Warning: This has not been published in a per-reviewed journal, and this theory has not been independently replicated.
The vaccines made by AstraZeneca and Johnson & Johnson are being linked to a very rare blood clotting disorder. German scientists think they have worked out why, and potentially how to stop it.
Why is this important? Well, several countries have halted the rollout of the AstraZeneca vaccine, and recently Belgium announced it would stop using the J&J vaccine to those under 41. Avoiding just a few cases of blood clots is disrupting vaccination programmes for thousands of people for whom it would not be a problem.
Why do some vaccines have this rare side-effect? A German study (not yet peer-reviewed) suggests that Covid-19 vaccines that use adenovirus vectors (cold viruses) send some of their payload into the nucleus of cells. When this happens, the cells can misread the instructions for making coronavirus proteins, potentially triggering blood clot disorders in a small number of recipients.
How does this happen? After entering the cell nucleus, parts of the spike protein splice or split apart, creating mutant versions which are unable to bind to the cell membrane. The spike protein, which is found on the surface of the Covid-19 virus, helps the virus to enter other host cells. These mutant versions then enter the body and trigger the rare blood clots.
Why does this not happen with all Covid-19 vaccines? The process is different for mRNA vaccines, such as those made by Pfizer and Moderna, as in these the genetic material of the spike protein is sent directly to the cell fluid and does not enter the nucleus. Researchers think that those using adenovirus vectors could alter the sequence of the spike protein "to avoid unintended splice reactions and to increase the safety of these pharmaceutical products."
How often does this happen? It is important to know that the blood clotting disorder is an extremely rare occurrence in people receiving a coronavirus vaccine. Latest data indicates that out of the 30.8 million doses of the AstraZeneca vaccine given in the UK between 9 Dec. 2020 and 5 May 2021, there have been more than 260 cases of blood clotting disorder (thrombosis with thrombocytopenia). This is the equivalent of 10.9 cases per million doses. The vast majority were reported after the first dose, and only eight after the second dose.
Source: Daily Express, 27 May 2021: German scientists claim to have finally cracked the vaccine blood clot puzzle.
