Few serious side effects have been reported since the inhaled flu vaccine FluMist became available nationwide two years ago. About 2.5 million people in the United States have received FluMist, which is intended for healthy people ages 5 to 49. Of 460 "adverse events" possibly linked to the vaccine and voluntarily reported to the U.S. Vaccine Adverse Event Reporting System through August, 40 were considered serious, including breathing problems in asthma patients and life-threatening allergic reactions.

The figures echo previous reports about FluMist's safety and show that the vaccine is safe when used as directed, government researchers said. The study appears in today's Journal of the American Medical Association. It was conducted by researchers from the Food and Drug Administration and the Centers for Disease Control and Prevention, the agencies that operate VAERS.

Unlike flu shots, FluMist contains live but weakened influenza virus—the reason it is not recommended for people at highest risk for flu complications, such as the elderly, children, and people with asthma or other chronic diseases. Overall, there were 73 reports of possible side effects in patients who should not have been given FluMist in the first place. Eight of the 12 reported asthma cases occurred in patients with a history of asthma.

Past shortages of injectable flu vaccine might have led some doctors to give FluMist to asthmatics and other patients for whom it is not approved, said Dr. Gary Noskin, chief of epidemiology at Northwestern Memorial Hospital in Chicago. Some may have thought the risks associated with the flu outweighed the risks from the vaccine, Noskin said. The number of cases of improper use "is really a small subset—it shows that the system is working," said the study's lead author, Dr. Hector Izurieta of the FDA. "I don't think it suggests people are using it in an arbitrary way."

No vaccine shortages are expected this flu season. FluMist, introduced in 2003, is thought to work at least as well as injections at preventing flu.