Swine flu is thought to have killed nearly 4,000 people in the United States, including more than 500 children, health officials said after a new counting method yielded an estimate six times higher than the last.
The new system is based on more precise figures provided by 10 states, the Centers for Disease Control and Prevention (CDC) said. The previous estimated death toll from H1N1 was 672.
While still imprecise, the new numbers provide "a bigger picture of what has been going on in the first six months of the pandemic," Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, told a press conference.
She said previous estimates were based on "laboratory confirmed cases of hospitalization and death, potentially giving an incomplete picture of the story of this pandemic."
According to the new estimates, the total deaths since the swine flu virus first appeared in April total about 3,900, the CDC said, noting that figures were rounded to the nearest 10. The CDC also posted the new set of figures on its website.
The new swine-flu death toll for children under 18 years of age is 540, four times higher than the previous estimate.
Still considered the tip of the iceberg compared to the real, full extent of the swine-flu pandemic, the new estimates are based on more precise data provided by hospitals in 10 US states, Schuchat said.
Those figures were extrapolated to the national level, she added.
The CDC cautioned that methodology was "not a predictive tool and cannot be used to forecast the number of cases, hospitalizations and deaths that will occur going forward over the course of the pandemic because they are based on actual surveillance data."
Schuchat said the estimated range of all H1N1 fatalities in the United States from April to mid-October was 2,500-6,100, with the mid-level range at 3,900.
Broken down by age group, the range was 300-800 deaths for children up to 17 years of age (mid-level range 540); 1,900-4,600 for ages 18-64 (2,920), and 300-700 for people above 65 years of age (440).
In all, 22 million Americans were infected by the swine flu virus during the period studied, with 98,000 hospitalized, according to the new CDC estimates.
Schuchat also said that 41.6 million doses of the H1N1 vaccine were made available Thursday for distribution around the country.
While the volume of vaccine deliveries shows the progress made in production facilities, it remains far below initial estimates and expectations, she added.
US health authorities have recently acknowledged greater shortfalls than anticipated in the vaccine supply, as long queues form outside authorized clinics and health centers in the inoculation drive.
Meanwhile, the global death toll from flu pandemic passed the 6,000 mark last week according to the World Health Organization.
The H1N1 virus has swept around the world since it was first identified in Mexico and the United States in April, spreading into at least 199 countries.
The pandemic is currently surging in the northern hemisphere with the onset of colder weather.