The Data Safety Monitoring Board (DSMB), a division of the National Institutes of Health that oversees new drug trials -- but is not involved in their approval -- said late Monday that it was concerned that AstraZeneca had provided and 'incomplete view" of the trial's efficacy data.
"We urge the company to work with the DSMB to review the efficacy data and ensure the most accurate, up-to-date efficacy data be made public as quickly as possible," the DSMB said in a statement published on the NIH website.
Dr. Anthony Fauci, who leads the NIH's National Institute of Allergy and Infectious Diseases, told ABC's Good Morning America program Tuesday that the AstraZeneca vaccine is "very likely a very good vaccine" but added that incomplete data "do nothing but really cast some doubt about the vaccines and maybe contribute to the hesitancy."
AstraZeneca said its numbers were based on a pre-specified interim analysis with a data cut-off of 17 February.
"We have reviewed the preliminary assessment of the primary analysis and the results were consistent with the interim analysis. We are now completing the validation of the statistical analysis," the drugmaker said in a statement.
"We will immediately engage with the independent data safety monitoring board (DSMB) to share our primary analysis with the most up to date efficacy data. We intend to issue results of the primary analysis within 48 hours."
AstraZeneca's U.S.-listed shares were marked 3.1% lower in early trading Tuesday to change hands at $49.60 each, a move that would extend the stock's six month decline to around 12%.
AstraZeneca had said on Monday that its newly-developed coronavirus vaccine scored a 79% efficacy rate in a large, late-stage north American trial, adding it would seek Emergency Use Authorization from the U.S. Food & Drug Administration over the coming weeks.
AstraZeneca said the phase 3 study of nearly 32,500 patients accrued just 141 symptomatic cases of COVID-19 in a 2:1 randomisation of vaccine to placebo set-up, with two doses administered over a four-week interval in the United States, Chile and Peru.
The headline 79% efficacy rate was supported by data showing "no increased risk of thrombosis or events characterized by thrombosis among the 21,583 participants receiving at least one dose of the vaccine", countering one of the key concerns in Europe, where reports of blood clot risks had resulted in the temporary suspension of its use in certain countries.