J&J, AstraZeneca COVID Vaccine Blood Clot Links May Be Tied to Their Design

Rare blood cots that have been reported by a handful of individuals following vaccination with the Johnson & Johnson (J&J) and Oxford-AstraZeneca vaccines may be linked to the viral vector platforms that the shots use, some experts believe.

On Tuesday, the U.S. Centers for Disease Control and Prevention (CDC) and the Food and Drugs Administration recommended a pause in the use of the Johnson & Johnson shot after six cases of a rare and severe type of blood clot were reported in individuals who had received the vaccine.

The type of blood clot—known as cerebral venous sinus thrombosis (CVST)—was seen in combination with low levels of blood platelets, among women between the ages of 18 and 48, according to the CDC and FDA. The symptoms occurred six to 13 days after vaccination.

While the condition appears to be extremely rare—six cases in the U.S. out of more than 6.8 million J&J vaccine doses administered—and a causal link has yet to be established, the FDA and CDC decided to recommend a pause out of an abundance of caution. The agencies are now reviewing the cases.

Meanwhile, J&J has been reviewing the cases with European health authorities and the company has taken the decision to proactively delay the rollout of its vaccine on the continent.

Adam Finn, a professor of pediatrics, at the University of Bristol in the United Kingdom, said in a statement provided to the Science Media Centre that it is possible the viral vector platforms used by the Johnson & Johnson and Oxford-AstraZeneca vaccines could be playing a role in the cases of the rare blood clots, although it is too soon to know for sure.

"The announcement of the recommended suspension of the one-dose Janssen [Johnson & Johnson] COVID vaccine by the authorities in the U.S. following six cases of cerebral venous sinus thrombosis with low platelets among nearly 7 million people given the vaccine echoes recent events in Europe around the Oxford-AstraZeneca vaccine," Finn said.

Both of the vaccines work in a very similar way, using two different modified adenoviruses—a common group of viruses that cause cold-like symptoms—to induce an immune response against the SARS-CoV-2 virus.

The adenoviruses (the viral vector) have been engineered so that they do not cause illness, while also carrying pieces of genetic material from the "spike protein" of SARS-CoV-2, which the body identifies and produces antibodies against.

The graphic below, provided by database research firm Statista, shows the countries with the highest rate of COVID-19 vaccination.

Statista graph CDC vaccine data
A Statista graph shows how many doses of the J&J vaccine have been administered in the U.S. compared to other shots. Statista/CDC

"The numbers of cases are, once again, very small but the combination of severe thrombosis with thrombocytopenia is characteristic of the cases that have been reported previously," Finn said. "Although it's too soon to draw any firm conclusions, this development does raise the possibility that at least some adenovirus vectors either of themselves or in combination with the SARS-CoV2 'S' protein gene can cause this idiosyncratic reaction in a very small proportion of individuals.

"This may help give us a clue towards understanding the mechanism or a way to prevent this problem from occurring. Given the importance of these vaccines for the timely control of the pandemic, investigation of this phenomenon is now an extremely urgent international priority."

Eleanor Riley, a professor of immunology and infectious disease, at the University of Edinburgh in the United Kingdom, said in a statement to the Science Media Centre that while a causal link between certain COVID-19 vaccinations, platelet abnormalities and blood clots has, so far, yet to be confirmed, "the index of suspicion is rising that these rare cases may be triggered by the adenovirus component" of the Johnson & Johnson and Oxford-AstraZeneca vaccines.

Experts stress that, despite the latest reports, the risks associated with contracting COVID-19 far outweigh any risks of being vaccinated for the vast majority of adults.

"Even if all of the cases were caused by the vaccine, the risk of less than one in a million would have to be set against the benefits of protection from COVID-19 disease; a disease which, in itself, causes clotting in many cases," Peter English, a retired consultant in communicable disease control, said in a statement.

Newsweek, in partnership with NewsGuard, is dedicated to providing accurate and verifiable vaccine and health information. With NewsGuard's HealthGuard browser extension, users can verify if a website is a trustworthy source of health information. Visit the Newsweek VaxFacts website to learn more and to download the HealthGuard browser extension.

Administration of a COVID vaccine
Stock image showing an individual receiving a coronavirus vaccine. Health agencies are investigating six cases of rare blood clots following administration of the Johnson & Johnson vaccine. iStock