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No extra blood clot risk after second dose of AstraZeneca COVID-19 vaccine: Study

The research, published in The Lancet on Tuesday, shows that the rates of the very rare clotting disorder, thrombosis with thrombocytopenia syndrome (TTS), following a second dose of the vaccine are comparable to those among unvaccinated population.

July 28, 2021 / 05:09 PM IST

The AstraZeneca COVID-19 vaccine is associated with a small risk of rare blood clots after the first dose, and no extra risk after the second shot, according to a study led and funded by the British-Swedish drugmaker.

The research, published in The Lancet on Tuesday, shows that the rates of the very rare clotting disorder, thrombosis with thrombocytopenia syndrome (TTS), following a second dose of the vaccine are comparable to those among unvaccinated population.

TTS is a very rare syndrome which occurs when a person has blood clots (thrombosis) as well as low platelet counts (thrombocytopenia).

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It is also referred to as vaccine-induced immune thrombotic thrombocytopenia (VITT).


COVID-19 Vaccine

Frequently Asked Questions

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How does a vaccine work?

A vaccine works by mimicking a natural infection. A vaccine not only induces immune response to protect people from any future COVID-19 infection, but also helps quickly build herd immunity to put an end to the pandemic. Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease, making the spread of disease from person to person unlikely. The good news is that SARS-CoV-2 virus has been fairly stable, which increases the viability of a vaccine.

How many types of vaccines are there?

There are broadly four types of vaccine — one, a vaccine based on the whole virus (this could be either inactivated, or an attenuated [weakened] virus vaccine); two, a non-replicating viral vector vaccine that uses a benign virus as vector that carries the antigen of SARS-CoV; three, nucleic-acid vaccines that have genetic material like DNA and RNA of antigens like spike protein given to a person, helping human cells decode genetic material and produce the vaccine; and four, protein subunit vaccine wherein the recombinant proteins of SARS-COV-2 along with an adjuvant (booster) is given as a vaccine.

What does it take to develop a vaccine of this kind?

Vaccine development is a long, complex process. Unlike drugs that are given to people with a diseased, vaccines are given to healthy people and also vulnerable sections such as children, pregnant women and the elderly. So rigorous tests are compulsory. History says that the fastest time it took to develop a vaccine is five years, but it usually takes double or sometimes triple that time.

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Rare cases of TTS have been reported after immunisation with the AstraZeneca vaccine, known as Covishield in India, which led to several countries restricting or stopping the use of the preventive.

The authors demonstrated that the estimated rate of TTS following a second dose of AstraZeneca was 2.3 per million vaccinees, comparable to the rate observed in an unvaccinated population.

The rate was 8.1 per million vaccinees after the first dose, they said.

The analysis was conducted using AstraZeneca’s global safety database, which captures all spontaneously reported adverse events from real-world use of its medicines and vaccines worldwide.

Reported cases of TTS globally were included up to the cut-off date of April 30 this year occurring within 14 days of administration of the first or second dose of AstraZeneca vaccine.

The results are in line with recent reports in the Medicines and Healthcare products Regulatory Agency (MHRA) Yellow Card Report, the UK system for collecting and monitoring information on safety concerns, which also show low rates of TTS after a second dose, AstraZeneca said.

"No specific risk factors or definitive cause for TTS following COVID-19 vaccination have been identified and AstraZeneca continues to perform and support ongoing investigations of potential mechanisms,” the company said in a statement.

“Furthermore, these very rare events can be avoided when symptoms are identified and treated appropriately," it said.

Co-developed by the University of Oxford, the AstraZeneca vaccine is based on a weakened version of a common cold virus (adenovirus) that causes infections in chimpanzees.

The preventive contains the genetic material of the SARS-CoV-2 virus spike protein, which the virus uses to enter and infect the human cells.

After vaccination, the surface spike protein is produced, priming the immune system to attack the SARS-CoV-2 virus if it later infects the body.

According to its manufacturers, the AstraZeneca vaccine has been granted emergency use in more than 80 countries across six continents.

Follow our full coverage of the coronavirus pandemic here.
first published: Jul 28, 2021 05:07 pm
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