With the U.S. Centers for Disease Control and Prevention hoping to have 120 million doses of H1N1 swine flu virus vaccine ready before flu season this fall, some are raising concerns over what they see as an effort to rush the drug through safety trials.
The source of many of these concerns is the probability that the mercury-containing preservative thimerosal will be an ingredient in some of the doses of the new vaccine. Concern over thimerosal has lingered for years, despite research that has overwhelmingly found it to be harmless.
"We have yet to find any evidence that thimerosal ever hurt anyone," said Dr. Andrew Pavia, chief of Pediatric Infectious Diseases at the University of Utah.
And while the heat is on manufacturers and governments to ensure that enough doses of the vaccines are available by this fall, when a possible swine flu resurgence is feared, government health officials say that by this time clinical trials will have taken place to determine the correct dosage and whether the vaccine should be delivered in multiple injections.
"I see no reason to anticipate major safety concerns with this vaccine," said Dr. Peter Hotez, a professor and chair of the Department of Microbiology, Immunology and Tropical Medicine at George Washington University. "The vaccine will be tested and licensed, and the FDA will not allow it to go forward unless the vaccine is shown to be safe and elicit an effective immune response."
Dr. John Treanor, a professor of medicine and immunology at the University of Rochester, added that the design and production of the swine flu vaccine should be much like that of the seasonal influenza vaccine, which is developed every year without the benefit of time- and cost-consuming clinical trials.
"It's like a new product each year, and when we use it and we don't do clinical studies when [seasonal flu vaccines] are licensed because of the long history of flu vaccine safety," Treanor said.
However, groups opposed to current vaccination practices continue to condemn thimerosal as a toxin responsible for the development of autism and related ailments in children. Additionally, the possibility that the swine flu vaccine could also contain an adjuvant, an ingredient that would allow more doses to be created from existing supplies of the vaccine, has also worried these groups.
"We don't have adequate safety studies on this vaccine before we are moving forward to market," said Lyn Redwood, president and co-founder of the group SafeMinds. "I'm really not convinced that we know for sure that the risk of the disease outweighs the risk of the vaccine, especially since this is a brand new additive that we have never used before in combination with thimerosal."
Some of the lingering fears surrounding a new swine flu vaccine may spring from a single black mark in the flu vaccine's 60-year history. During the 1976-77 flu season, a vaccine developed to prevent the spread of a strain of the swine flu was linked to an as-yet-unexplained increase in cases of a rare neurological condition known as Guillain-Barre syndrome in those who received immunizations.
Medical professionals maintain there was no strong evidence to support the theory that the vaccine caused the neurological disorder. And they said overblown safety concerns should not prevent people, particularly those at high risk of infection such as children and pregnant women, from receiving this year's swine flu vaccine.
"Public perception plays a very important role in how vaccination campaigns succeed," Treanor said. "It will be important to provide transparency and up-to-date safety information."
Donna Cary, a spokeswoman for Sanofi-Aventis, the company that will manufacture the swine flu vaccine, said that they have received an order from the Department of Health and Human Services for a bulk amount of vaccine but that the formula is still to be determined. However, they expect they will be manufacturing two varieties of the vaccine.
"The anticipation is that we will be producing both a thimerosal-free and a vaccine containing thimerosal," Cary said. "Because of the quantities they're talking about, multi-dose vials will be used."
Using thimerosal and multi-dose vials makes vaccines cheaper to manufacture and distribute, Treanor said.
Still, Pavia said, there will likely be enough doses of thimerosal-free vaccine for very young children whose parents fear that the chemical will have some negative effect. Taking such steps, he said, may further ensure than those who need this important vaccine will receive it.
ABC News' Lisa Stark contributed to this report.