While drug makers prepare a swine flu vaccine in anticipation of a possible outbreak this fall, one of the issues yet to be resolved is whether the shots will contain an adjuvant.
"It's something that allows the immune system to respond with higher levels of effectiveness," said Dr. David Fedson, formerly a professor of medicine at the University of Virginia and former medical director for the pharmaceutical company Aventis Pasteur.
Adjuvants can include various forms of aluminum and are typically used with other vaccines in the United States, including vaccinations for hepatitis A and B, diphtheria-tetanus-pertussis and Haemophilus influenzae type b (Hib). In a flu vaccine, the adjuvant would be a water-oil mixture.
By using them in various vaccines, doctors hope to reduce the amount of the vaccine itself that is needed.
"Hopefully, in the future, they're going to lead to the ability to get a better immune response with much less vaccine," Dr. Andrew Pavia, chair of the public health committee of the Infectious Diseases Society of America and a professor of professor of pediatrics at the University of Utah, said in an interview with ABC News last September.
The ability to stretch a supply of swine flu vaccine -- and an adjuvant's ability to help to do that -- is not trivial. Fears have arisen about whether there will be enough swine flu vaccine available. Adjuvants can allow dosing to be much smaller.
For example, Fedson points to trials for a bird flu vaccine in which 90 micrograms of an antigen -- a flu virus's "signature" that allows for an immune response -- could be reduced to 3.75 micrograms when an adjuvant was introduced, effectively enabling 24 times more doses.
"By adding an adjuvant, you gain what is known as an antigen sparing effect," Fedson said.
While flu vaccine doses typically use 15 micrograms of antigen, adjuvants could increase that significantly.
"Being able to produce four times as many vaccines is a huge advantage in terms of public health," Fedson said.
At the same time, vaccines have long been plagued by safety concerns -- whether legitimate or not -- and the use of adjuvants is only likely to add to that. No flu vaccine approved for use in the United States has ever contained an adjuvant.
"It wasn't felt to be necessary, because the flu vaccines that have been used for decades in this country … were rather broadly protective," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "We felt that the immune response of the vaccine was pretty good to begin with."
Fears surrounding the swine flu vaccine are often attributed to the vaccine for the 1976 swine flu epidemic when a few hundred people came down with Guillain-Barré syndrome after receiving the swine flu vaccine.
While the bad publicity led to the halting of the vaccine program after 40 million Americans had received the vaccine, no link has ever been proven between the vaccine and Guillain-Barré.
But concerns about what might happen with the vaccines remain, and the addition of adjuvant is likely to add more fuel to the fire.
The initial trials of the swine flu vaccine for the United States will be with the unadjuvanted form, while later trials with an adjuvanted form may take place if deemed necessary.