How does someone have clots yet no platelets?  Clots and vaccines??  Thrombosis with thrombocytopenia??? 

The links between AstraZeneca’s Covid vaccine and this unique thrombotic reaction still have so many unanswered questions, and women under 60 are a little nervous.

Here is some of what we know:  The condition is Thrombosis with thrombocytopenia.   Which, almost seems counterintuitive.   How to I clot without platelets??? 

As always in my posts, let’s go back...... 

Some terms:

Thrombocytes are platelets; small cell fragments present in blood and serve to assist in stopping bleeding. They form clots.

Clot : is a clump of platelets packed together like snow flakes in a snowball.

Thrombus is a clot that is strengthened by gumming it together with strands of protein called Fibrin. 

Fibrin is a sticky stringy thread like substance that forms when liquid proteins in the blood called coagulation Factors (specifically Factor X(10), prothrombin and fibrinogen) are activated or triggered in a process called Coagulation.

So recap - To clog up a blood vessel that is injured and bleeding, we form a clot and strengthen it with coagulation.

Together this makes a stable clot.   That is called a thrombus.

Now Thrombocytopenia means a reduced number of platelets.   Normally 150-400 thousand/ micro-litre of blood.  So technically a platelet count below 150 is regarded as Thrombocytopenia.  Recognise that clotting (involving platelets) and coagulation (involving a chemical protein chain reaction), are separate processes that, together, stop bleeding.  The technicality of the marriage between platelets and coagulation is a magnetic attraction.  Romantic I know. 

But platelets have a chemical they release called Platelet factor 4.  PF4 is positively charged.  A natural protein called Heparin circulates in blood preventing those coagulation proteins activating.   That’s why we call heparin a blood thinner - it isn’t, it’s actually a coagulation stopper (anticoagulant).

Heparin is negatively charged.

So.... remember the magnet?  Negative sticks to positive right?

Well heparin sticks to Platelet Factor 4.

Boom! 

Heparin is switched off.  Blood in the vicinity of activated platelets starts to form Fibrin which wraps around the clot like a fishnet around a bait ball of prawns, forming the clot into the thrombus.  The change in charge also further activates platelets to activate .

So.......what triggers the syndrome .

Well we think it’s the immune (inflammatory) reaction where platelets are first triggered to activate and release PF4.  Clots then set off a chain reaction of where clots trigger fibrin formation (coagulation) which in turn, triggers more clots.   As clots form, platelets die off reducing their population to low levels - thrombocytopenia.  It is similar to a rare condition called Heparin induced Thrombocytopenia (HIT).

Like this AZ vaccine stimulus, HIT is also thought to be the result of an immune reaction to complexes formed when negatively charged heparin molecules bind to a positively charged platelet factor 4.  Its like dropping a cigarette butt into a forest of tinder dry leaves, it creates its own microclimate and spreads like wild fire.

The syndrome also has some similarities to the effect of some snake venoms.   Brown snakes particularly - cause both clotting (early); though to be the cause of early snake bite deaths (inside the first few hours), then, because the platelets are depleted, they start bleeding. It’s called consumptive coagulopathy.

In AstraZeneca vaccine recipients affected by this rare clotting syndrome, the number of platelets crashes.  They had platelets, but they got used up forming clots triggered by the immune response, now leaving thrombi behind in their wake.  It appears within two weeks, usually between day 4 and 20.

The weird thing is these AZ clots tend to form directly in Gut vessels, brain and lungs.  Not in the deep veins which is typical of other thrombosis disorders.  There are so many unanswered questions here.

Why this vaccine and not the others?  Dunno

Why not flu shots and other vaccines ?  Dunno.

Why women > men ? Dunno