Results from AstraZeneca's recent COVID-19 vaccine trial "may have included outdated information" that "provided an incomplete view of the efficacy data," a federal health institute said early Tuesday.
The unusual statement, issued just after midnight by the National Institute of Allergy and Infectious Diseases, came hours after AstraZeneca announced encouraging results from its Phase 3 trial based in the United States.
Interim analysis of the trial showed the vaccine made by AstraZeneca and Oxford University was safe and 100 percent effective in stopping severe and fatal cases of COVID-19, AstraZeneca said. After months of questions about the shot's safety and efficacy, these results were seen as a boost and the first step toward it being approved in the U.S., perhaps as early as April.
But later Monday, the NIAID said it was notified about a "concern" relating to the data included in this trial by the data and safety monitoring board, a panel of independent experts that reviews safety and efficacy data for vaccines in the United States.
"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," it said, referring to AstraZeneca and using the acronym for the data and safety monitoring board.
NBC News has reached out to AstraZeneca for comment.
The NIAID did not provide any more information on how the outdated and incomplete data might affect the trial results for this vaccine, which has been taken by tens of millions of people around the world and approved or given emergency approval in more than 70 countries. The agency did not suggest this shot is anything other than safe and effective against COVID-19.
Dr. Anthony Fauci, director of the NIAID and President Joe Biden's chief medical advisor, said it was "really what you call an unforced error because the fact is this is very likely a very good vaccine," speaking on ABC's "Good Morning America" on Tuesday. "This kind of thing does, as you say, do nothing but really cast some doubt about the vaccines and maybe contribute to the hesitancy."
The situation is "a total head scratcher," tweeted Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children’s Hospital and dean of the National School of Tropical Medicine at Baylor College of Medicine.
Like other experts he noted that it was rare and perhaps even unprecedented to see such an open disagreement between a drugmaker the country hosting and funding the clinical trial.
Though the U.S. is now vaccinating rapidly using vaccines made by Pfizer, Moderna and Johnson & Johnson, the results released Monday had provided a wider reassurance about the AstraZeneca shot.
But instead these concerns will come as yet another setback for this vaccine, which despite positive results in clinical trials and real-world studies has faced a tough experience in terms of public relations at least.
Some European countries paused their rollouts after reports there had been extremely rare cases of blood clots among people who had taken it. They resumed vaccinations days later after the European Medicines Agency, the continents' top regulator, said there was no proven link between the vaccines and the clots.
But experts said the damage may have already been done: withholding a life-saving vaccine during a resurgent wave of the virus in Europe, while damaging public confidence in it.
European countries had previously said there was not enough data to approve the vaccine for people over the age of 65, before reversing that call too after studies suggested it was in fact very effective.
This article was originally published on NBCNews.com.