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

"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.