The reliability of early-stage trial results of Russia’s COVID-19 vaccine, Sputnik-V, are being called into question in an open letter penned by a team of 26 scientists. These scientists are saying they observe patterns in the study data that look “highly unlikely,” including reports that several participants receiving the experimental vaccine in the study had identical antibody levels.

The scientists, most of whom are from universities in Italy, addressed their concerns to the editor of The Lancet, the journal that published Russia’s early-stage trial results of its vaccine. The concerns were based on the published summary of the trial data rather than the original data itself.

Moscow’s Gamaleya Institute, which developed Sputnik-V, has publicly rejected the criticism, saying “the published results are authentic and accurate” and were assessed by five Lancet reviewers. The institute rebukes the open letter by noting it had submitted the raw trial data to the journal in their entirety.

This isn’t the first time Sputnik-V has drawn criticism from global scientists. In early August, the vaccine candidate was approved by Russian health regulators before large-scale phase 3 trials of the vaccine had been conducted. Since then, the Gamaleya Institute has launched a 40,000-participant phase 3 trial of the vaccine. So far, approximately 31,000 patients have enrolled in that trial.