How to Make Sense of the AstraZeneca Vaccine Data Drama

U.S. health officials raise concerns about the company’s latest efficacy data

Emily Mullin
Medium Coronavirus Blog
3 min readMar 23, 2021


Photo: Svetlana Ustinov/Getty

In a highly unusual move, a U.S. health agency raised concerns on Tuesday that AstraZeneca had not provided the most current data about its Covid-19 vaccine in a press release it issued Monday.

In the release, which garnered excitement from public health experts, the company said its vaccine was 79% effective at preventing Covid-19 symptoms and 100% effective at preventing hospitalization and death in a large trial of more than 32,000 participants in the United States. Five people enrolled in the trial developed severe illness, all in the placebo group.

But the true efficacy might actually be lower than that reported by AstraZeneca. In a statement issued early Tuesday morning, the National Institute of Allergy and Infectious Diseases said the company “may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data.” After AstraZeneca released its data on Monday, the trial’s data and safety monitoring board sent a letter to NIAID with its concerns.

All human trials of a drug, vaccine, or other medical intervention have a monitoring board made up of outside experts, such as doctors and statisticians, who are not affiliated with the company or institution running the trial. Its job is to monitor a trial’s progress and review safety and efficacy data over the course of the trial. A monitoring board can recommend that a trial be stopped early if it has concerns about the safety of a drug or if it becomes abundantly clear that an intervention is or isn’t working.

According to the Washington Post, which obtained a copy of the letter, the monitoring board met with the company in February and March and said it saw data showing the vaccine may be 69% to 74% effective.

In its statement, NIAID urged AstraZeneca to work with its monitoring board to “review the efficacy data and ensure the most accurate, up-to-date efficacy data be made public as quickly as possible.”

“If you look at it, the data really are quite good. But when they put it into the press release, it wasn’t completely accurate.”

In its response on Tuesday, AstraZeneca said that the numbers in its March 22 press release were based on an interim analysis with a cutoff date of February 17. The company said it will immediately be in touch with the monitoring board and plans to issue results containing more recent data in 48 hours.

Speaking on Good Morning America on Tuesday, NIAID director Anthony Fauci said AstraZeneca’s error in not releasing the most up-to-date data is unfortunate given that the vaccine is safe and seems to work. “This is very likely a very good vaccine,” he said. “If you look at it, the data really are quite good. But when they put it into the press release, it wasn’t completely accurate.”

The debacle could further erode public confidence in AstraZeneca’s vaccine, which is already facing scrutiny in Europe. The vaccine was authorized for use in Europe at the end of January, but several countries recently put a pause on administering the vaccine over fears that a small number of people developed blood clots after receiving the shot.

The World Health Organization and European Medicines Agency have urged countries to continue using the vaccine, saying there’s no evidence that the AstraZeneca vaccine causes blood clots. The letter by the trial’s data safety and monitoring board did not mention any safety issues or potential side effects, according to the Washington Post.

Still, the new doubts raised about the vaccine’s efficacy could slow down its uptake in Europe and elsewhere.

Ultimately, the Food and Drug Administration will decide whether to allow AstraZeneca’s vaccine for emergency use in the United States. “They will independently go over every bit of data themselves and not rely on any interpretation from anyone, including the company,” Fauci said.



Emily Mullin
Medium Coronavirus Blog

Former staff writer at Medium, where I covered biotech, genetics, and Covid-19 for OneZero, Future Human, Elemental, and the Coronavirus Blog.