• The Data Safety Monitoring Board "expressed concern that AstraZeneca may have included outdated information from that trial," the U.S. National Institute of Allergy and Infectious Diseases said.
• The NIAID is led by White House chief medical advisor Dr. Anthony Fauci and is part of the National Institutes of Health.
• Shares of AstraZeneca slipped nearly 1% during Tuesday trading in London and were down 2% in premarket trading in New York.

LONDON — A U.S. health agency said Tuesday that AstraZeneca may have included outdated information in trial results of its Covid-19 vaccine, potentially casting doubt over published efficacy rates.

The announcement came just one day after the findings of a large U.S. trial showed that the vaccine was safe and highly effective and throws into question whether AstraZeneca can seek U.S. clearance for the vaccine next month as planned.

The Data Safety Monitoring Board "expressed concern that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data," the U.S. National Institute of Allergy and Infectious Diseases said in a statement.

"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 NIAID said it was notified of the concerns late Monday, along with AstraZeneca and the Biomedical Advanced Research and Development Authority. Led by White House chief medical advisor Dr. Anthony Fauci, the NIAID is part of the National Institutes of Health.

Fauci told ABC News' "Good Morning America" on Tuesday that the Oxford-AstraZeneca shot is "likely a very good vaccine." However, he added, the DSMB became concerned that the data in AstraZeneca's public statement "were somewhat outdated and might, in fact, be misleading a bit."

In response, AstraZeneca said that the figures published Monday "were based on a pre-specified interim analysis with a data cut-off of 17 February."

"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," the company said in a statement.

Shares of AstraZeneca slipped nearly 1% during Tuesday trading in London and were down 2% in premarket trading in New York.

U.S. trial results showed that the coronavirus vaccine developed by AstraZeneca and the University of Oxford is 79% effective in preventing symptomatic illness and 100% effective against severe disease and hospitalization.

The findings were welcomed as "surprisingly positive" and "good news for the global community." It was thought the trial data could help to bolster public confidence after a flurry of countries had temporarily suspended their use of the vaccine amid safety concerns.

U.S. criticism

AstraZeneca said it planned to prepare for the primary analysis to be submitted to the U.S. Food and Drug Administration for emergency use authorization before mid-April.

Data from the late-stage human trial study was based on more than 32,000 volunteers across 88 trial centers in the U.S., Peru and Chile.

"It is not unknown for a DSMB to disagree with investigators over interpretation of trial results," said Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.

"It is usually done in private, so this is unprecedented in my opinion," he added. "It does not leave me concerned particularly unless they had found a safety issue that was being hidden, which does not appear to be the case."

The rollout of the Oxford-AstraZeneca vaccine had been halted in several countries after reports of blood clots in some vaccinated people. Health experts sharply criticized the precautionary measure, citing a lack of data, while analysts expressed concern about the impact on vaccine uptake as the virus continues to spread.

Germany, France, Italy and Spain are among those to have resumed use of the shot after Europe's drug regulator said its initial investigation of possible side effects concluded the shot is safe and effective, adding the benefits of administering the vaccine still outweigh the risks.

AstraZeneca said Monday that the independent DSMB had found no increased risk of blood clots.

Ruud Dobber, executive vice president of AstraZeneca's biopharmaceuticals business unit, told CNBC's "Squawk Box" on Monday that it was "very pleasing to see that even with a magnifying glass the Data Safety Monitoring Board didn't see any imbalance between the vaccinated group and the placebo group."

"So, that gives us a lot of confidence," he added.

Late last year, AstraZeneca faced criticism from U.S. health experts over the results and methods used in its phase 3 vaccine trials.

Analysts at U.S.-based health care and biotech investment bank SVB Leerink said at the time that they believed the vaccine would "never be licensed in the U.S."

AstraZeneca pushed back against the criticism, saying the studies "were conducted to the highest standards" and that more data would follow.