A U.S. health agency on Tuesday said that AstraZeneca may have included outdated information in trial results of its Covid-19 vaccine, potentially casting doubt over published efficacy rates.

It comes 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The Data Safety Monitoring Board “expressed concern that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data,” the U.S. National Institute of Allergy and Infectious Diseases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.

On Monday, 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.

AstraZeneca did not immediately respond to a CNBC request for comment.

Shares of AstraZeneca slipped more than 1% during Tuesday trading in London.

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.

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 Saftey 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.

AstraZeneca vaccine found to be 79% effective in U.S. trial
Late last year, AstraZeneca faced criticism from U.S. health experts over the results and methods used in their 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.

Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said on Monday that U.S. health regulatory authorities “are reluctant, even in a pandemic, to rely totally on data obtained outside the U.S.”

Evans said the AstraZeneca-led trial data was published “to provide convincing evidence of efficacy and safety in a sufficiently large number of U.S. patients.”

“One hopes that any political statements around this good news are avoided,” he added.