London: The Oxford-AstraZeneca vaccine, also known as Covishield, may be associated with a very small risk of developing a blood condition characterised by low platelet counts, according to a nationwide study in the UK.
The increased risk of the condition, known as idiopathic thrombocytopenic purpura (ITP), is estimated to be 11 per million doses, similar to figures seen in vaccines for flu, and measles, mumps, and rubella (MMR), the researchers said.
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, they said.
The team, led by researchers at the University of Edinburgh in the UK, noted that people at most risk from ITP tended 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.
The researchers were unable to establish a definitive link between other forms of clotting -- including the rare form called cerebral venous sinus thrombosis or CVST -- due to the very low number of cases in vaccinated people included in the study.
The study of 5.4 million people in Scotland, of whom 2.5 million had received their first vaccine dose, is the first analysis of ITP, clotting and bleeding events following vaccination for an entire country.
The authors of the study, published in the journal Nature Medicine on Wednesday, emphasised that the increased chance of developing ITP after receiving the vaccine remains smaller than the risk of developing it because of COVID-19.
They noted that the rare risk should not deter the roll out of the vaccine programme.
The same risk of ITP was not found for the Pfizer-BioNTech vaccine. Other vaccines were not included in the study.
The researchers analysed a dataset as part of the EAVE II project, which uses anonymised linked patient data to track the pandemic and the vaccine roll out in real time.
They investigated data up to April 14, 2021 for people in Scotland who had received the first dose of either vaccine.
By this date, over 1.7 million had an Oxford-AstraZeneca jab and some 800,000 had a dose of the Pfizer-BioNTech vaccine.
Experts recommend that recipients of the Oxford-AstraZeneca vaccine should be made aware of the slight increased risks of ITP, but also stress that the risk of developing these disorders from COVID-19 is potentially much higher.
UK's Medical and Healthcare products Regulatory Agency (MHRA) had previous reported low platelet counts in combination with blood clots following vaccination with the AstraZeneca vaccine, estimated to occur at a rate of approximately 13 per million first doses.
The researchers, including those from the universities of Strathclyde, Aberdeen, Glasgow, Oxford, Swansea and St Andrew's, also looked at health records dating back to September 2019 to investigate any previous issues with ITP, clotting or bleeding disorders.
The data was then compared with those who were yet to be vaccinated to determine if any clotting events were outside what would have been expected pre-pandemic.
The analysis indicated that there was a slight increase in ITP in the second week following vaccination for those who received the AstraZeneca vaccine and possibly also increased risk of arterial clotting and bleeding events.
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