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Coronavirus | After blood clots, Canada limits AstraZeneca vaccine to those over 55

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Following reports of an extremely rare adverse reaction to the AstraZeneca vaccine which causes blood clots, several countries have set age limits for the use of the vaccine and Canada is the latest to follow suit.

Health Canada, a Canadian federal department responsible for helping citizens maintain and improve their health, on March 29 recommended that the AstraZeneca vaccine be used only on people above 55 years. This guidance was given after scientists in Europe said they had identified a mechanism that was responsible for the vaccine causing blood clots in rare cases.

Earlier on March 24, Health Canada had issued a label change and guidance on the AstraZeneca vaccine following reports from European studies of rare but serious events of blood clots associated with low platelet count (thrombocytopenia). The Canadian federal body has asked the manufacturers to conduct an assessment of the risks and benefits of the vaccine by age and gender in Canada. Until then, it has recommended pausing the use of the vaccine on people under 55 years of age.

Canada is not the first country to recommend an age limit for use of the vaccine. According to a report in Science, France has restricted the use of this vaccine to people aged 55 and higher; the age limit is 65 in Sweden and Finland and 70 in Iceland — the rationale being this is the age group most likely to suffer the severest impact of Sars Covid-2 infection.

Puzzling combination

The combination of symptoms — blood clots and low platelet count, sometimes with bleeding — is puzzling, because normally it is the blood platelets that help in clotting of blood and therefore they would be present in large numbers when blood clots.

Researchers said in a preprint posted on Research Square that these unusual symptoms actually resemble a rare side-effect of the blood thinner heparin which is known as heparin induced thrombocytopenia (HIT). In this context, researchers Greinacher et al draw a parallel to HIT and call this vaccine-induced prothrombotic immune thrombocytopenia (VIPIT) and say it may be treated in a manner similar to treatment of HIT.

The symptoms include shortness of breath, chest pain, leg swelling, persistent abdominal pain, sudden onset of severe or persistent worsening headache, or blurred vision or skin bruising other than at the site of vaccination, and onset by four to twenty days of vaccination.

Need to vaccinate

Given the wide rollout of the AstraZeneca vaccine in India, some scientists believe the decision of the Canadian Agency need not be emulated.

According to Vineeta Bal, retired scientist from the National Institute of Immunology, Delhi, and currently with IISER Pune, “Given that the number of infected [in India] is increasing rapidly, and that there are not very many reports of reinfections, but new people getting infected, there is an urgent need to vaccinate the people. This [VIPIT] is a very rare side-effect and when crores of people around the world have been vaccinated, this particular adverse reaction has been seen in about 30 people. I hope our government does not take a cue from others and issue a directive, as that would only confuse people.”

While other countries have a choice of several vaccines, India has only two right now. There is also no data on such adverse effects in India. “I would recommend both vaccines equally and not worry about decisions made by other countries which are based on a risk-benefit analysis,” Prof. Bal adds.