(AFP) -

The United States said Thursday it has begun implementing a massive campaign to vaccinate millions of Americans against swine flu, with the first 600,000 doses to be distributed in coming days.

Anne Schuchat, deputy director of the US Centers for Disease Control and Prevention (CDC), said 25 areas across the country -- among the 50 states and four main metropolitan areas identified in the CDC program -- placed their initial vaccine orders Wednesday and will receive them by Tuesday.

"We are transitioning from the planning phase to the implementation phase," Schuchat told reporters. "This is really just the beginning."

The initial rollout, about two weeks earlier than expected, is welcome news to health experts who had expressed fears that the vaccine might arrive too late in the flu season to provide protection to the millions of Americans believed to be at risk.

The first doses will be in nasal spray form, safe for adults up to age 49 and for children two years and older, and the vaccine should be effective against A(H1N1) within eight days.

Schuchat expects that many of the first people receiving the vaccine will be health care workers on the front lines of the battle against a virus that has claimed more than 3,900 lives worldwide since it was uncovered in April.

The US government announced last week it expected to have six or seven million doses available in the first week of October -- figures confirmed by Schuchat Thursday -- and production will be quickly ramped up after that. A total of 250 million doses eventually will be available in about 90,000 sites throughout the country.

"We are planning to have enough vaccine for everybody who wants to be vaccinated," Schuchat said.

Swiss pharmaceutical firm Novartis announced Thursday it had begun delivering a first batch of swine flu shots to the United States, which ordered 979 million dollars worth of Fluviron H1N1 vaccine from the company.

While spray vaccines will be the first to reach vaccination sites, one of the most vulnerable groups, pregnant women, will have to wait until later this month for the injection version.

Schuchat expressed concern about the "striking" toll that pandemic A(H1N1) influenza is having on pregnant women, saying that 28 expectant mothers have died of swine flu in the United States this year.

Clinical trials for pregnant women tested with the vaccine are expected to be complete by the end of October, but last week the government urged expecting women not to wait because years of data on seasonal flu vaccines suggested there was little elevated risk from the dosage.

Schuchat also moved to assuage concerns about the safety of the vaccines.

"The H1N1 vaccine is being produced exactly the same way that the seasonal flu vaccines are produced, with exactly the same careful oversight," she said.

"No corners have been cut at all."

Schuchat also said some 300,000 courses of antiviral treatment in the form of liquid Tamiflu for children were released Wednesday.

She said some of the treatment will have expiration dates that have passed, but that the Food and Drug Administration extended the usability of the drugs after determining through testing "that the anti-viral medicines are still potent and safe to use."

The announcements come as a new report warned that hospitals in 15 states could reach capacity if 35 percent of Americans get H1N1.

As many as 168,025 people in California alone could need hospitalization and states may be forced to restrict non-flu-related discretionary hospitalizations due to limited bed availability and the enormous strain on intensive care units, according to a report released Thursday by Trust for America's Health.

The government has warned that swine flu could infect up to half the US population of 300 million this fall and winter and cause up to 90,000 deaths.