The clots occurred in two to three out of every 100,000 people who received the vaccine.
The cause of rare blood clots in a COVID-19 vaccine that killed eight people in Australia may have been found in a new study by local researchers.
The AstraZeneca vaccine was one of the two primarily used in Australia during the early stages of the pandemic, but caused blood clots in about two to three people out of every 100,000 who received it.
The condition was called vaccine-induced immune thrombocytopenia and thrombosis, 173 confirmed or probable cases of which caused by the vaccine were recorded.
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In many cases, the symptoms were minor and people made a quick recovery, however other cases were far more serious and, in aight instances, fatal.
A team of researchers led by Australian scientists at Flinders University in Adelaide found that in some people, the immune system can confuse a normal adenovirus protein - something used in the AstraZeneca vaccine - for a human blood protein called platelet factor four (PF4).
Their study was released in the latest edition of the New England Journal of Medicine.
The presence of PF4 causes the body to produce antibodies that trigger blood clotting, which can cause serious injury or death.
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Through linking the two, scientists believe they can now manufacture vaccines to avoid this rare symptom.
"This was the missing link that explains how a normal immune response can, in very rare cases, become harmful," Dr Jing Jing Wang from Flinders University said.
"By modifying or removing this specific adenovirus protein, future vaccines can avoid this extremely rare reaction while continuing to provide strong protection against disease."
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Symptoms of thrombosis included severe headache, blurred vision or shortness of breath, with these occurring from four to 42 days after receiving the first dose of the vaccine.
To minimise the risk of the rare side-effect, the Australian government recommended the AstraZeneca vaccine only be given to people over 60 years old.
This is because younger people have a higher risk of producing a stronger antibody response and developing severe blood clots.
Despite this, many people below 60 years old received at least one dose of the AstraZeneca vaccine due to shortages of the Pfizer vaccine amid the push to reach certain vaccination targets and lift lockdown restrictions during the pandemic.
The AstraZeneca vaccine hasn't been available in Australia since 2023.
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