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Two major adenoviral-vectored vaccines—Johnson & Johnson and AstraZeneca—have been paused because of blood clots. Is there a connection?

This article was published on
April 14, 2021

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SciLine reaches out to our network of scientific experts and poses commonly asked questions about newsworthy topics. Reporters can use these responses in news stories, with attribution to the expert.

SciLine reaches out to our network of scientific experts and poses commonly asked questions about newsworthy topics. Reporters can use these responses in news stories, with attribution to the expert.

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Expert Comments: 

Dave O’Connor, PhD

This is the million (or billion) dollar question. It is tempting to link these events since both vaccines are based on similar platforms, but more data will be needed to know for sure. I think it is essential to remember that some serious, rare side effects are often linked to large-scale vaccine efforts. The Yellow fever vaccine, one of the most successful in history, causes roughly one-in-a-million adverse neurological outcomes. So two things can be true at once: the Johnson & Johnson and AstraZeneca vaccines offer lifesaving potential for ending the pandemic and should continue to be used aggressively throughout the world, while a very small number of people who receive them could be adversely impacted. Figuring out how to identify those at risk of serious complications and how to manage their care is what I hope comes out of a (hopefully brief) safety pause.

Beth Kirkpatrick, MD

Based on what we know today, these two vaccines appear to be associated with this phenomenon, which appears thus far to be an autoimmune phenomenon related to antibodies directed against a component of the blood that helps to normally clot the blood (platelets). Exactly how and why this is happening, and if any individual is predisposed, is not yet understood. The Russian vaccine, Sputnik, is another adenovirus-vectored vaccine. Less is known about the Sputnik vaccine.

Darryl Falzarano, PhD

A very small number of people who have received adenovirus-based vaccines appear to have a coagulation disorder that is comparable to a rare clotting syndrome observed as a reaction to treatment with heparin. This has not been observed with other vaccine platforms as of yet, which may suggest that this is not being caused by the SARS-CoV-2 spike antigen and is in fact due to the adenovirus vector; however, it is important to note that this is a very small number of people and a causal relationship has not been established. There are other adenovirus-vectored vaccines, including the Sputnik V vaccine from Russia and the CanSino vaccine from China. I have not seen reports of blood clots with these vaccines, but am unsure how many have been vaccinated with these platforms, and you need to vaccinate 4 million people or so be able to detect rare events at the frequencies observed with the Johnson & Johnson or AstraZeneca adenovirus platforms.

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