Germany BioNTech said on Thursday it was not in talks with Thonburi Healthcare Group Plc about a sale of its vaccines.
"We are not in negotiations with the company you mentioned below about vaccine supply," BioNTech said in an emailed statement to Reuters.
Earlier on Thursday, Thonburi Healthcare Group Plc said it would sign a private order for 20 million doses of the Pfizer-BioNTech coronavirus vaccine, news of which saw the hospitals operator's share price leap over 13%.
Company chairman Boon Vanasin told Reuters a deal would be signed on Thursday, with BioNTech, the German firm that jointly developed the mRNA vaccine with Pfizer.
Neither Pfizer nor BioNTech had confirmed the deal.
Shares of the hospital group rose as much 13.45% at 12.40pm with the benchmark index up 0.62% on earlier reports of the plans.
Shares of THG.BK were still up 12.61% at 3.15pm, shortly after BioNTech's denial.
mRNA in demand
The country is in the early stages of its mass Covid-19 vaccinations programme and is struggling to suppress its most severe wave of infections.
The government has relied heavily on the AstraZeneca and Sinovac vaccines but is seeing increased appetite for mRNA type vaccines, after concern about efficacy. The majority of the mRNA vaccines ordered will not arrive until the final quarter.
Mr Boon said a licensed importer would make an announcement of the deal. "I want it to conclude this week or next," he said.
Mr Boon, who has been a vocal critic of the government's vaccine policy, said he had asked for a delivery of five million doses this month.
The state drugmaker, the Government Pharmaceutical Organization, on Wednesday said it had filed a defamation suit against Mr Boon over his criticism of its role in the procurement of Moderna vaccines.