WASHINGTON (Reuters) - The first U.S. roll-out of vaccines against the new swine flu virus will be 3.4 million doses of MedImmune's nose spray, the Centers for Disease Control and Prevention said on Friday.
The CDC's Dr Jay Butler said the vaccines would be distributed the first week of October.
"Initially we anticipate that 3.4 million doses of vaccine will be available," Butler told a telephone briefing.
"We anticipate being able to start receiving orders for the vaccine by early October," Butler added. The U.S. government is providing the H1N1 vaccine for free to about 90,000 distributors, including doctor's offices, retail chains and state health departments.
"We estimate that the amount of vaccine that will be available will increase through October," Butler said, adding that eventually delivery would rise to about 20 million doses a week.
The United States has ordered 195 million doses of H1N1 swine flu vaccine from five companies -- MedImmune, a unit of AstraZeneca, Sanofi-Aventis, Australia's CSL, GlaxoSmithKline and Novartis.
It has recommended that about 160 million people, roughly half the population, get vaccinated first -- pregnant women, healthcare workers, children and people with chronic conditions such as diabetes or asthma who are most at risk of getting very ill from flu.
There is no enforcement of this, however, and it will be up to the people giving the vaccine to decide who goes first.
MedImmune's vaccine is not approved for people with asthma, people over age 50 or very young children, mostly because it has not yet been tested widely in these groups. State health officials say that might affect who gets the first doses.
Some of the other vaccines contain thimerosal, a preservative that scientists say is safe but which worries some people, and the state of Washington, for instance, says infants and pregnant women may not be given thimerosal-containing vaccines, another factor that could affect distribution.
The CDC officials said they are racing to try to stay ahead of the virus, which is now active in all 50 states. It spread around the world to cause a pandemic within a few weeks in April and May.
"The flow of vaccine the first week or so may be slower than we like," Butler said.
So far, the virus is causing moderate disease, with a death rate similar to that seen in seasonal flu. Every year, seasonal influenza kills between 250,000 and 500,000 people globally and around 36,000 in the United States.
But H1N1 may infect far more people than seasonal flu does, because so few people have any immunity to it. It also causes symptoms in a far younger age group than does seasonal flu, which is worse among the elderly.
"We expect that if the H1N1 (virus) remains the predominate strain, that more younger people might be affected than we have seen in the past," the CDC's Dr Daniel Jernigan said.
"There is some increase in the rate of hospitalization for younger children and for adults, but it is not up at the levels that we would see for seasonal flu."
He said it was very unusual to have so much flu spreading this time of year in the United States. It is usually more common in the United States from January to March.
"It's about twice as much, at least for what we would expect this time of year," Jernigan told the briefing.
"If you talk to doctors they will tell you, 'boy I am seeing a lot of flu for this time of year,'" he added.
Editing by Eric Beech