Scottish and U.S. scientists have found a way to prevent flu infections that could, in theory, be used to fight an epidemic long before a vaccine is ready.

The method, successfully tested only in mice thus far, is a nasal spray of engineered proteins that coat the receptors in the nose and throat to which flu viruses attach.

Mice that got the spray as long as a week before being given a lethal dose of viruses from the 2009 pandemic flu were fully protected, according to the team at the University of St. Andrews in Scotland that invented the proteins.

“We think it has potential,” said Dr. Robert Webster, a leading flu expert at St. Jude Children’s Research Hospital in Memphis. If one of these viruses “like H5N1 or H7N9 gets away, you could use it in a family or a community.”

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Webster did some of the testing and was a co-author of the study, published Monday by Proceedings of the National Academy of Sciences.

Lethal bird flus like H5N1, H7N9 and H10N8 have killed some people, mostly in the poultry trade, but they have not mutated to become easily transmissible between humans. If they did, scientists fear, many people would die during the roughly six months that it takes to create, grow, purify and package a vaccine.

As many as 20 million doses of some potential flu vaccines are stockpiled, but they might not turn out to be close matches for a new strain.

The new proteins will need lengthy testing, in ferrets and in small numbers of humans, to be sure they are safe, experts said. Many treatments that work in mice do not in humans.

It could be easy to make large quantities of the new proteins, because they can be grown in E. coli, a common bacteria, according to Garry Taylor, one of the St. Andrews scientists who invented them.

Right now, little can be done to fight a flu epidemic before a vaccine is ready, “so the government is desperate for new antivirals,” said Dr. Arnold Monto, a flu expert at University of Michigan School of Public Health.

Tamiflu and other antivirals are only partly effective and must be given immediately after infection. A new drug, Fludase, is being tested.