An AstraZeneca plant in Wales was evacuated and the bomb disposal unit called after a "suspicious package" was found.
The industrial estate where the Oxford University/AstraZeneca vaccine is put into vials and syringes was partially evacuated while the package was investigated.
Wockhardt UK, the company that operates the site, said in a statement: "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 said a bomb squad was at the site and advised the public to avoid the area.
"We are currently dealing with an ongoing incident on the Wrexham Industrial Estate."
Wales' First Minister Mark Drakeford said the government was investigating with local police and the military.
"We are working with local police and the military to find out more about this incident.
"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."
Wrexham MP Sarah Atherton said she had spoken with Wockhardt UK and was "assured that they are doing everything to keep their employees and the businesses around them safe".
Meanwhile AstraZeneca is locked in an escalating feud with the European Union over coronavirus vaccine shortages.
The European commission claimed the British-Swedish pharmaceutical company had pulled out of a meeting over the supply breakdown, only for an AstraZeneca spokesman to say this was not the case and the firm would attend the meeting.
AstraZeneca has warned the EU of a possible 60 per cent shortfall in supply after lower than expected yields at its Belgium plant.
New Zealand has an agreement to purchase 7.6 million doses from AstraZeneca.