Slovakia’s drug regulator has said that the Russian Sputnik V doses it received differ from the same vaccine reviewed by the EU’s drug overseer and the U.K.-based medical journal The Lancet.

The State Institute for Drug Control (SUKL) did not explain how the mix-up might have occurred.

“Batches of [Sputnik V] vaccine used in preclinical tests and clinical studies published in The Lancet journal do not have the same characteristics and properties as batches of vaccine imported to Slovakia,” the SUKL said a statement.

The SUKL had already said the day before that lingering questions about the efficacy and risks of the Russian vaccine due to inadequate data from the producer were preventing use of the doses.

EU member Slovakia received 200,000 batches of Sputnik V, which has still not been cleared by the European Medicines Agency (EMA), last month.

The Russian Health Ministry, which supervises the Gamaleya Institute where Sputnik V was developed, did not immediately respond to a Reuters request for comment.

“On March 30, 2021, the agency officially sent an evaluation report to the Health Ministry, in which it stated that it was not possible to make a conclusion on the balance of benefits and risks of the Sputnik V vaccine,” Reuters quoted SUKL as saying on April 7.

The regulator cited “an amount of missing data from the producer, inconsistency of dosage forms, and [the] impossibility of mutually comparing batches used in various studies and countries.”

A news report on April 7 also suggested that European regulators were concerned about possible ethical problems during clinical trials by the Russian developers of Sputnik V.

As its inspection continues, reports say the EMA’s approval will also be contingent on “good clinical practice” (GCP) standards.

Russian backers of the vaccine, which was registered to great Kremlin fanfare last year despite concerns about underlying data and unfinished clinical trials, have denied the problems should stand in the way of safe rollout.

Slovakia’s prime minister, Igor Matovic, was forced to resign last week under a cloud of questions about his administration’s order for 2 million doses of Sputnik V.

In the neighboring Czech Republic, Prime Minister Andrej Babis on April 7 announced the dismissal of a health minister who was reportedly resisting pressure — including public complaints by President Milos Zeman — to order Sputnik V.

Sputnik V is already being used in EU member Hungary and other countries around the world.

The German state of Bavaria recently announced an agreement to buy 2.5 million doses of Sputnik V pending approval by European regulators.

With reporting by AFP and Reuters