US health experts are alarmed and outraged that the Chinese government appears to be withholding samples of the deadly, rapidly evolving bird flu virus, H7N9, from US research labs, according to a report by The New York Times.

The samples are critical for studying the virus and developing life-saving treatments and vaccines in preparation for potential outbreaks or pandemics. Usually, countries share viral samples “in a timely manner” without any fanfare under an agreement established by the World Health Organization to address such potential flu threats. That usually means a matter of months.

But according to the Times, China has failed to share the samples for more than a year, despite persistent requests from government officials and researchers, including those at the Centers for Disease Control and Prevention. Moreover, scientists and experts worry that, as the US and China continue to butt heads on trade agreements, the issue of sharing biological samples and other medical-related materials could worsen.

“Jeopardizing US access to foreign pathogens and therapies to counter them undermines our nation’s ability to protect against infections which can spread globally within days,” Michael Callahan, an infectious-disease specialist at Harvard Medical School, told the Times.

Likewise, Andrew Weber, who oversaw biological defense programs at the Pentagon during the Obama administration, told the paper:

Countries don’t own their viral samples any more than they own the birds in their skies. Given that this flu virus is a potential threat to humanity, not sharing it immediately with the global network of WHO laboratories like CDC is scandalous. Many could die needlessly if China denies international access to samples.

H7N9 is a type of bird flu that emerged in China in 2013 and has since sparked a string of sporadic epidemics annually in Asia. Its official case count totals 1,565 people in that time, with 766 reportedly infected in an outbreak that lasted from October 2016 to September 2017. Luckily, in the outbreaks so far, the virus hasn’t seemed to spread readily from person to person, limiting its reach. The people who fall ill from the virus tend to report recent exposure to poultry markets or places contaminated by birds.

But, when people do fall ill with the virus, the infection tends to be severe. In past outbreaks, about 39 percent of those sickened died. And, like many flu viruses, it has the potential to mutate rapidly.

That combination of rapid mutation and deadliness is what has experts worried about its potential to ignite a global pandemic.