To protect people against the next flu pandemic, scientists need to know what flu strains are circulating and how they are changing. But such efforts can be stymied if countries don’t share flu samples, and now, the Chinese government appears to be withholding samples of the dangerous bird flu virus H7N9 from the United States, according to news reports.

For more than a year, China has not provided samples of H7N9 to the United States, despite persistent requests from officials and research institutions, according to The New York Times.

Experts say samples of H7N9 are needed to develop vaccines against the virus and treatments for it.