Study explores COVID-19 vaccination blood clots

A new study has uncovered fundamental differences in how the AstraZeneca and Pfizer COVID-19 vaccines impact the immune system — and could explain why some patients experienced blood clots.

The COVID-19 Vaccine Immune Responses Study (COVIRS), led by SAHMRI and Flinders University and published in Cell Reports Medicine, tracked the immune responses of 102 adults of varying ages, all living in South Australia where there was no community transmission of SARS-CoV-2 at the time.

Participants had their bloods taken immediately after each dose of either the Oxford/AstraZeneca or Pfizer/BioNTech COVID-19 vaccines to assess early immune response. They were also tested 28 days after every immunisation to evaluate B and T cell activity.

The vaccines are alike in the sense that they induce the immune system to recognise and protect against the SARS-CoV-2 virus, but they differ in how this is achieved.

The Pfizer vaccine uses mRNA technology to induce host cells to produce the SARS-CoV-2 Spike protein, while the AstraZeneca vaccine uses a harmless virus (a chimpanzee adenovirus) to encode the Spike protein.

Professor David Lynn, Study lead, Head of SAHMRI’s Systems Immunology Laboratory and Professor at the Flinders University College of Medicine & Public Health, used a method of biological analysis known as ‘multiomics’ to examine immune responses in many different ways in thousands of blood samples.

Lynn said this comprehensive analysis of immune responses to these different vaccines has revealed lots of new information that’ll help inform future vaccine design.

“After the first dose, we were surprised to find the Oxford/AstraZeneca vaccine elicits an unexpected memory-like response in the immune system, recognising the vaccine as if it’s something it’s seen before,” Lynn said.

“This response is targeted against the adenovirus vector in the vaccine, not the Spike protein, and the intensity of this response correlates with the expression of proteins that act as a precursor to thrombosis, or blood clotting.”

“While vaccine-induced immune thrombotic thrombocytopenia (VITT) is an extremely rare side effect associated with the Oxford/AstraZeneca vaccine that none of the participants developed during the study, this research offers a potential explanation for the connection between the Oxford/AstraZeneca vaccine and the cases of VITT that’ve been reported.”

The study also found those who’d only had two doses of the Oxford/AstraZeneca vaccine generally produced lower amounts of antibodies and less of a specialised type of T-cell that helps with antibody production; compared to those who had two doses of the Pfizer/BioNTech vaccine. However, this was rectified once they had their third booster dose of an mRNA vaccine, illustrating the importance of booster doses.

The study added evidence to the notion that COVID-19 vaccines offer some people more effective protection than others.

The reasons for this variability are not well understood, but Lynn said age is a factor, with older people generally having a lower immune response after two doses. A third booster dose was highly effective at overcoming this.

Immune responses induced immediately after vaccination predicted the subsequent B and T cell response to the vaccine measured a month later.

“One to two days after initial vaccination we measured gene expression responses in the blood, which correlated with adaptive immune responses that mediate protection 28 days later,” Lynn said.

Researchers also discovered that feeling unwell after a vaccine dose may in fact be linked to its effectiveness.

“People who showed symptoms of fatigue and fever immediately after the third dose were more likely to have better T-cell responses. T-cells play a vital role in vaccine efficacy as they can directly kill viral cells,” Lynn said.

Image credit: iStock.com/Jair Ferreira Belafacce