Global pharmaceutical giant AstraZeneca has said that its AZD1222 vaccine against Covid-19, which was made under licence in India as Covishield, could cause low platelet counts and formation of blood clots in “very rare” cases.
AstraZeneca has accepted a link between the vaccine and Thrombosis with Thrombocytopenia Syndrome (TTS), a medical condition characterised by abnormally low levels of platelets and the formation of blood clots.
What has led AstraZeneca to make this submission? What is known about the vaccines and TTS? And should Indians who received Covishield shots be worried?
What exactly has AstraZeneca said?
AstraZeneca spoke about TTS in court documents following a suit brought against the company in the United Kingdom. The pharmaceutical giant is being sued over health claims related to the vaccine that it developed in collaboration with the University of Oxford amid the Covid-19 pandemic in 2020.
The UK-based media organisation The Telegraph reported recently that a “case was lodged last year by Jamie Scott, a father of two, who was left with a permanent brain injury after developing a blood clot and a bleed on the brain that has prevented him from working after he received the vaccine in April 2021”.
The report said that in all, “fifty-one cases have been lodged in the High Court (in the UK), with victims and grieving relatives seeking damages estimated to be worth up to £100 million”.
In court documents from February, AstraZeneca denied that “TTS is caused by the vaccine at a generic level”. However, it admitted to the possibility of TTS as a result of its vaccination in “very rare cases”.
What are the symptoms of TTS?
A range of symptoms are associated with TTS, including breathlessness, pain in the chest or limbs, pinhead-size red spots or bruising of the skin in an area beyond the injection site, headaches, numbness in body parts, etc. TTS could mean a restriction in the flow of blood due to clotting.
The website of Johns Hopkins Medicine says, “Thrombosis can block the blood flow in both veins and arteries. Complications depend on where the thrombosis is located. The most serious problems include stroke, heart attack, and serious breathing problems.”
Why have these concerns surfaced now, four years after the outbreak?
These concerns have in fact, been reported earlier. But this is the first time that AstraZeneca has confirmed the link between TTS and its vaccines.
Before the Covid-19 vaccines were rolled out in India, the Indian government had issued a fact sheet in January 2021, in which it specifically said that Covishield (the Indian variant of AstraZeneca’s vaccine) should be given with “caution” to individuals with thrombocytopenia, that is, a condition of low platelet counts. Covishield and Covaxin were the two Covid vaccines given to Indians.
Then, in March 2021, the European countries France, Germany, Italy, Netherlands, Denmark, Norway, Iceland, Austria, Estonia, Bulgaria, Romania, Estonia, Lithuania, Luxembourg, and Latvia temporarily paused the use of the AstraZeneca vaccine after a few cases of blood clotting were reported.
The following month, the World Health Organisation (WHO) said TTS was being reported in some cases after vaccinations with Covishield and Vaxzevria (which was the other trade name for AstraZeneca’s vaccine).
However, the WHO had added, “Based on latest available data, the risk of TTS with Vaxzevria and Covishield vaccines appears to be very low. Data from the UK suggest the risk is approximately four cases per million adults (1 case per 250,000) who receive the vaccine, while the rate is estimated to be approximately 1 per 100,000 in the European Union (EU).”
Were incidents of blood clots reported in India as well?
The Indian government said in May 2021 that 26 potential thromboembolic events, or formation of clots in blood vessels, had been reported since Covishield began to be administered.
India started administering the vaccines on January 16, 2021, and considering the total number of doses that had been administered until then, the number of potential thromboembolic events worked out to 0.61 cases per million doses, or a percentage of 0.000061.
According to more recent data, the government’s committee on Adverse Events Following Immunization (AEFI) said at least 36 cases of TTS have been vetted and confirmed to have been caused due to Covishield. The committee’s last report, which was made public in June last year, showed that of these cases 18 deaths were reported. Almost all of these TTS cases pertain to 2021 ,which was the first year of Covid-19 vaccination in the country.
The Union Health Ministry had termed this possibility as “minuscule”, and said that Covishield “continues to have a definite positive benefit-risk profile” with “tremendous potential to prevent infections and reduce deaths” due to Covid-19.
In the case of Covaxin, which was India’s indigenous vaccine manufactured by Bharat Biotech, it said “no potential thromboembolic events” had been reported.
The Ministry had added that scientific literature suggests “this risk (of blood clotting) is almost 70% less in persons of South and Southeast Asian descent in comparison to those from European descent”.
In 2023, the WHO incorporated vaccine-induced immune thrombotic thrombocytopenia (VITT) into its classification of TTS.
The Indian government said in May 2021 that 26 potential thromboembolic events, or formation of clots in blood vessels, had been reported since Covishield began to be administered. (Express file photo by Partha Paul) 
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