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Will the AstraZeneca COVID-19 Vaccine Be Safe to Take?

astrazeneca vaccine

Lisa Maree Williams / Stringer / Getty Images

Key Takeaways

  • There is no association between the AstraZeneca vaccine and developing blood clots. Blood clotting is a rare event reported in less than 1% of people who are currently vaccinated in Europe.
  • You are nine times more likely to develop blood clots from a COVID-19 infection.
  • The AstraZeneca vaccine released data on their Phase 3 clinical trial, finding it was 79% effective in preventing symptomatic infection and 100% effective in preventing severe illness and hospitalization. However, the National Institute of Allergy and Infectious Diseases immediately raised concerns about incomplete data, prompting another independent review.
  • The vaccine is not yet authorized in the United States.

The AstraZeneca COVID-19 vaccine has had its fair share of good news and bad news over the past week. The good: clinical trial results show it's quite effective at preventing COVID-19 infection. The bad: health organizations are questioning those results, and people are associating it with blood clots.

On March 22, AstraZeneca released the results of its Phase 3 US clinical trial. The trial recruited 32,449 volunteers, including 141 who had symptomatic cases of COVID-19.

The vaccine was 79% effective at preventing symptomatic COVID-19 infection. It was also 100% effective in preventing severe disease and hospitalization.

These results mean it's on par with the currently-authorized vaccines in the U.S. when it comes to preventing the worst cases of COVID-19. It's only slightly less effective than Moderna (94.5%) and Pfizer (95%) at completely preventing infection, and slightly more effective at disease prevention than Johnson & Johnson (66%).

For adults age 65 and older, the AstraZeneca vaccine was 80% effective. The results were consistent among people of all races and ethnicities—but most participants were White. About 79% of trial volunteers were White, 8% were Black/African American, 4% Native American, 4% Asian, and 22% were Hispanic/Latino.

U.S. Government Raises Concerns of Outdated Trial Results

The trial results would have been the final step towards applying for emergency use authorization with the U.S. Food and Drug Administration (FDA).

But less than 24 hours later, the National Institute of Allergy and Infectious Diseases (NIAID) released a statement about concerns that the AstraZeneca trial data is outdated. An independent review board reported the company may have been cherry picking results, putting the true efficacy closer to 69% or 74%, according to the New York Times.

AstraZeneca gave a brief response Tuesday morning, confirming the results only showed data before February 17. The company plans to have data reviewed by an independent data safety monitoring board, offering new results within the next two days.

The AstraZeneca vaccine is currently approved in more than a dozen countries, including those in the European Union, United Kingdom, and India. The vaccine is currently not authorized for use in the United States. Before the released results, White House Chief Medical Officer Dr. Anthony Fauci said he expected AstraZeneca vaccine approval in April, according to CNBC.

Does it Really Cause Blood Clots?

Claims that the vaccine may cause blood clots caused vaccinations to be temporarily suspended in Europe while the European Medical Agency reviewed the data.

On March 18, the European Medicines Agency released a statement reporting unusual blood clotting cases after vaccination. About seven cases of multiple blood vessels and 18 cases of blood clotting in the brain were reported from the 20 million people in Europe who were vaccinated. The people affected were mainly women under the age of 55.

However, the preliminary review found no direct link between the AstraZeneca vaccine and an elevated blood clotting risk. There is an extremely rare chance the vaccine may be associated with blood clots from thrombocytopenia (a low platelet count), but this needs further analysis.

Blood Clotting Is More Likely From COVID Infection Than Vaccination

Nitin Desai, MD, CEO and CMO of COVID PreCheck, says the risk of blood clotting is extremely rare and more likely to be seen in people who have COVID-19 infection, not vaccination. He says autopsy findings show COVID-19 infection had a nine times higher risk of blood clotting than autopsies of non-COVID infection.

Blood clots happen when there is an imbalance between bleeding and clotting in the body. This can trigger conditions such as cerebral venous sinus thrombosis (CVST) and disseminated intravascular coagulation (DIC), where abnormal clotting causes massive bleeding.

Desai says blood clotting is not only triggered by infection, but also by something like birth control pills. The pill chemically disrupts specific proteins in the blood, making users potentially more prone to blood clots. Clots can appear in the brain, legs, and lungs.

“COVID itself is disrupting the clotting mechanism to the degree that physicians are considering giving blood thinners anticoagulants to hospitalized COVID patients who are moderately ill,” Desai tells Verywell. 

Rare instances of blood clotting may not be unique to the AstraZeneca vaccine.

“We may have seen blood clotting in one or two deaths from the Pfizer vaccine as well,” Desai says. “In Miami, there was a physician who was healthy and of middle age who came to the emergency room for a stroke following his second dose.” The doctor received the Pfizer vaccine and developed acute immune thrombocytopenia, according to the New York Times. Pfizer says this isolated incident is likely not connected to the vaccine.

Given the extreme rarity of blood clotting after vaccination, Desai says the AstraZeneca vaccine is safe to take and should continue to be offered.

“I don’t think AstraZeneca should be left out of consideration," he says. "If you can have 10 million more people vaccinated by adding AstraZeneca, I would say yes to that. Plus, I think the CDC is closely watching all the Phase 3 clinical trial results and the European experience before considering emergency use authorization."

What This Means For You

The risk of developing blood clots from the AstraZeneca vaccine is highly unlikely and should not be a reason to avoid vaccination if it is approved in your country. The European Medicines Agency says the AstraZeneca vaccine benefits far outweigh the risk of not getting it.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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  1. Moderna, Inc. European medicines agency begins rolling review of Moderna's mRNA vaccine candidate against COVID-19 (mRNA-1273) Updated November 17, 2020.

  2. Garde D. Pfizer and BioNTech to submit Covid-19 vaccine data to FDA as full results show 95% efficacy. Stat. Updated November 18, 2020.

  3. U.S. Food and Drug Administration. Vaccines and Related Biological Products Advisory Committee Meeting. Updated February 26, 2021.