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Rare clot incidents linked to J&J, AstraZeneca COVID-19 vaccines bring adenovirus vector platform under scanner

Both Sputnik V and J&J used a type of adenovirus called AD26. AstraZeneca-Oxford University's COVID-19 vaccine also uses chimpanzee adenovirus to deliver the antigen.

April 15, 2021 / 08:22 PM IST
Johnson & Johnson's COVID-19 vaccine

Johnson & Johnson's COVID-19 vaccine

The Gamelya Centre, the Russian research lab that developed Sputnik V COVID-19 vaccine, on April 14 issued a statement stating that a comprehensive analysis of adverse events during clinical trials and over the course of mass vaccinations with the Sputnik V showed that there were no cases of clots or cerebral venous sinus thrombosis (CVST).

The statement comes in the background of US pausing the use of Johnson & Johnson’s COVID-19 vaccine following cases of six women developing a rare blood-clotting disorder. The Centers for Disease Control and Prevention (CDC) panel is investigating the matter. Sputnik V is based on the similar adenovirus vector platform, which J&J vaccine also uses.

In fact, both Sputnik V and J&J used a type of adenovirus called AD26. AstraZeneca-Oxford University's COVID-19 vaccine also uses chimpanzee adenovirus to deliver the antigen, consisting of S-protein. AstraZeneca-Oxford University's COVID-19 vaccine, too, has come under scrutiny in Europe for rare events of clots.

These events; however, rare has brought the entire adenovirus vector platform under the spotlight.

The Gamelya defended the use of adenovirus vector platform. It says all of the above vaccines based on adenoviral vectors have significant differences in their structure and production technology and felt that there is no reason and no justification to extrapolate safety data from one vaccine to safety data from other vaccines.

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COVID-19 Vaccine

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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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It said that the vaccine from J&J uses human adenovirus serotype Ad26 and full-length S-protein stabilised by mutations. In addition, it is produced using the PER.C6 cell line (embryonic retinal cells), which is not widely represented among other registered products.

"Sputnik V is a two-component vaccine in which adenovirus serotypes 5 and 26 are used. A fragment of tissue-type plasminogen activator is not used, and the antigen insert is an unmodified full-length S-protein. Sputnik V vaccine is produced with the HEK293 cell line, which has long been safely used for the production of biotechnological products," Gamelya said.

"The quality and safety of Sputnik V are, among other things, assured by the fact that, unlike other vaccines, it uses a 4-stage purification technology that includes two stages of chromatography and two stages of tangential flow filtration. This purification technology helps to obtain a highly purified product that goes through mandatory control including the analysis of free DNA presence," Gamelya added.

The company quoted a study published in The New England Journal of Medicine on April 9, 2021, that discussed the cause of the clots or thrombosis in some patients vaccinated could be due to insufficient purification that leads to the emergence of significant quantities of free DNA.

Insufficient purification or use of very high doses of target DNA/RNA can result in adverse interaction of a patient’s antibodies that activate thrombocytes with elements of the vaccine itself and free DNA/RNA, which can form a complex with the PF4 factor.

A leading virologist of vaccine company who declined to be named told Moneycontrol that adenoviruses are benign, easy to engineer and safe, therefore, becoming first choice as vectors for vaccine developers.

"The adenovirus vectors have been in use to develop vaccines for Ebola, but it is still a new platform, we are coming to know about the risks only after the mass vaccination with the vaccines based on this platform," the virologist above said.

Meanwhile, China's CanSino Biologics, which also uses the human adenovirus platform, tried to distance itself from the controversy stating that it had not found any serious blood clot cases in people inoculated with its single-dose COVID-19 vaccine.
Viswanath Pilla is a business journalist with 14 years of reporting experience. Based in Mumbai, Pilla covers pharma, healthcare and infrastructure sectors for Moneycontrol.
first published: Apr 15, 2021 08:20 pm

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