Here's What You Need to Know About the Oxford AstraZeneca Vaccine

After some ups and downs in rollout across other parts of the world, the Oxford AstraZeneca COVID vaccine will soon undergo FDA review for emergency use in the U.S.

AstraZeneca vaccine explainer
Photo: Getty Images

Three COVID-19 vaccines are already authorized by the Food and Drug Administration (FDA) for emergency use in the U.S. — Pfizer, Moderna, and Johnson & Johnson — and a fourth may not be far behind. On March 22, AstraZeneca announced promising results of a U.S. clinical trial of more than 30,000 people. The biopharmaceutical company reported that its COVID vaccine candidate, created in partnership with the University of Oxford, appears to be 79 percent effective at preventing symptomatic COVID-19 and 100 percent effective against "severe or critical disease and hospitalization" from the virus, according to a press release. In the same release, AstraZeneca stated its plans to submit these findings to the FDA for emergency use authorization (EUA) in the U.S.

As of now, the Oxford AstraZeneca vaccine is already authorized for use in the European Union, the UK, a number of Latin American countries, India, and Morocco, according to the company. But the global rollout of its COVID vaccine has been a little rocky so far. Several countries in Europe, for example, recently temporarily suspended the AstraZeneca vaccine due to an investigation into a possible link between the vaccine and blood clots. The World Health Organization (WHO) has since said that the data on all approved COVID vaccines and candidates "do not suggest any overall increase in [blood clots] following administration of COVID-19 vaccines."

There's also some concern that AstraZeneca's vaccine may not offer as much protection against B.1.351, the COVID variant that originated in South Africa and appears to spread more readily than other strains of the virus. (

But most recently, the National Institue of Allergy and Infectious Diseases (NIAID) released a statement saying that an independent data-safety board monitoring the study "expressed concern that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data."

Shortly thereafter, AstraZeneca put out a news release stating that, after analyzing more of its data, the company concluded that its COVID vaccine is 76 percent effective at preventing symptomatic COVID-19 (compared to 79 percent in the initial press release) and confirmed it is 100 percent effective against severe COVID-related illness and hospitalization.

The back-and-forth on AstraZeneca's data has led some people to question the trial's credibility — and, in turn, the vaccine's effectiveness. "Vaccine confidence is often a very tenuous thing that should be respected and protected," says Abisola Olulade, M.D., a board-certified family medicine physician at Sharp Rees-Stealy Medical Group. "All of this has only put that in jeopardy, which doesn't bode well for this and future diseases. Vaccine hesitancy has been very appropriately cited by the WHO as one of the top 10 global health threats. We have to do everything we can to combat it, and this undermines that effort."

Anthony Fauci, M.D., director of the NIAID, told Good Morning America that he believes the NIAID's statement about AstraZeneca's data was "rather harsh." "This is very likely a very good vaccine," continued Dr. Fauci. "This kind of thing does nothing except cast some doubt about the vaccine and maybe contribute to hesitancy."

Abisola Olulade, M.D.

Vaccine confidence is often a very tenuous thing that should be respected and protected.

— Abisola Olulade, M.D.

For now, AstraZeneca and Oxford are still preparing to submit their findings to undergo review for EUA in the U.S, and it's ultimately up to the FDA to review the data and make a decision. In the meantime, here's some clarity on what to know about the AstraZeneca vaccine.

How does the Oxford AstraZeneca vaccine work?

Much like Johnson & Johnson's COVID-19 vaccine, AstraZeneca's vaccine uses adenovector vaccine technology. That means it uses an inactivated virus (adenovirus, which causes the common cold) as a vector to deliver proteins (in this case, the spike protein from SARS-CoV-2, which attaches itself to cells in your body) that your body will recognize as a threat and create antibodies against so you're protected from future infections, explains Dr. Olulade. Reminder: You cannot get infected by an inactivated form of a virus, including COVID. (Another FYI: The flu shot works similarly.)

In terms of dosing, the Oxford AstraZeneca vaccine calls for two shots. The U.S. trial of the vaccine included two doses given at a four-week interval, but previous studies "have shown that an extended interval of up to 12 weeks demonstrated greater efficacy," according to AstraZeneca's press release.

One of those studies — a preprint published in The Lancet — looked at data from vaccine trial participants across the UK, Brazil, and South Africa and found that a single AstraZeneca shot is 76 percent effective at preventing symptomatic COVID within 90 days of that first dose. Interestingly, though, the timing of the second dose seems to make a significant difference in the vaccine's effectiveness, as AstraZeneca's press release noted. The study showed that the vaccine was just under 55 percent effective against COVID infection when doses were given within less than six weeks, but that number jumps to 82 percent if the second dose is given after 12 or more weeks, according to the research.

In other words, the longer you go between doses (with a cutoff around 12 weeks between doses, which is the UK's current strategy with the AstraZeneca vaccine), the more protection you might get from symptomatic COVID, says Brittany Busse, M.D., associate medical director at WorkCare.

How effective is the Oxford AstraZeneca vaccine?

AstraZeneca's most recent press release states that its COVID vaccine is 76 percent effective at preventing symptomatic COVID-19 and 100 percent effective against severe COVID illness and hospitalization from the virus 15 days or more after receiving two doses given four weeks apart. The data are based on a large-scale U.S. trial of more than 30,000 people, and the results showed "no safety concerns related to the vaccine," according to the release. The findings are yet to be peer-reviewed; the company said it will submit the data for peer-reviewed publication "in the coming weeks."

The U.S. trial doesn't necessarily paint a full picture of the vaccine's effectiveness, though. Back in December 2020, The Lancet published data from four ongoing large-scale trials in the UK, Brazil, and South Africa showing that, on average, AstraZeneca's vaccine was about 70 percent effective at protecting against symptomatic COVID-19. More specifically, the vaccine's effectiveness against COVID-19 infection ranged from 62 percent to 90 percent, depending on the dosing. An error of dose miscalculation in this data led some participants to accidentally receive a half-dose followed by a full dose instead of the standard two full doses. Interestingly, the half-dose followed by a full dose appeared to provide more protection against symptomatic COVID (about 90 percent effectiveness) than two full doses (62 percent effectiveness). However, the error is likely "irrelevant" because, whichever way you look at the data, "we still have efficacy that meets the thresholds for approval with a vaccine that's over 60 percent effective," Mene Pangalos, Ph.D. head of AstraZeneca's non-oncology research and development, told The Wall Street Journal.

COVID-19 Variants: AstraZeneca's vaccine appears to be around 74 percent effective against symptomatic infection from the UK COVID strain, according to data from the UK published in The Lancet — pretty on-par with the numbers seen across other sets of data on the vaccine. However, AstraZeneca's vaccine doesn't seem to protect as well against infection from the South African COVID strain: A study published in The New England Journal of Medicine (NEJM) suggests this vaccine has "no efficacy" against mild to moderate disease from the variant, though it does appear to offer complete protection against severe illness and hospitalization from the strain. That said, though, the median age of participants in the NEJM study was only 30 years old — a detail that seems to have "contributed to the absence of severe COVID-19 cases," considering older populations are at higher risk of severe COVID illness, says Dr. Olulade. The study itself states that its findings "are inconclusive with respect to whether [AstraZeneca's vaccine] may protect against severe COVID-19 caused by infection with the B.1.351 variant." (

COVID-19 Transmission: Unlike the data for other vaccines currently authorized for emergency use in the U.S., some research on AstraZeneca's vaccine suggests it could help contain the spread of COVID. In the preprint from The Lancet, researchers working in the UK regularly tested their vaccine trial participants for COVID and found a 67 percent reduction in positive tests after people received just one dose of the AstraZeneca vaccine. While the study authors noted that "transmission studies per se were not included in the analysis," these data suggest that AstraZeneca's vaccine "may have a substantial impact on transmission." So, while that research sounds promising, "there is no conclusive evidence of reduced COVID spread at this point and no consensus from the CDC and WHO on this," says Dr. Olulade.

Severe COVID-19 Illness and Deaths: The December 2020 paper published in The Lancet showed that more than 21 days after their first dose (whether it was a miscalculated half-dose or a standard full dose), none of the participants who'd received the vaccine were hospitalized with COVID. For comparison, in the control group (i.e. those who received a placebo instead of the vaccine), 10 participants were hospitalized, two of whom were found to have severe COVID, including one person that died, according to the paper. Similarly, AstraZeneca has stated that the U.S. trial of its vaccine showed 100 percent effectiveness against severe COVID and hospitalization from the virus.

What are the side effects of the AstraZeneca vaccine?

Like the Pfizer, Moderna, and Johnson & Johnson vaccines, AstraZeneca says you could have mild flu-like symptoms after being vaccinated (fatigue, chills, fever, headache, muscle aches), as well as temporary pain and tenderness at the injection site.

You might have also heard something about blood clots potentially related to the AstraZeneca vaccine — but there's no need to panic. "Some countries halted the vaccine administration due to reports of blood clots in people who had received it," says Dr. Olulade. "The European Medicines Agency (EMA), however, stated that the vaccine is not associated with an increased risk of blood clotting and that the benefits of the vaccine outweigh the risks."

"The number of blood clots is insignificant and no more prevalent than blood clots would be in the same population over the same period of time, even if those people had not received the vaccine," notes Dr. Busse. "There have only been 37 cases of blood clot when over 17 million doses of this vaccine have been given (incidence of about 0.0002 percent), while [blood clot] has an incidence of 1 in 1,000 in the general adult population (incidence of 0.001 percent)." Translation: Linking blood clots to the vaccine "is an unfair association," she says.

The Bottom Line On AstraZeneca's COVID Vaccine

Between reports of blood clots and skepticism about AstraZeneca's reporting of its data, you're probably wondering whether the vaccine will actually get authorization from the FDA. At this point, the decision is up to the agency and its independent vaccine advisory committee.

"Just know that the FDA is thorough and wouldn't approve a vaccine based on poor data, especially since we have three good candidates and enough doses for all Americans," says Purvi Parikh, M.D., an immunologist with Allergy & Asthma Network who is also a co-investigator with the COVID vaccine trials. "Due diligence will be done."

It's also worth emphasizing that all of the currently authorized vaccines in Europe and the U.S., including AstraZeneca, seem to be 100 percent effective in preventing death and very severe disease from COVID, notes Jonathan Baktari, M.D., CEO of

"Overall I think there are many great options for vaccination that will prevent severe disease," echoes Dr. Busse. "All people should receive whichever vaccine is available, and I do not think we should wait for one vaccine over another."

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it's possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.

Was this page helpful?
Related Articles