Vaccine-related blood clotting explained
With news spreading of the death of a second person due to an extremely rare blood clotting disorder after receiving the AstraZeneca vaccine, Australians are still feeling hesitant about getting the jab.
According to the Therapeutic Goods Association (TGA), the blood clotting condition, known as thrombosis with thrombocytopenia syndrome (TTS), is likely linked to the AstraZeneca vaccine.
Of the 3.3 million doses of the AstraZeneca vaccine that have been administered so far, 48 people have developed blood clots, with 31 recovering after a hospital stay.
Despite the concerns over the vaccine, experts have said we’ve come a long way in understanding the disorder - as well as how to detect it and treat it.
How are blood clots detected?
The first step in detecting the blood clot relies on getting the timing right, said Vivien Chen, a haematologist specialising in coagulation disorders at the University of Sydney.
“The first entry point is being a patient within the right time frame after receiving the AstraZeneca vaccine,” Dr Chen said.
“The first dose appears to be of particular concern.”
TTS symptoms - including severe headaches that don’t go away, abdominal pain, blurred vision, and leg pain or swelling - appear within four to 30 days after receiving the AstraZeneca vaccine, with most occurring within six to 14 days.
If you experience any of these symptoms after getting vaccinated, it’s recommended you go to your GP in order to get a blood test known as a platelet count.
What are platelets?
Platelets are fragments of cells that come from the bone marrow and play a vital role in blood clotting, including preventing excessive bleeding following a cut or scrape.
Normal platelet levels hover between 150,000 and 450,000 per microlitre of blood.
A platelet count lower than 150,000 results in a condition called thrombocytopenia, which usually results in excessive bleeding.
But when it follows a vaccination, abnormal blood clotting can occur.
For a small number of people, the immune system triggers antibodies that recognise COVID-19 proteins and platelets. These antibodies can then activate the platelets, cause them to cluster and go into blood-clotting mode.
“The consequences of this is the platelets are getting used up in the blood clot and their count drops,” Dr Chen said.
Next steps
If the platelet count is low, doctors will look at blood samples to find protein fragments called D-dimers, which are produced by the body to break down blood clots.
If the level of D-dimers is five times higher than that of normal blood, it can be a sign of TTS.
When a low platelet count and high D-dimer level have been detected, clinicians then look for antibodies that target a protein called platelet factor 4, or PF4. The PF4 antibodies are also tested to determine whether they can activate platelets and cause TTS.
Detection speeds
Vaccine-related clots can be detected via blood tests as soon as symptoms appear - whether that’s four or 20 days after getting the jab - but not before that.
“In most cases, [antibodies] can only be found after people feel unwell and present at hospital,” said Jose Perdomo, a haematologist at the University of New South Wales.
Patients presenting at the emergency department of a metropolitan hospital with TTS symptoms can receive results from the blood-screening tests within one hour. But, wait times can be longer in regional hospitals.
Once initial tests have confirmed platelet and D-dimer levels, patients can commence treatment while waiting for their PF4 antibody test results.
Treatment
Two different types of treatment are combined to battle TTS.
To start with, patients start taking blood-thinning medication to slow down the formation of blood clots.
Patients also receive a large dose of immunoglobulins - proteins that are part of the body’s defence system - to calm down the immune system.
“This essentially swamps the antibody system, so that vaccine-induced antibodies can’t activate the platelets,” Dr Chen said.
This combined treatment can help return blood platelet levels to normal within a few days, according to Dr Chen.
“In more severe cases, it can take longer but it’s a rapid response to commencement of new therapy,” she said.
She also said this treatment is safe to use even if the blood clots end up being unrelated to the vaccine.
What to do if symptoms appear
Though the risk of getting a blood clot following the vaccine is very low, Dr Chen said it’s important to be aware of the symptoms and take action as soon as they appear.
If you have a headache that won’t go away after taking painkillers, you should visit your GP.
But if you notice more severe symptoms - including numbness, difficulty speaking, or passing blood in bowel movements - head straight to the emergency department instead.