Izurieta HS, Haber P, Wise RP, et al. Adverse Events Reported Following Live, Cold-Adapted, Intranasal Influenza Vaccine. JAMA. 2005; 294:2720-2725.

In its first two seasons of use, the intranasal influenza vaccine FluMist did not lead to "unexpected serious risks," according to a study of adverse events reported to the U.S. Vaccine Adverse Event Reporting System (VAERS).

Among 2.5 million vaccine recipients in the 2003-04 and 2004-05 seasons, there were 460 adverse events, 40 of them (9%) were considered serious. No deaths were reported. The most frequently reported events were respiratory, including influenza-like illness, pharyngitis/tracheitis, pneumonia, and sinusitis.

Allergic events (12%) included throat swelling, rash, urticaria and edema. There were seven reports of possible anaphylaxis. "The interval from vaccination to symptom onset was less than three hours for all cases, and 20 minutes or less for five of the cases," the authors stated. "Although no one had a reported prior history of anaphylaxis or known allergy to any of the vaccine components, five cases had prior history of hypersensitivity including contact dermatitis, hypersensitivity to other drugs, and seasonal allergy."

Numerous other events were reported, including two cases of Guillain-Barré syndrome and one case of febrile seizures in a 4-year-old boy. The authors cautioned that there are inherent limitations in the surveillance data, including possible underreporting and reporting bias. Some events may occur coincidental to the vaccination, but may not be caused by it.

But there are two issues raised by the FluMist reports. Concerns about the potential secondary transmission of influenza from the live, attenuated vaccine cannot be answered by VAERS data, the authors said. There were 22 reports of secondary transmission of the virus to non-vaccinated individuals, but a laboratory analysis of the viruses occurred in only one case. In that case, involving a pediatrician and her 4-year-old child, the child had a wild-type influenza A (H3N2) strain that did not genetically match the FluMist strain.

"The fact that more than half of these reports were of transmission to a health care worker who administered the vaccine may reflect preferential reporting related to concerns of the medical community regarding this potential risk," the authors stated.

The VAERS analysis revealed the importance of following the approved labeling of the product. Twenty-six reports occurred among people who were not in the recommended age group for the vaccine (5 to 49) and 52 had chronic conditions. Eight of 12 reports of asthma were among people with a prior history of asthma.

"The reports of asthma exacerbations in vaccinees with prior asthma history highlight the potential risks of not following the approved indications and support the need for continued close surveillance for asthma exacerbations following use of this vaccine," the authors said.