For over a year, the Chinese government has withheld lab samples of a rapidly evolving influenza virus from the United States — specimens needed to develop vaccines and treatments, according to federal health officials.

Despite persistent requests from government officials and research institutions, China has not provided samples of the dangerous virus, a type of bird flu called H7N9. In the past, such exchanges have been mostly routine under rules established by the World Health Organization.

Now, as the United States and China spar over trade, some scientists worry that the vital exchange of medical supplies and information could slow, hampering preparedness for the next biological threat.

The scenario is "unlike shortages in aluminum and soybeans," said Dr. Michael Callahan, an infectious disease specialist at Harvard Medical School.

"Jeopardizing U.S. access to foreign pathogens and therapies to counter them undermines our nation's ability to protect against infections which can spread globally within days."

Experts concur that the world's next global pandemic will likely come from a repeat offender: the flu. The H7N9 virus is one candidate.

Since taking root in China in 2013, the virus has spread through poultry farms, evolving into a highly pathogenic strain that can infect humans. It has killed 40 percent of its victims.

If this strain were to become highly contagious among humans, seasonal flu vaccines would provide little to no protection. Americans have virtually no immunity.

"Pandemic influenza spreads faster than anything else," said Rick A. Bright, the director of Biomedical Advanced Research and Development Authority, an agency within the Department of Health and Human Services that oversees vaccine development. "There's nothing to hold it back or slow it down. Every minute counts."