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n expert panel that advises the US government on vaccines said on Wednesday that the nasal mist influenza vaccine used by millions should not be administered this coming flu season.

The vaccine, FluMist, has not worked in recent years, the panel was told by scientists from the Centers for Disease Control and Prevention. The expert panel reviewed numerous studies that found no evidence the vaccine protected people who had the FluMist puffed up their nostrils over the past three flu seasons.

The panel, the Advisory Committee on Immunization Practices, wants MedImmune, the manufacturer of the vaccine, to identify the problem with the vaccine and fix it. MedImmune is a division of pharmaceutical giant AstraZeneca.

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Dr. Joseph Bresee, chief of epidemiology for the CDC’s influenza division, said the problems with FluMist have perplexed CDC scientists and the company, and both are working to try to learn more.

“We don’t really know why we’re observing the low vaccine effectiveness that we are observing,” Bresee told STAT.


MedImmune was involved in the discussions that led to Wednesday’s vote, but had little to say in its immediate aftermath.

The company presented data from its own research that contest what the CDC and others have reported. But in a statement, AstraZeneca said it would work with CDC “to better understand its data to help ensure eligible patients continue to receive the vaccine in future seasons in the US.”

More than 90 million doses of FluMist have been distributed over the past 14 years, according to the company’s website. It is particularly popular with children because it can be given without needles.

It is the only noninjected flu vaccine licensed in the United States.

The American Academy of Pediatrics supported the decision, calling the science behind it compelling.

“We do understand this change will be difficult for pediatric practices who were planning to give the intranasal spray to their patients, and to patients who prefer that route of administration,” Dr. Karen Remley, the group’s CEO, said in a statement.

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“The AAP will be working with CDC and vaccine manufacturers to make sure pediatricians and families have access to appropriate vaccines, and to help pediatricians who have already ordered intranasal vaccines,” she added.

Medimmune had been expecting to provide 14 million doses of the vaccine this fall — about 8 percent of the nation’s flu vaccine supply.

First licensed in the United States in 2003, the vaccine appeared to be an attractive and effective alternative to injectable flu vaccine. It is licensed for use in people from the ages of 2 to 49.

FluMist is made with live but attenuated — or weakened — flu viruses. When puffed into a nostril, they essentially trigger an infection. But because the vaccine viruses are attenuated, the immune system quells the invaders before a person can become sick.

Injectable flu vaccine is made with killed flu viruses.

Studies have suggested FluMist works better in children than adults. Experts have theorized that with years of exposure to influenza viruses, adult immune systems may shut down the vaccine-induced infection before good levels of antibodies have been generated.

But the vaccine was always thought to be highly effective in children. So the evidence that it has not been working in that population for the past three years has been a puzzle.

Bresee said there are a bunch of theories being explored. One relates to the fact that a few years ago, MedImmune switched the composition of the vaccine.

It used to be a trivalent, meaning it protected against three types of flu viruses. But there has been a move in the flu vaccine market to add a fourth virus, making vaccines quadrivalent.

FluMist’s performance problems track with that switch, though it could be a coincidence.

“The quadrivalent has only been out for the past three years, and in that time we haven’t been able to measure [the] effectiveness of this vaccine,” Bresee said.

Another theory is that if children are getting vaccinated year after year, they build up antibodies that are shutting down the vaccine viruses before they can induce a good immune response.

Alex Hogan/STAT The vaccine triggers the immune system to create antibodies, preparing the body for viral battle.

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