Explained: Why Canada has stopped use of AstraZeneca vaccine for those below 55 years

Canada’s National Advisory Committee on Immunization (NACI) has recommended that the AstraZeneca Covid-19 vaccine not be used for individuals below the age of 55.

The committee stated that until a type of an adverse event associated with the vaccine is investigated, its usage should for those below the age of 55 be stopped. Even so, individuals above the age of 55 can continue getting the AstraZeneca vaccine in Canada, considering that the adverse event is more common in people below the age of 55, the committee said.

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In the past few weeks, several European countries temporarily paused the administration of the Oxford-AstraZeneca vaccine after reports that some recipients developed blood clots. In response, the WHO has maintained that the benefits of the vaccine outweigh the risks and has recommended the use of the vaccine continue.

So, why has Canada taken this step?

Canada has decided to suspend the usage of AstraZeneca because of an adverse event referred to as Vaccine-Induced Prothrombotic Immune Thrombocytopenia (VIPIT). “A combination of thrombosis and thrombocytopenia, in some cases accompanied by bleeding, has been observed very rarely following vaccination with the AstraZeneca COVID-19 Vaccine,” Health Canada said in a release.

This adverse event has led to rare cases where people have experienced the formation of serious blood clots. This is because VIPIT is associated with the creation of antibodies that can activate platelets, which then stimulate the formation of clots and result in thrombocytopenia (a condition characterised by lower than normal levels of platelets in the blood).

Symptoms to look out for this adverse event include shortness of breath, chest pain, leg swelling, persistent abdominal pain, neurological symptoms including sudden onset of severe or persistent worsening headaches or blurred vision, skin bruising (other than at the site of vaccination) or petechiae (unraised, round red spots under the skin caused by bleeding).

So far, NACI has observed that the cases associated with VIPIT have been primarily among women under the age of 55. However, cases among men have also been reported and the adverse event occurs most commonly between 4 and 16 days after the vaccine has been administered.

How can the vaccine lead to VIPIT?

It is not clear how exactly the vaccine triggers VIPIT and NACI has said the adverse event has not been associated with an mRNA vaccine till date. The AstraZeneca vaccine is not an mRNA vaccine. mRNA (messenger RNA) vaccines work by encoding instructions that inform the body’s cells to create the spike protein of the SARS-CoV-2 virus. The creation of this spike protein should then trigger the immune system to develop antibodies to fight the disease.

The AstraZeneca vaccine, on the other hand, uses a weakened version of a common cold virus called the adenovirus from chimpanzees. Once injected, this weakened version of the virus which has been made to resemble the coronavirus (which does not cause people to fall ill) should trigger an immune system response and lead to the creation of antibodies.

NACI says that the rate of this adverse event is still under investigation. As per information given by the European Medicines Agency (EMA), as of March 18 the adverse event was occurring in 1 per 1,000,000 people vaccinated with the AstraZeneca vaccine. However, the Paul-Ehrlich Institut in Germany reported a rate of 1 per 100,000.

What are the latest efficacy findings of the AstraZeneca vaccine?

According to The University of Oxford and AstraZeneca, interim results from phase 3 clinical trials conducted on 32,000 participants across the US, Chile and Peru show that the vaccine had an efficacy of 79 per cent against symptomatic Covid-19. More importantly, the efficacy in the cases of severe or critical symptomatic Covid-19 was 100 per cent.

This means that the possibility of getting symptoms of Covid-19 reduced by 79 per cent in those vaccinated in these trials compared with those who weren’t vaccinated. It also means that the vaccine was able to keep everyone who was inoculated with it from developing severe and critical symptoms that would require hospitalisation.