At entry into the study, the participants were divided at random into two groups: half are receiving two doses of a 15-microgram vaccine and the other half are receiving two doses of a 30-microgram vaccine. The two injections of vaccine are spaced three weeks apart.

Safety is being monitored closely in the trial, by the study investigators and by an independent panel of experts known as a safety monitoring committee. To date, the vaccine appears to be well-tolerated, and no safety concerns related to the vaccine have arisen.

The vaccine used in this clinical trial was manufactured by Sanofi Pasteur in its plant in Swiftwater, Pa., in the same manner as the company's injectable seasonal influenza vaccine. Like the seasonal flu vaccine, the 2009 H1N1 flu vaccine contains a purified portion of the killed virus and therefore cannot cause infection. The vaccine does not contain the preservative thimerosal or an immune boosting substance known as an adjuvant.

Source: NIH/National Institute of Allergy and Infectious Diseases