Regulators have delayed approval of an H5N1 bird flu vaccine from GlaxoSmithKline, designed to be used in a pandemic.
A spokesman for Britain's biggest drugmaker said the delay was not related to recent controversy over links between a similar flu vaccine made by the company and narcolepsy.
Rather, the U.S. Food and Drug Administration (FDA) decided it needed more time to assess the product "due to an administrative matter that has recently been rectified", GSK said in a statement on Monday.
"GSK and the FDA are actively working together to complete the review in a timely manner," it added.
There is growing evidence of a link between GSK's earlier H1N1 flu vaccine, Pandemrix, and an increase in narcolepsy cases among children who received it in Europe - a fact which has raised questions as to whether the FDA should approve the similar H5N1 product.
Both vaccines contain AS03, a new adjuvant, or booster, that turbo-charges the body's immune response to a vaccine.
A 14-member panel of advisers to the FDA voted unanimously in November to recommend the H5N1 vaccine to protect against bird flu. The panel considered early studies from Europe showing an increase in the number of narcolepsy cases but concluded that the potential benefit of the vaccine outweighed the risk.
Since then, however, new data, including study results from Britain, suggest the scale and strength of the narcolepsy link to Pandemrix during the 2009-10 H1N1 swine flu pandemic could be greater than first thought.
Officials at the FDA were not immediately available to comment on the case.