The New England Journal of Medicine published a study focusing on the safety profile of the H1N1 vaccine.
It was conducted in China where 2200 subjects received a dose of H1N1. 2103 (95.6%) received the second dose. They ranged in age from 3y to 77y the subjects were randomized to either vaccine or placebo. Patients were removed from the study once they achieved an increase in antibody titer of 4x baseline. Which is considered adequate protective levels. Those that did not achieve this level of antibody received a second dose at day 21.
The results did show good efficacy of the vaccine in adults with just a single dose. Children and the elderly did need the second vaccine dose to get the needed antibody level.
One adverse result was seen in the study, it was development of atrial fibrillation in a man who was receiving placebo. There were no serious adverse results in any of the vaccine recipients. Local pain at the injection site was the main complaint.
This is the New England Journal article: A Novel Influenza A (H1N1) Vaccine in Various Age Groups