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FluMist: Intranasal Flu Vaccine

Background: Worldwide, influenza epidemics cause hundreds of thousands of excess deaths annually. Pandemics still carry the threat of severe morbidity and mortality affecting millions. Patients with cardiac, respiratory, and other underlying diseases are at particular risk for serious illness. Nevertheless, a maximum of 50% to 65% of U.S. residents who should be vaccinated receive the vaccine -- partly due to fear of injections. An intranasal influenza vaccine, recently FDA approved, may improve immunization rates.

Vaccine Composition: FluMist is a trivalent live attenuated vaccine containing 2 Type A and 1 Type B strains, assessed to be the strains projected to circulate in the coming year. It induces serum antibodies to all 3 strains. The virus is attenuated by multiple passages in eggs and is further constituted by recombination with influenza mutants, so that its growth is enhanced in the relatively cool nasopharynx, but systemic replication is inhibited. The antigenic composition must be changed annually.

Target Population: FluMist is approved for appropriate candidates aged 5-49 years. Because an increase in asthmatic attacks was noted in children <5 years, the vaccine is not approved for this population. Safety and efficacy were not tested in adults older than 49. The vaccine is contraindicated in children or adolescents on aspirin therapy, immunocompromised patients, persons with known egg allergy, and individuals with a history of Guillain-Barré syndrome, asthma, or reactive airways disease.

Immunization Schedule and Efficacy: The vaccine is administered from a single-dose mister containing 0.5 mL. It must remain frozen at or below -15^oC but can be thawed in a refrigerator up to 24 hours before use. Previously unvaccinated children 5-8 years old receive 2 doses 60 days apart; anyone else requires only 1. FluMist was evaluated in 20,228 individuals. In children, efficacy in preventing influenza due to a strain in the vaccine was 87.4% (95% confidence interval, 59.4-97.9) during the first year and 86.9% (95% CI, 70.8-94.1) following revaccination the second year. In adults, there was a 19.5% reduction in severe febrile illness and a 23.7% reduction in febrile upper respiratory infection as compared with placebo.

Adverse Effects: No serious adverse effects were noted in 5- to 49-year-olds. Runny nose/nasal congestion, sore throat, tiredness/weakness, cough, chills, and sinusitis were significantly more common with FluMist than with placebo. Vaccine virus is shed regularly in the nares and can spread to unvaccinated contacts.

Comment: This long-awaited vaccine appears to be effective in preventing influenza in people 5-49 years old and may be slightly more effective than the current parenteral subunit vaccines. Administration is simple, and adverse effects are minimal. However, there are several highly significant limitations to its widespread use: It cannot be given to children younger than 5 years, immunocompromised patients, or aged adults -- all high-risk groups. Current influenza vaccines are safe for these groups but sometimes in short supply; use of FluMist for lower-risk people might increase vaccine availability for high-risk patients. Furthermore, FluMist must be stored in a manual-defrost freezer to avoid intermittent warming to above -15^oC. We're closer, but not yet there, in the development of a perfect vaccine against this potentially deadly virus.

— Stephen G. Baum, MD

Published in Journal Watch Infectious Diseases August 8, 2003

Citation(s):

FluMist package insert, June 2002 (http://www.fda.gov/cber/label/inflmed061703lb.pdf)

First nasal mist flu vaccine approved. FDA News 2003 Jun 17 . (http://www.fda.gov/bbs/topics/NEWS/2003/NEW00913.html).

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