German scientists say they've figured out why AstraZeneca's and Johnson & Johnson's COVID-19 vaccines can cause rare blood clots

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Pharmacist Madeline Acquilano fills a syringe with the Johnson & Johnson Covid-19 Vaccine before inoculating members of the public at Hartford Hospital in Hartford, Connecticut, on March 3, 2021. - Some 7,400 vials of the Johnson & Johnson Covid-19 single shot vaccine were delivered and an initial offering of the vaccine was given to ten members of the public. (Photo by Joseph Prezioso / AFP) (Photo by JOSEPH PREZIOSO/AFP via Getty Images)
A pharmacist at Hartford Hospital in Connecticut in fills a syringe with the Johnson & Johnson COVID-19 vaccine. Joseph Prezioso/Getty Images
  • Scientists say they've figured out how AstraZeneca's and J&J's COVID-19 vaccines cause blood clots.

  • They say the vaccines can cause "mutant" proteins to escape cells, triggering a response.

  • The hypothesis hasn't been proved or scrutinized by experts, and other theories exist.

  • See more stories on Insider's business page.

German scientists say they've figured out why some COVID-19 vaccines can cause unusual blood clots in rare cases.

The team of scientists from Goethe University in Frankfurt, Germany, said Wednesday that the delivery mechanism for AstraZeneca and Johnson & Johnson's vaccines was responsible for causing blood clots after immunization.

All four COVID-19 vaccines widely used in the West work by delivering a piece of genetic code into the body. This teaches the body to create a harmless part of the virus, which triggers an immune response. The body can then stage the same response if it later detects the real virus.

Vaccines that use mRNA technology, including Pfizer's and Moderna's, deliver the genetic material to the fluid part of the cell. Adenovirus-based vaccines, such as J&J's and AstraZeneca's, deliver it to the central part of the cell, called the nucleus, the scientists said in a study, which hasn't been scrutinized by other experts in a peer review.

According to the scientists, this is where the issue lies. While the harmless part of the virus is being made in the nucleus, bits of genetic material can break off, creating "mutant" proteins that leave the cell and travel around the body. Blood vessel walls respond to these proteins, which triggers blood clots, they said.

Unusual blood clots, particularly in the brain, are a very rare side effect of both vaccines, according to regulators. For AstraZeneca's vaccine, 86 cases of unusual blood clots were reported out of 25 million people who had received the shot as of March 22, according to the European Medicines Agency. For J&J's vaccine, there were eight reports of serious blood clots as of April 13, when more than 7 million people had received the shot, according to the EMA.

Young women have had the clots more often than other groups. The scientists suggested that older people are less likely to get the clots partly because their immune responses are weaker. They added that women naturally have stronger immune responses than men.

The German scientists' hypothesis hasn't been proved. Researchers from Bristol University said in October that the protein made by the vaccine inside human cells accurately reflected the genetic instructions.

Other hypotheses also exist. One is that the blood cells in the body that cause clotting, called platelets, are "activated" post-vaccination and cause clots.

Jonathan Ball, professor of molecular virology at the University of Nottingham, told Insider in a statement Thursday that the new research posed a "really interesting hypothesis."

The data highlights that production of mutant spike proteins "may well occur," he said.

"But it stops short of providing a concrete link with promotion of blood clotting. Nevertheless, it is certainly something worth investigating further," Ball added.

Rolf Marschalek, a professor at Goethe University who was involved in the research, said the group had been in touch with J&J already, per the Financial Times. J&J "is trying to optimize its vaccine now," he told the FT.

Marschalek said AstraZeneca had not yet been in touch.

Janssen, the pharmaceuticals unit of J&J that produced the vaccine, said in a statement to Insider on Thursday: "We are supporting continued research and analysis of this rare event as we work with medical experts and global health authorities. We look forward to reviewing and sharing data as it becomes available, and, currently, we believe it is too soon to draw conclusions."

An AstraZeneca spokesperson said in a statement to Insider on Thursday: "We are continuing to work with regulators, recommending bodies and the scientific community around the world to understand more about the epidemiology and the possible mechanisms behind these extremely rare events."

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