In-Depth: Is AstraZeneca's vaccine linked to blood clots?

AstraZeneca vaccine shows strong immune response in elderly, appears safe in study
Posted at 7:07 PM, Mar 17, 2021
and last updated 2021-04-18 21:39:52-04

SAN DIEGO (KGTV) -- The World Health Organization said Wednesday it continues to support the use of the AstraZeneca vaccine while regulators try to determine if there’s a link between the shot and a handful of reports of serious blood clots.

More than a dozen countries, primarily in Europe, have temporarily stopped using the vaccine while scientists review safety data. The European Medicines Agency will meet to discuss the topic Thursday.

Dozens of other countries, including the UK, Canada and Australia, have dismissed concerns about blood clotting and continued to roll out the vaccine.

The AstraZeneca vaccine is not currently authorized in the U.S., although the government has stockpiled millions of doses in anticipation of a review by the Food and Drug Administration.

“In extensive vaccination campaigns, it is routine for countries to signal potential adverse events following immunization. This does not necessarily mean that the events are linked to vaccination itself, but it is good practice to investigate them,” the WHO said in a statement. “At this time, WHO considers that the benefits of the AstraZeneca vaccine outweigh its risks and recommends that vaccinations continue.”

More than 17 million people in the UK and Europe have now gotten the AstraZeneca vaccine. The company says a total of 37 people developed blood clots. A few of those cases resulted in death.

With the vaccine likely coming up for a review by the FDA in the next few weeks, should we be worried?

“I think it's very, very unlikely that this vaccine is responsible for the clotting that's been seen,” said Dr. Peter Chin-Hong, an infectious disease expert at UC San Francisco.

“In fact, forming clots, or thrombosis as we say in medicine,” he added, “it's not something we've seen historically with other vaccines.”

Still, several European countries say they want to investigate as a precaution. They’ll start by comparing the rate of clotting in vaccinated people against what’s called the “background rate.” That’s how often you would expect blood clots in the general population.

In the U.S., about 1,000 to 2,000 people get a blood clot every day for a variety of reasons.

“When you're vaccinating literally millions of people, things are going to happen. People are going to die from other causes. People are going to have clots, just like they would in the general population,” said Dr. Christian Ramers of Family Health Centers of San Diego.

In a statement, AstraZeneca said the rate of blood clots in people who got its vaccine is much lower than the hundreds of cases that would be expected in the general population. The company noted the rates of blood clots are similar to other licensed COVID-19 vaccines.

Dr. Ramers said the numbers change slightly when you start considering the exact types of blood clots that have occured.
Regulators are particularly focused on seven cases of a rare blood clot in the brain that popped up in Germany after 1.6 million people got vaccinated. The typical background rate of that kind of clot, called sinus vein thrombosis, is about five cases per million, although studies on the incidence rate differ.

“Regardless of how you slice and dice it, this is a very, very rare thing,” said Dr. Chin-Hong. “It's really difficult to disentangle what's background rates and what's vaccine-associated with something so super rare.”

Scientists will be reviewing each person’s medical history for clues. In the meantime, dozens of countries continue to roll out the shots. The vaccine is authorized in more than 70 countries.

“Overall, remember that you’re having to balance risks and benefits,” said Dr. Ramers. “There are still thousands of people in Europe dying in many, many countries.”

Vaccine safety concerns usually turn out to be unsubstantiated, but they can undermine public confidence. This kind of erosion of trust led to the demise of a highly effective vaccine for Lyme disease in the early 2000s. After a class-action lawsuit in 1999 claimed the vaccine was linked to arthritis, the FDA reconvened a panel of experts in 2001 to review the safety data.

The agency concluded it should stay on the market, but by then it was too late. The vaccine’s developer pulled the drug off the market in 2002 because of a sharp decline in sales.