An international group of scientists has reiterated "serious concerns" over what they call a lack of transparency, missing data and questionable findings in the clinical research behind Russia's homemade Sputnik V vaccine. [...]

Brazil's regulator last month rejected the vaccine after reviewing documentation which said the jab carried a live version of adenovirus, a common cold-causing virus.

In this week's letter to The Lancet, the group pointed to "very peculiar" and "highly coincidental results" in terms of the jab's efficacy across different age cohorts, noting limited variance of between just 90% and 92.7% across the five age bands studied.

In a response published simultaneously in The Lancet, Gamaleya defended the efficacy findings, stating: "The homogeneity of the values only confirms the fact that ... the effectiveness of the vaccine does not differ between age groups."

The letter was authored by nine scientists and academics from institutions in Russia, the U.S., France, Italy and the Netherlands. The group -- led by Temple University's Enrico Bucci, who has raised concerns about the research underpinning Sputnik V on multiple occasions -- also criticized Russia for ignoring requests to share the study's underlying data.

"We have a serious concern regarding the availability of the data from which the investigators draw their conclusions," they wrote. "We have made multiple independent requests for access to the raw dataset, but these were never answered. Data sharing is one of the cornerstones of research integrity. It should not be conditional."