The bomb squad is at the Wrexham factory making the Oxford/AstraZeneca vaccine after a suspicious package was received.
Wockhardt UK in Wrexham is carrying out the "fill and finish" stage of the manufacturing process and is set to produce around 300 million doses of the vaccine each year.
This morning a suspicious package was received - triggering a partial evacuation.
A spokesman 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."
North Wales Police has confirmed it is dealing with an “ongoing incident” on Wrexham Industrial Estate.
A spokesman said: “The roads are currently closed and we would ask the public to avoid the area until further notice.”
First Minister Mark Drakeford said that they were working to ensure the safety of those on the site and the vaccines.
"We are working with local police and the military to find out more about this incident," he said.
"Thank you to the security personnel who are on-site to protect lives and ensure the safety of our vaccine supply.
"This highlights the vital role they play in keeping us all safe."
