By Maggie Fox

WASHINGTON (Reuters) Nov 20 - As US health officials struggle to vaccinate tens of millions of Americans against the pandemic of swine flu, some are looking regretfully at one easy way to instantly double or triple the number of doses available: using an adjuvant.

These additives, often as simple as an oil and water mixture, broaden the body's response to a vaccine, reducing the amount of antigen needed.

Adjuvants are widely used in European flu vaccines as well as in Canada. But not in the United States -- even though the federal government has spent nearly $700 million buying them.

The reason -- people might not trust them.

"If we really do want pregnant women to trust this vaccine or even parents, we have to think about what is acceptable to them," Dr. Anne Schuchat of the US Centers for Disease Control and Prevention said in an interview.

"We have so much vaccine hesitancy in this country," agreed Jeff Levi of the non-profit Trust For America's Health. "To add...a new element could well have undermined the efficacy of this campaign," Levi told a hearing this week before a Congressional subcommittee.

This frustrates the World Health Organization, which says the global capacity to make influenza vaccines is about 3 billion doses a year -- not enough to cover the population of 6.8 billion people. WHO has hoped rich countries would donate leftover H1N1 vaccine to others.

The US Health and Human Services Department was ready to try adjuvants had the pandemic been worse. The H1N1 virus has infected an estimated 22 million Americans and killed 3,900, but it so far does not appear to be any deadlier than seasonal influenza.

The worry is that it is affecting younger adults and children instead of the elderly usually targeted by flu, and has the potential to mutate into something more deadly.

"If things had been worse and this would have been a more severe pandemic, we may well have needed to go that way anyway," Levi said.

Instead, the US has stuck with what CDC director Dr. Thomas Frieden has repeatedly called the "tried and true" approach -- the same formulation used in seasonal flu vaccines. Five companies have contracts -- Sanofi-Aventis, CSL, Novartis, AstraZeneca unit MedImmune and GlaxoSmithKline.

Polls show that only about half of Americans plan to be vaccinated against H1N1. Of those who do not, about half say they worry about safety.

Even so, long lines have formed as people try to get the 50 million or so vaccine doses that have rolled out of factories. Drug companies have struggled with an unpredictable virus that does not grow well in eggs, as well as changes to US orders that slowed down packaging.

Studies suggest the supply that is out now could have been tripled.

In September, GlaxoSmithKline found a single shot of its H1N1 vaccine protected 98% of volunteers, using an adjuvant and just 5.25 micrograms of antigen. A standard dose without adjuvant uses 15 micrograms of antigen.

Vaccine makers urged Congress this week to help federal agencies find ways to approve the use of adjuvants, and to assure skeptical Americans about their safety.

Dr. Vas Narasimhan, president of Novartis Vaccines USA, noted adjuvants had been licensed for use in Europe for 10 years and tested in 200,000 people.

"Adjuvanted vaccines produce higher immune response than unadjuvanted vaccines, particularly in the elderly and young children," Narasimhan told a hearing this week.

He said they may protect better than standard vaccines against viruses that have drifted a little -- the single biggest reason that flu vaccines must be re-formulated every year.

They may also eventually help require less vaccination. "Adjuvanted vaccines have been shown to more broadly prime patients' immune response -- up to seven years later -- requiring fewer vaccinations to the newly circulating strain," he said.

The National Institute of Allergy and Infectious Diseases is intrigued. Last month it awarded $60 million to researchers and companies to develop new adjuvants