Fraudsters have offered governments across the EU a total of about 1 billion non-existent Covid-19 vaccine doses for €14 billion ($16.87bn), the bloc's anti-fraud agency said on Monday.
Middlemen are asking for payment in advance and giving no delivery details, said Ville Itala, head of the European anti-fraud office.
Mr Itala said pharmaceutical companies insist that they sell directly to governments.
"It's quite massive," he said. "They send an offer, which is not a real one. In fact, there is as such no vaccines attached to the offer from what we have seen so far."
The scams the agency was investigating came from professional criminals and individual opportunists in a long chain of intermediaries often leading outside the EU, Mr Itala said.
In Italy, authorities said they were investigating an offer for Covid-19 vaccines made by unidentified intermediaries to Veneto regional authorities amid warnings of growing vaccine fraud.
Mr Itala said the agency was not yet aware of counterfeit vaccines being pushed on the EU market but that was bound to come.
"Where there is a lot of money, the fraudsters find their way there," he said.
"There can be financial consequences but there is also another aspect – trust of the people.
"That's why it's important now to prevent this, so that people can trust that if they go to take the vaccine, they are the real ones."
The agency was working with EU governments, law enforcement agencies and pharmaceutical companies to bring criminals to justice.
It said it was not aware of an EU government falling for such scams.
Increasing fraud adds to problems in the EU's botched inoculation campaign, which has been marred by cuts in promised deliveries, introduction delays and some social resistance, especially to the AstraZeneca shot.
The bloc has so far administered vaccines to only about 5 per cent of its adult population – way behind Britain, Israel, the UAE and the US – compared to its target of 70 per cent by the end of summer.