Two-Dose AstraZeneca Vaccine Is 76 Percent Effective in U.S. Clinical Trials

Studies show strong results among adults 65 and older

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By:

Dena Bunis

En español

Published March 24, 2021

As the number of Americans getting a COVID-19 vaccine is rapidly escalating, a fourth one could be available in the United States if the federal government authorizes the use of a two-dose AstraZeneca product that revised company data shows is 76 percent effective in protecting against symptomatic coronavirus illness.

The new information from AstraZeneca's stage 3 clinical trials, released on March 25, also shows that the vaccine was found to be 85 percent effective among adults 65 and older — and like the other COVID-19 vaccines now available in the U.S., it was 100 percent effective in preventing hospitalizations and deaths from the virus. The trials included 32,449 participants, most from the U.S. but some from Chile and Peru.

Hours after AstraZeneca had released its clinical trial results, the National Institute of Allergy and Infectious Diseases (NIAID) announced that it was notified by the Data and Safety Monitoring Board (DSMB) “that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data.” DSMBs are independent panels of experts that monitor clinical trials. NIAID urged AstraZeneca to work with the monitoring board to review the effectiveness data and make sure that the most up-to-date information be made public “as quickly as possible.” In a statement issued early on March 23 the company said that the data it published on March 22 was an “interim analysis” that reflected clinical trial data as of Feb. 17. AstraZeneca officials said they have reviewed their data, that their “primary” analysis is consistent with the “interim analysis.” It released its updated results early on March 25.

As of March 24, 26 percent of the U.S. population has received at least one COVID-19 vaccination, and 70 percent of Americans age 65 and older have gotten at least one shot.

AstraZeneca's vaccine has been the subject of some recent controversy over reports of people getting blood clots after being inoculated. About a dozen countries in Europe temporarily halted using the AstraZeneca vaccine, but most have resumed its use after the European Medicines Agency — the equivalent agency of the FDA — said that the incidence of blood clots among people who had received the AstraZeneca vaccine was actually lower than the expected rate in the general population.

The company said that there were no cases of people getting blood clots after being inoculated during its clinical trials. But its news release did not list any side effects that occurred during the trials. A more detailed review of the safety results of the stage 3 trials is expected when AstraZeneca applies for an emergency-use authorization (EUA) with the U.S. Food and Drug Administration, which it says it will do within weeks.

An international view

The U.S. government has already contracted for 300 million doses of the AstraZeneca product and recently sent 4 million doses from the U.S. stockpile to Mexico and Canada. President Joe Biden said earlier this month of the three vaccines already approved in the U.S. that he expects there to be enough doses available to vaccinate all adults in the country by the end of May. Experts predict much of the AstraZeneca vaccine will be sent overseas.

"I suspect that the AstraZeneca vaccine will play a limited role in the United States, but I think this is really important globally,” said William Moss, M.D., executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health. Already the product has been distributed in 70 countries worldwide.

“We have to take an international perspective on this because we need to control this pandemic globally in order to control the development of variants,” said William Schaffner, M.D., an epidemiologist and professor of preventive medicine and health policy at Vanderbilt University. But that doesn't mean it might not be needed in the United States, he says. “Each additional manufacturer is an insurance policy,” Schaffner says. “Suppose there were — I don't know, God forbid — a fire in the manufacturing facility of any one of these manufacturers, you like to have excess capacity, just in case you need it."

How COVID-19 vaccines are made

AstraZeneca's vaccine was developed using the same technology as the Johnson & Johnson product, which received an EUA on Feb. 27 for its one-dose product. The J&J vaccine was found to be 66 percent effective in its multi-country trials and 72 percent effective among U.S. participants.

There are differences and similarities between the two. AstraZeneca's vaccine requires two shots instead of the one needed for J&J, but like the J&J product, AstraZeneca's vaccine can be transported and stored at regular refrigerator temperature.

All COVID-19 vaccines are designed to stimulate people's immune systems to produce antibodies to the protein on the surface of the coronavirus, commonly referred to as the spike protein.

The AstraZeneca and J&J vaccines were created using what's called an adenovirus — a harmless virus that can no longer replicate — to send a genetic code to our cells. The adenovirus goes into the nucleus of our cells and uses our DNA to send the genetic code. That code triggers immune responses that will allow our bodies to make antibodies to the spike protein located on the surface of the coronavirus. Those antibodies will help fight a COVID-19 infection if the real coronavirus enters our body.

The Pfizer-BioNTech and Moderna vaccines, which received EUAs in December 2020, use what's known as mRNA, to provide the genetic code our cells need to produce the antibodies. Both of these vaccines require two shots and were shown to be 94 to 95 percent effective.

The FDA has said that a COVID vaccine would have to be at least 50 percent effective in order to be authorized. Once AstraZeneca files an EUA application, an independent panel of experts would review a more detailed set of data than was released on Monday and discuss the application at a public meeting.

"I think all of the vaccines, this one included, are in the same ballpark in preventing the most serious disease, keeping you out of the hospital and the intensive care unit and preventing you from dying,” Schaffner said. “From that point of view, I think they're all comparable."

Dena Bunis covers Medicare, health care, health policy and Congress. She also writes the “Medicare Made Easy” column for the AARP Bulletin. An award-winning journalist, Bunis spent decades working for metropolitan daily newspapers, including as Washington bureau chief for the Orange County Register and as a health policy and workplace writer for Newsday.

Editor's note: This story has been updated to reflect new information.

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