Last checked at 
(Adds details from CDC, background, byline, dateline)
By Ben Hirschler and Julie Steenhuysen
LONDON/CHICAGO, June 23 (Reuters) - U.S. health officialshave advised doctors not to use AstraZeneca's FluMist inthe upcoming flu season based on three years of U.S. datashowing that the nasal spray vaccine is not effective atpreventing influenza.
The decision, announced late Wednesday, was based on areview by the Advisory Committee on Immunization Practices - apanel of experts that advises the U.S. Centers for DiseaseControl and Prevention - showing the vaccine did not offeradequate protection.
As a result of the move, the CDC said it will be workingwith manufacturers throughout the summer to ensure there isenough alternative vaccine supply.
FluMist Quadrivalent is currently the only licensed fluvaccine that does not require a shot, making it a favored choiceby parents of young children.
In the past year, among children aged 2 to 17, FluMist wasonly 3 percent effective, meaning it offered "no protectivebenefit," the CDC said. That compared with conventional flushots, which were 63 percent effective against any flu virusamong children in this age group.
AstraZeneca said it would take an $80 million writedown onstocks of its Flumist Quadrivalent vaccine as a result of thedecision.
It said the CDC data contrasted with its own studies as wellas preliminary independent findings by public health authoritiesin other countries suggesting the vaccine was 46 to 58 percenteffective overall against flu strains during the 2015-2016season.
U.S. sales of FluMist in 2015 totaled $206 million, or justunder 1 percent of group revenue.
The CDC said FluMist made up about 8 percent of the totalprojected supply of 176 million doses of flu vaccine for theupcoming flu season.
"AstraZeneca is working with the CDC to better understandits data to help ensure eligible patients continue to receivethe vaccine in future seasons in the U.S.," the company said ina statement on Thursday.
"The distribution and use of the vaccine in other countriesare progressing as planned for the forthcoming influenza season,pending the annual release process from relevant regulatoryauthorities."
Despite the setback, AstraZeneca said it maintained its 2016financial outlook of a low-to-mid single digit decline inrevenue and core earnings, at constant exchange rates.
However, Deutsche Bank analyst Richard Parkes said the U.S.vaccine problem would likely drag down consensus earningsforecasts for the current year by around 2 percent and there wasa 1-2 percent risk to future forecasts, if the issue was notresolved.
Shares in AstraZeneca gained 0.3 percent while the Europeandrugs sector rose 0.4 percent.
(Reporting by Ben Hirschler and Julie Steenhuysen; Editing bySusan Fenton and Marguerita Choy)
