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WASHINGTON (Reuters) - The U.S. Centers for Disease Control and Prevention is preparing its broadest and most ambitious vaccination effort yet for the coming influenza season, experts said on Thursday.

The new recommendations will cover 218 million Americans, or 73 percent of the population, for the 2006-2007 influenza season, the experts told reporters. They also hope to persuade more of these people to get vaccinated and will tell them that getting vaccinated is still useful late in the season.

The hope is to both reduce flu deaths -- 36,000 every year on average -- and to coax vaccine makers back into the uncertain U.S. market.

There will not be enough vaccine for this many people but historically most Americans who should be vaccinated do not.

But the CDC expects a record number of doses to be available -- up to 120 million, said Dr. Nicole Smith of the CDC's influenza division. The most ever available before has been 95 million doses.

Smith said the CDC was expected to extend its recommendations to include children aged up to 5 years and all their contacts -- including siblings, parents and caregivers. Last year only children aged 6 months to 2 years were on the priority list.

Vaccinating schoolchildren could provide indirect protection to other groups, Smith added, including the elderly. Other studies have shown that a fairly new vaccine that protects young children against pneumococcal bacteria -- which cause pneumonia, bronchitis and other conditions -- has greatly reduced rates of disease in older adults as well.

One question was whether to go ahead and recommend that everyone be vaccinated, or concentrate on getting more members of higher-risk groups vaccinated, Smith told the briefing, sponsored by The National Foundation for Infectious Diseases.

Dr. Ardis Hoven, a member of the American Medical Association's Board of Trustees, noted that only 40 percent of health-care professionals, including doctors, nurses and technicians, get vaccinated against flu every year, even though they know it can protect not only themselves, but their families and the patients they care for.

At Hoven's hospital at the University of Kentucky College of Medicine, 1,000 doses of flu vaccine went unused last year and had to be sent back to the manufacturer.

Part of the problem is that makers cannot deliver all their influenza vaccines right at the start of flu season in October. If people ask for a vaccine and cannot get it immediately, they often fail to come back looking for one later.

Interest in vaccination tends to wane after the Thanksgiving holiday at the end of November, Hoven said, even though the flu season has not even properly geared up. "We know that it peaks in February, but we are seeing cases in March, we are seeing cases in April," she said.

The CDC and NFID say people benefit from getting vaccinated as late as February.

Hoven and the other experts hope that having more vaccine available, and getting more Americans vaccinated, will encourage drug companies to make flu vaccines. Currently just four companies supply the U.S. market, and there have been severe shortages in the past five years.

"We have got to increase the number of manufacturers and we have got to stabilize the vaccine supply," Hoven said.

The issue has become more critical with the spread of H5N1 avian influenza, which experts say has a greater potential than any flu virus in 30 years to cause a pandemic. It does not yet easily infect people but has killed 122 people in 10 countries and spread rapidly among birds.

"Preparation for delivery of 120 million vaccines is good preparation for any pandemic," Hoven said.