Late Wednesday, a federal expert committee on immunization voted against using the nasal spray after preliminary data from the CDC presented to the committee showed that FluMist, made by MedImmune, was only 3 percent effective among children ages 2 through 17 during the 2015-2016 season.

“This 3 percent estimate means no protective benefit could be measured,” the CDC said. The nasal spray also showed poor effectiveness during the two previous seasons, the CDC said.

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The reason for the nasal spray’s “recent poor performance” is not known, the CDC said.

By comparison, flu shots were 63 percent effective for the same age group. The CDC has recommended annual flu shots for everyone ages 6 months and older for the last six years. During the 2014-2015 season, federal health officials had recommended the nasal spray vaccine for young children.

AstraZeneca, which owns MedImmune, said its research and that of others contradict the CDC’s effectiveness data. In a statement, it said the other research findings showed FluMist to be 46 to 58 percent effective overall against the circulating flu strains last year.

The company said it is working with the CDC to better understand the agency’s data “to help ensure eligible patients continue to receive the vaccine in future seasons in the U.S.”

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How well a flu vaccine works can range widely from season to season and can be affected by several factors, including the characteristics of the person getting vaccinated, the similarity between vaccine viruses and circulating viruses, and which vaccine is used. Flu vaccines have to be made every year.

The nasal spray vaccine is made from a live but weakened flu virus; shots are made from a killed virus. Vaccines containing live viruses can cause a stronger immune response than vaccines with inactivated virus.

Vaccine manufacturers had projected that as many as 171 million to 176 million doses of flu vaccine, in all forms, would be available for the United States during the coming flu season. The makers of the nasal spray had projected a supply of as many as 14 million doses, or about 8 percent of the total projected supply. FluMist, which was first licensed in 2003, is the only non-injection-based flu vaccine on the market.

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The panel's recommendation must still be reviewed and approved by the CDC’s director before it becomes policy. The final recommendations on the prevention and control of influenza with vaccines will be published in late summer or early fall.

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