The European Medicines Agency (EMA) says it has been hit by a cyber-attack, APA reports citing BBC.
The agency is currently working on approval of two Covid-19 vaccines, which it expects to conclude within weeks.
The EMA did not say what the nature of the cyber-attack was, if it was successful, or if it was linked to the vaccine approval process.
There has been a string of warnings about hacking threats against vaccine-makers and public health bodies.
The agency authorises the use of medicines across the European Union.
It is trying to decide if the Pfizer/BioNTech jab - which has just begun being rolled out in the UK - and another made by Moderna, are safe for use in EU countries.
"EMA has been the subject of a cyber-attack," it said in a brief statement on its website.
"The Agency has swiftly launched a full investigation, in close co-operation with law enforcement and other relevant entities."
But it declined to provide any further details while the investigation is being carried out.
A spokesperson for the agency said it was still "functional".
The use of cyber-attacks against bodies involved in the vaccine rollout has been a feature of recent months.
Security services warned in the summer that Russian intelligence had been targeting organisations attempting to develop a successful vaccine.
In October, one pharmaceutical company based in India was the victim of a significant cyber-attack.
And in recent days, IBM said that the cold storage supply chain used to transport viable vaccines, had come under cyber-attack - probably by a nation state.
The EMA has not yet said there is any link between the cyber-attack and its vaccine work.
But it comes the day before the agency is due to update MEPs on the progress of the vaccine assessments.
Members of the EU's Public Health Committee are due to quiz the agency's executive director "on how close the most advanced vaccines are to receiving authorisation", on Thursday.
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