German scientists say they've figured out why certain coronavirus vaccines trigger rare and potentially deadly blood clots. Experts urge caution because the theory—that the issue lies with how the AstraZeneca and Johnson & Johnson "adenovirus vector" vaccines are delivered—isn't proven or peer-reviewed, and there are other possibilities. But in a preprint study, Goethe University scientists say adenovirus vectors—harmless viruses used to send genetic instructions to cells to produce the spike protein of the coronavirus, which triggers an immune response—deliver these instructions to the nucleus, the central part of the cell, rather than the fluid part, "where proteins are usually produced," per Bloomberg. Vaccines using mRNA technology, like those by Pfizer-BioNTech and Moderna, deliver genetic material of the coronavirus spike protein to the fluid part of the cell, not the nucleus.
"RNA genes are not optimized for gene transcription inside the nucleus," lead author Rolf Marschalek tells Deutsche Welle. There, parts of the spike protein DNA can break apart, causing mutations which prevent proteins from bonding to the cell, per CBC News. Proteins can then flow through the bloodstream, triggering inflammatory reactions. Researchers point to "highly specific blood flow conditions in veins that drain from the brain." On the bright side, Marschalek tells the CBC that adenovirus vector vaccines (Russia's Sputnik V is another) can be modified to avoid "splice reactions." J&J's Janssen Pharmaceuticals says it's "too soon to draw conclusions," per Bloomberg. The outlet cites one molecular virologist as saying the data "highlights that production of this truncated spike may well occur, but it stops short of providing a concrete link with promotion of blood clotting." (Read more coronavirus vaccine stories.)