Two covid vaccines share a common thread that might account for blood clotting in rare cases.
Note: I am not a doctor and am not advising anyone on a particular course of action. This piece is intended to inform the general reader.
The common thread between the Oxford-AstraZeneca covid vaccine and a similar vaccine from Johnson and Johnson, is the way that instructions for making virus spike proteins are delivered to human cells. Both vaccines use an adenovirus to do the work. Adenoviruses cause common colds in humans and the viruses used for the vaccine have been weakened and deliver Covid instructions almost exclusively.
The blood clotting problem reported to be associated with these vaccines is very rare, only a few for J&J and a couple hundred for AstraZeneca which has been distributed more widely. But it appears the problems may be related. It remains to be proven but the attendant low platelet counts associated with these clots in some people suggests that in addition to making antibodies to the virus proteins, the vaccines can, in some people, also stimulate antibodies to platelets. More specifically, it suggests that the platelet antibodies may be intended for the adenovirus which amounts to a cross reaction.
Antibodies work by attaching to their targets and forming small clumps containing virus particles held together by antibodies, in this case covid viruses. The clumps are found and consumed by white blood cells thereby eliminating the virus from the system.
Cross reactive antibodies are not uncommon in medicine. One of the most well known involves antibodies that the immune system makes to Group A Streptococcus, a bacterium that causes scarlet fever. Rheumatic fever can follow on many weeks later as antibodies that were generated against the Streptococcus attack body tissues. This is especially dangerous if the antibodies attack heart tissue. Antibiotics can kill the Strep and prevent Rheumatic fever which is why strep throat is treated so seriously.
If in rare cases the immune system can make antibodies to platelets, the prognosis is uncertain. Since the body needs platelets to remain healthy and avoid hemorrhaging from small cuts, the bone marrow continues to make a steady supply of them. Whether the immune system will eventually cease making cross reacting antibodies is an important question that we don’t have answers for yet.