The Oxford-AstraZeneca vaccine may be associated with a condition characterised by low platelet counts, in rare cases, according to a study by an international team of researchers.
A low number of platelets- blood cells that help prevent blood loss when vessels are damaged-can result in no symptoms or can lead to an increased risk of bleeding or, in some cases, clotting.
The research team, led by the University of Edinburgh, analysed more than 1.7 million people who had their first jab of Oxford-AstraZeneca jab and 800,000 who took the Pfizer-BioNTech vaccine in Scotland.
The small increased risk that affects the blood is known as idiopathic thrombocytopenic purpura (ITP).
"The data indicated that there was a slight increase in ITP in the second week following vaccination for those who received the Oxford-AstraZeneca vaccine and possibly also increased risk of arterial clotting and bleeding events," the researchers said.
"The 11 cases of ITP per million vaccine doses is similar to numbers seen for Hepatitis B, MMR and flu vaccines, which range from 10 to 30 cases of ITP per million doses," they added.
The team found no adverse events in relation to ITP, clotting or bleeding in their analysis for the Pfizer-BioNTech vaccine.
The results, published in the journal Nature Medicine, also showed people most at risk from ITP tend to be older -- a median age of 69 years old -- and had at least one underlying chronic health problem such as coronary heart disease, diabetes, or chronic kidney disease.
"This very small risk is important, but needs to be seen within the context of the very clear benefits of the vaccines and potentially higher risks of these outcomes in those who develop Covid-19," said Professor Aziz Sheikh, Director of the University of Edinburgh's Usher Institute.
But, the recipients of the Oxford-AstraZeneca vaccine should be made aware of the slight increased risks of ITP, the experts noted while also stressing that the risk of developing these disorders from Covid-19 is potentially much higher.
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