A deadly strain of avian flu, known as H7N9, could infect the world, but the Chinese authorities refused applications for laboratory samples from the UK and the US.
Health employers are worried that the UK will struggle to make a vaccine without a sample of the virus.
Professor Ian Jones, an expert in virology at the University of Reading, said: "If the virus is jumping, you want to be ahead with a vaccine.
"If the virus were to jump, it would become a pandemic strain."
H7N9 is supposed to be only a few mutations away from the possibility of spreading from person to person, reported The Telegraph.
At this moment it can only be spread from birds to humans.
This means that a person must come into contact with infected birds in order to become infected with the disease.
China has given no reason for its refusal to supply samples and experts are not sure of the reasons.
It is not known whether the last H7N9 sample that China shared in 2016 is sufficiently up-to-date to make a vaccine against newer forms.
So far there have been 1,625 cases of the H7N9 virus in humans and about 623 people have died.
Most people who develop infections with H7N9 become seriously ill.
Up to 40 percent of cases result in death.
Director of the Worldwide Influenza Center in London, Dr. John McCauley, said that while vaccines can also be made using available gene sequences from H7N9, vaccines are usually developed using the virus itself.
He said: "With the data on the gene sequence you can develop a vaccine – it's good to do, but it's not the way vaccines are currently being made and regulated.
"There is a better sense of trust with the actual virus in hand."
The infectious disease specialist at Harvard Medical School, Dr. Michael Callahan, told the New York Times: "Disrupting US access to foreign pathogens and therapies to combat them undermines our nation's ability to protect itself against infections that can spread globally within days. "
H7N9 has been around since 2013.
Experts believe that it could be as deadly as the Spanish flu from 1918 that killed up to 100 million people.