A plant where doses of the Oxford-AstraZeneca vaccine are produced and stored has been evacuated after receiving a suspicious package.
North Wales Police said officers were at Wrexham Industrial Estate in Ash Road, Holt.
Vaccine producer Wockhardt UK said: “Upon expert advice we have partially evacuated the site.”
Police said a cordon had been put in place near the Wockhardt plant and the public have been asked to keep away.
There are no reports of any injuries.
The BBC understands that the bomb disposal unit is on site.
In a statement Wockhardt UK said: “Wockhardt UK in Wrexham this morning received a suspicious package to site.
“All relevant authorities were immediately notified and engaged.
“Upon expert advice we have partially evacuated the site pending a full investigation.
“The safety of our employees and business continuity remain of paramount importance.”
The Wrexham plant has the capability to produce around 300 million doses of the vaccine a year.
John Roberts, who runs CMS Wrexham Ltd, next door to the plant, said he heard a “big bang” at about 11:35 GMT.
“We’re next door to Wockhardt. Three of us were talking then we heard a hell of an explosion or a bang,” he said.
“I went outside, couldn’t see anything. I looked the other side and two blokes were on the roof.
“The next thing the police had blocked off the road and were looking in the bushes.”
His son Mark Roberts said: “The police just closed the road off and we’ve heard there’s a bomb disposal unit.
“They’ve been here about an hour or so – we’re on tenterhooks.
“Boris Johnson toured the factory around December time, so I wonder if that’s raised the profile, as it’s where they make the Oxford vaccine.”
Visiting the plant in November, Prime Minister Boris Johnson it could provide “salvation for humanity”.
Wockhardt UK entered an agreement in August to help prepare the vaccine for distribution.
When the company’s contract was announced, Ravi Limaye, managing director, said: “We are immensely proud to have been selected to partner with the UK government on this project.
“We have a sophisticated sterile manufacturing facility and a highly skilled workforce.”
The Welsh Government said there had been “no adverse effects” on the coronavirus vaccine roll-out.