The Hindu Explains | How safe is AstraZeneca’s COVID-19 vaccine?

What has the European Medicines Agency found after a probe and why should side effects be monitored?

Updated - March 24, 2021 07:12 pm IST

Published - March 21, 2021 05:05 am IST

The story so far: Safety concerns around the Oxford-AstraZeneca vaccine led several European countries to temporarily stop administering the shot last week. On Thursday, the European Medicines Agency (EMA) cleared the vaccine as safe for use for mass administration, saying the “benefits” of the vaccine in combating COVID-19 “continue to outweigh the risk of side effects”.

What were the concerns in Europe?

Starting with Denmark, sporadic reports started to emerge of blood clots in a small number of recipients of the Oxford-AstraZeneca vaccine.

Also read | AstraZeneca vaccinations resume in Europe after clot scare

The vaccine is a recombinant one, i.e., it is formed by mixing genetic material from different organisms — in this case from an adenovirus that causes common colds and affects chimpanzees and the spike protein from the coronavirus. It was expected that some recipients of the vaccines would experience side effects such as rashes, swellings and flu-like symptoms. During the trials, some had experienced even serious reactions such as transverse myelitis, or an inflammation of the spinal cord. However, the decision to roll out a vaccine was premised on the overall benefits. Germany, too, reported such “thrombotic events” and experts noted that some clots were of a very rare kind, located in the brain’s veins that prevented efficient draining of the blood. Some were accompanied by a deficiency of blood platelets.

How many such cases were observed?

AstraZeneca said as of March 8, it had received reports of 37 instances of blood clots after 17 million inoculations across the EU and the U.K. This was well below the number of cases of blood clots — 1,000 to 2,000 — reported every day in the general population, as per studies based on the U.S. population. It was also unclear whether it was the vaccine itself that caused the clots.

What was the final verdict after the investigation?

After investigation, the EMA reported that the “vaccine is not associated with an increase in the overall risk of blood clots (thromboembolic events) in those who receive it”. It said there was no evidence of problems related to specific batches of the vaccine or particular manufacturing sites. However, the EMA said the vaccine “may be associated with very rare cases of blood clots associated with thrombocytopenia, i.e., low levels of blood platelets (elements in the blood that help it to clot) with or without bleeding, including rare cases of clots in the vessels draining blood from the brain”. Even before its report, the EMA and the World Health Organization (WHO) had not called for a halt to vaccination .

Also read | Post-vaccine blood clots a form of rare cerebral thrombosis: European Medicines Agency

On Thursday, the EMA said it had concluded its investigation and declared the vaccine safe. “This vaccine is safe and effective in preventing COVID-19, and its benefits continue to be far greater than its risks,” said Dr. Sabine Straus of the EMA’s Pharmacovigilance Risk Assessment Committee. Dr. Straus said because the vaccine is effective in preventing COVID-19, which in itself is a cause of blood clots, vaccination likely reduces the risk of thrombotic events overall.

While the EMA ruled that vaccination did not increase the overall risk of clots, there were “uncertainties”. Some very rare case reports described “specific, unusual events” of a combination of blood clots, low blood platelet levels, and bleeding. There were also instances of tiny clots developing in multiple blood vessels in the first 7-14 days after vaccination. So, some “association” between vaccines and clots could not be entirely ruled out. In the end, the very small risk of clots far outweighed the threat from COVID-19. However, the risk must be conveyed in factsheets accompanying the vials of the vaccine and those getting vaccinated must be told to seek medical care if they fall ill after vaccination.

Also read | European Union threatens AstraZeneca with export ban

How does this affect India?

Covishield, which is based on the AstraZeneca vaccine, constitutes almost 90% of the over 4 crore doses so far administered in India. Trials on safety and efficacy of the vaccine are based entirely on data from the United Kingdom, South Africa and Brazil. Bridging studies (to show that a vaccine performs equivalently on Indians) have been announced but no results have been made public or are available in scientific literature. Events as rare as vaccine-linked clots in the brain are one in a million, meaning that it requires millions to be vaccinated for them to be detected, if at all.

India’s ‘Adverse Event Following Immunization’ panel has not reported adverse events of this kind. Thrombotic events following vaccination have been reported in India but they were said to be unrelated to vaccination.

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