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.
BBC