and was declared a global flu pandemic on 11 June.
Cases are currently surging in the northern hemisphere with the onset of colder weather.