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.