The FDA said the drug cannot be used to prevent flu infection in this age group. The drug is currently approved as both a flu treatment and preventative flu drug for children ages 1 and older, and adults. It aims to help lessen the length and severity of the flu.

Tamiflu was approved in 1999 and is distributed in the United States by Genentech, part of Roche. It was co-developed by Gilead Sciences. Its most common side effects include vomiting and diarrhea.

The FDA said its expanded use is based on extrapolating data from previous study results in adults and older children, and supporting studies by the U.S. National Institutes of Health and Roche.

Tamiflu, which had peak sales of $3 billion in 2009 because of the H1N1 swine flu epidemic, is approved by regulators worldwide but some researchers claim there is little evidence it works and have asked Roche to hand over data so they can study its effectiveness.

(Reporting By Caroline Humer; Editing by Grant McCool)

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