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New Delhi: Pfizer and Moderna vaccines generate more robust long-term populations of T cells capable of recognizing multiple variants of the SARS-CoV-2 virus in individuals who had been infected with covid-19 prior to vaccination compared to those who had never been infected, a new research showed.

The research was published in the scientific journal eLife, on Wednesday and was carried out by Gladstone Institutes. “Overall, our data support the idea that vaccines are eliciting a very robust T cell response in healthy individuals. But they also suggest there may be some ways to improve them further, by getting more of the vaccine-elicited T cells to park themselves in the respiratory tract," said Gladstone Associate Investigator Nadia Roan, senior author of the study.

The scientists explained human immune system has two main arms that help fight returning infections. Antibodies produced by the immune system’s B cells can quickly recognize a virus, target it for destruction, and prevent infection. T cells, on the other hand, identify and destroy already infected cells. While antibodies are more effective at stopping initial infection completely, T cells generally last longer after an initial infection or vaccine and can help quell disease in its early stages, preventing severe symptoms. T cells, however, are diverse and difficult to study, said the scientists.

“It’s not just the number of T cells that matter; It’s the quality—whether the T cells are the type that can actively destroy virus-infected cells," said Roan, associate professor of urology at UC San Francisco who used a technology called CyTOF that can measure the levels of nearly 40 different proteins on the surface and inside of T cells. The technology during the study identified exactly which subsets of T cells were able to recognize SARS-CoV-2 before and after vaccination.

The researchers used the approach to study blood samples from 11 people who had received an mRNA vaccine (either Pfizer/BioNTech or Moderna) against SARS-CoV-2. Blood samples were collected from each individual before vaccination, approximately 2 weeks after their first vaccine dose, and approximately 2 weeks after their second dose. Six of the individuals had also previously recovered from mild covid-19.

Scientists found that all fully vaccinated individuals in the study had T cells that responded to three different variants of SARS-CoV-2—the ancestral virus first detected in Wuhan, China; the B.1.17 variant first detected in the United Kingdom; and the B.1.351 variant first detected in South Africa. The Delta variant of the virus was not included in the paper but the scientists are currently analyzing data on it.

In people who had never been infected with SARS-CoV-2, researchers found that the T cell response became stronger—in both quantity and quality of T cells—after the second vaccine dose. However, in those who had previously had a covid-19 infection, there was little change between the first and second dose of the vaccine.

“This doesn’t necessarily mean there is no benefit to a second dose in convalescent individuals. It means there is no additional effect on T cells in ways that we could capture, but there may still be other effects on the immune system, such as within B cells, after the second dose," said Roan.

The researchers also found that, while all individuals had a robust T cell response, T cells from those who had previously contracted covid-19 had molecular markers suggesting the immune cells could last longer and migrate more effectively to the respiratory tract.

The potentially more effective T cell defense in the respiratory tract may explain why breakthrough infections are less frequent in people with a prior covid-19 infection compared to other vaccinated individuals. The new data, Roan said, also suggest that improving the ability of T cells to migrate to the respiratory tract after vaccination may improve the effectiveness of vaccines at preventing breakthrough infections.

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