Does AstraZeneca’s Covid-19 Vaccine Cause Blood Clots? What We Know So Far
The AstraZeneca Covid-19 vaccine may soon be available in the United States, but there are concerns about blood clots. Here's what doctors need you to know.
A Covid-19 vaccine on hold
Covid-19 vaccines can help establish herd immunity, stop the virus’s spread, and end the pandemic. Sounds pretty great, right?
But as thrilled as most people are about the availability of Covid-19 vaccines, they also want to avoid side effects.
That’s especially true of the AstraZeneca Covid-19 vaccine, which has recently been linked to a rare but potentially dangerous side effect.
The vaccine is not yet approved for use in the United States, and following reports of blood clots among people who received the AstraZeneca Covid-19 vaccine, several countries have partially or completely paused their rollouts, especially in younger people.
On April 7, the European Medicines Agency (EMA, roughly equivalent to the U.S. Food and Drug Administration) made its most definitive statement yet, announcing that blood clots should be classified as a very rare side effect of the vaccine.
These results will likely affect whether the vaccine is approved in the United States.
This one is different from the Moderna and Pfizer vaccines
The AstraZeneca vaccine, developed in conjunction with Oxford University and now branded Vaxzevria, would be the fourth Covid-19 vaccine to join the U.S. market.
Like the Johnson & Johnson vaccine, Vaxzevria uses an inactivated virus to prompt cells to produce the infamous spike protein found on SARS-CoV-2—the Covid-19 virus.
Your immune system reacts to that spike by manufacturing antibodies to fight the virus. For Vaxzevria, developers used an inactivated adenovirus that infects chimpanzees, which is similar to viruses that cause mild colds in humans.
The first two vaccines to receive Food and Drug Administration (FDA) approval, from Pfizer-BioNTech and Moderna, are mRNA vaccines.
They use genetic material called RNA to deliver a similar set of instructions to cells. The end result is the same: Your body produces antibodies that will fend off SARS-CoV-2 should you encounter it.
Side effects from the Johnson & Johnson, Moderna, and Pfizer-BioNTech vaccines have largely been the expected sore arm and mild flu-like symptoms.
Some people have had anaphylactic (allergic) reactions to the shots, but all recovered quickly.
The vaccine is effective
Trial results released at the end of March found that the AstraZeneca vaccine was 76 percent effective at preventing symptomatic Covid-19 and no one in the trials developed a severe case of Covid-19 or required hospitalization.
This is similar to the Johnson & Johnson vaccine, which had 74.4 percent overall effectiveness in the United States and prevented hospitalizations and deaths. The Pfizer and Moderna vaccines were at least 90 percent effective at preventing symptomatic Covid-19.
The Johnson & Johnson vaccine only requires one dose. The others require two doses given several weeks apart. The bottom line is that all four vaccines are pretty effective in preventing Covid-19.
“[AstraZeneca’s vaccine is] good but perhaps not in the first rank on the efficacy side,” says H. Dirk Sostman, MD, president of the Houston Methodist Academic Institute. (It’s still well ahead of the typical flu vaccine, which is only 40 to 60 percent effective.)
The safety concerns with Vaxzevria
A small number of people have developed rare blood clots with the AstraZeneca shot. Exactly what that number is, however, isn’t clear; one of the NEJM studies found five cases out of 130,000 vaccinations.
But the actual risk factor is still murky, says Stephan Moll, MD, professor of medicine in the division of hematology at the University of North Carolina School of Medicine in Chapel Hill.
Most of the cases have been in women under 60 years of age, and they’ve happened within two weeks of getting the vaccine. Some people have even died.
The blood clots have been found in blood vessels in the brain or abdomen. In the NEJM studies, researchers call the condition vaccine-induced immune thrombotic thrombocytopenia.
Sometimes the clot is accompanied by bleeding; other times it’s not. One theory is that antibodies prompted by the vaccine attach to platelets in the blood, forming dangerous clumps.
(Here’s who should skip the Covid-19 vaccine.)
While the link between clots and the vaccine isn’t conclusive, it’s getting there: “As more and more reports come in, it becomes more realistic that we’re not just a one-off situation,” says Kathryn Edwards, MD, a spokesperson for the Infectious Diseases Society of America and scientific director of the Vanderbilt Vaccine Research Program in Nashville, Tennessee. “I think the more we hear about this, the more likely that there is a link.”
It’s also possible that there may be a link between dangerous blood clots and other adenovirus-vectored vaccines.
“That would become an even bigger question,” says Dr. Edwards. In fact, Dr. Sostman says platelet issues with other vaccines are not unheard of, though they don’t always lead to clots.
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Benefit vs. risk
Some people wonder if the world really needs another Covid-19 vaccine.
Experts feel that we do: “In contrast to the World Series, this is a time we need everyone to win,” says Dr. Edwards “We’re going to need a lot of vaccines to vaccinate the world’s people. We need all of these vaccines to be proven safe.”
Some of the advantages of the AstraZeneca shot is that it doesn’t have to be kept at ultra-low temperatures, making it easier to store and transport.
To truly tame this virus, we need most of the world’s more than 7.7 billion people to be vaccinated. Vaxzevria is already approved in more than 70 countries and widely available, which Dr. Moll says is a huge advantage in the race toward an immunized global population.
“It’s a risk-benefit thing,” says Dr. Sostman. For some populations who are at lower risk of blood clots from the vaccine, the AstraZeneca vaccine could be a safe option—much safer than contracting Covid-19.
In the United States
Vaxzevria isn’t approved in the United States: AstraZeneca has not yet submitted an application to the FDA, but the company may do so this month or next.
It’s been waiting for an analysis of the results of a U.S. clinical trial involving 32,000 participants.
If the FDA does end up approving the vaccine, says Dr. Sostman, it could follow the lead of other countries in recommending it be reserved for older folks—in studies, people under the age of 60 are the ones at elevated risk of blood clots.
“I think the point is that we have relatively little need for this vaccine here in the United States, so the bar for emergency use is going to be high,” he says. “Restricting it to older age groups could be one way of making the best risk-to-benefit case for the vaccine.”
You have choices
Thankfully, you can select from different Covid-19 vaccines, although the current expert advice is to get whatever vaccine is offered to you whenever it’s offered to you, says Dr. Moll.
Everyone—vaccine officials and the public—needs to assess the risks and benefits carefully, says Dr. Edwards. Those risks and benefits may vary depending on which other vaccines are available. “What we’re trying to prevent is not a common cold,” she says. “People are dying from Covid-19.”
Next, check out the Covid-19 vaccine myths you can safely ignore.
- Centers for Diseases Control and Prevention: "Trends in Number of COVID-19 Vaccinations in the U.S."
- European Medicines Agency: "AstraZeneca's COVID-19 vaccine: EMA finds possible link to very rare cases of unusual blood clots with low blood platelets"
- The Ohio State University: "Explaining Johnson & Johnson's, AstraZeneca's new COVID-19 vaccines"
- AstraZeneca: "AZD1222 U.S. Phase III primary analysis confirms safety and efficacy"
- American Medical Association: "What doctors wish patients knew about the Johnson & Johnson vaccine"
- H. Dirk Sostman, MD, president, Houston Methodist Academic Institute
- Centers for Disease Control and Prevention: "Interim Estimates of Vaccine Effectiveness of BNT162b2 and mRNA-1273 COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Health Care Personnel, First Responders, and Other Essential and Frontline Workers — Eight U.S. Locations, December 2020–March 2021"
- Stephan Moll, MD, professor of medicine, division of hematology, University of North Carolina School of Medicine, Chapel Hill
- University of Minnesota Center for Infectious Disease Research and Policy: "More global reports of blood clots after AstraZeneca COVID vaccination"
- AstraZeneca: "Update on the safety of COVID-19 Vaccine AstraZeneca"
- The Lancet: "Oxford–AstraZeneca COVID-19 vaccine efficacy
- Kathryn Edwards, MD, spokesperson, Infectious Diseases Society of America, and scientific director, Vanderbilt Vaccine Research Program, Nashville, Tennessee
- U.S. Census Bureau: "U.S. and World Population Clock"
- Reuters: "Exclusive: AstraZeneca to seek U.S. authorization for COVID-19 vaccine this month or early next - sources"