The firm’s Italian Unit said in an email that the shipment is being held while it awaits export permission and describes the checks as “routine”.
The freight of meningitis vaccine is said to have drawn suspicions due to some unlabelled vials.
The global firm, which has its headquarters based in Brentford, currently does not produce any of the approved COVID-19 jabs.
But earlier today, Boris Johnson confirmed its North East facility would help to manufacture the new Novavax vaccine once it’s given the green-light by the health authorities.
The incident in Italy comes amid growing tensions between the UK and the European Union over the transportation and supply of vaccines.
Brussels has threatened to halt exports of COVID-19 jabs to the UK in the wake of a shortfall in the number of vaccines for the EU27.
Authorities in Italy have been on the lookout for unauthorised shipments of COVID-19 products in recent days.
Last week, officials seized 29 million doses of AstraZeneca vaccine from a plant in the town of Anagni.
Talks between the UK and EU remain ongoing in order to find a solution after officials in Brussels grew frustrated at vaccines being shipped from the continent to the UK.
Britain continues to race ahead in its inoculation programme and has given first doses to more than 30 million people.
Brussels is reportedly seeking to cut a deal in order to receive a slice of vaccines destined for Britain which are being produced at a pharmaceutical plant in The Netherlands.
Downing Street said discussions with the EU were ongoing and that “openness and global co-operation” were important in tackling the pandemic.
A spokesman for the Prime Minister added the UK does not currently have a surplus of vaccines and the British people would take priority.
He said: “Our first priority is to protect the British public.
“The vaccine rollout is continuing to that end. We don’t currently have a surplus of vaccines but we will consider how they are best allocated as they become available.”
At the European Commission’s daily press briefing, a spokeswoman refused to be drawn on the specifics of meetings, but described the discussions with the UK as covering a “broad range of matters”.
At a Downing Street briefing, Mr Johnson confirmed the UK had reached a deal with GlaxoSmithKline to provide “fill and finish” capacity for the Novavax vaccine.
The UK has reached an agreement for between 50-60 million doses, subject to approval by the Medicines and Healthcare products Regulatory Agency (MHRA).
According to results of a phase three trial in the UK, the coronavirus jab offers 100 percent protection against severe disease, including all hospital admission and death.
The firm said a study of more than 15,000 people found the jab was 86 percent effective against the Kent variant and 96 percent effective in preventing cases caused by the original strain of the coronavirus.