Sunday, December 5

People Who Test Negative for Covid-19 Despite Exposure May Have “Immune Memory” | Coronavirus

We all know that person who, despite the fact that his entire family contracted Covid-19, never tested positive for the disease. Now scientists have found an explanation that shows that a proportion of people experience an “abortive infection” in which the virus enters the body but is cleared by the immune system’s T cells at the earliest stage, which means that CRP and antibody tests show a negative result.

About 15% of healthcare workers who were tracked during the first wave of the pandemic in London, England, seemed to fit this scenario.

The discovery could pave the way for a new generation of vaccines targeting the T-cell response, which could produce much more durable immunity, the scientists said.

Leo Swadling, an immunologist at University College London and lead author of the paper, said: “Everyone has anecdotal evidence of people exposed but not succumbing to infection. What we didn’t know is whether these people actually managed to avoid the virus entirely or if they cleared it naturally before it was detectable through routine testing. “

The latest study intensively monitored healthcare workers for signs of infection and immune responses during the first wave of the pandemic. Despite the high risk of exposure, 58 participants did not test positive for Covid-19 at any time. However, blood samples taken from these people showed that they had an increase in T cells that reacted against Covid-19, compared to samples taken before the pandemic took hold and compared to people who had not been. exposed to the virus at all. They also had increases in another blood marker of viral infection.

The work suggests that a subset of people already had memory T cells from previous infections from other seasonal coronaviruses that cause common colds, protecting them from Covid-19.

These immune cells “sniff out” proteins in the replication machinery, a region of Covid-19 shared with seasonal coronaviruses, and in some people this response was rapid and powerful enough that the infection was cleared at the earliest stage. “These pre-existing T cells are ready to recognize SARS-CoV-2,” Swadling said.

The study adds to the known spectrum of possibilities after exposure to Covid-19, ranging from escaping infection entirely to serious illness.

Alexander Edwards, associate professor of biomedical technology at the University of Reading, said: “This study identifies [a new] Intermediate outcome: enough exposure to the virus to activate part of your immune system, but not enough to experience symptoms, detect significant levels of virus, or elicit an antibody response. ”

The finding is particularly significant because the T-cell arm of the immune response tends to confer longer-lasting immunity, usually years rather than months, compared to antibodies. Almost all existing Covid-19 vaccines focus on preparing antibodies against the vital spike protein that helps SARS-CoV-2 enter cells. These neutralizing antibodies provide excellent protection against serious diseases. However, immunity declines over time and a potential weakness of spike-based vaccines is that this region of the virus is known to mutate.

In contrast, the T cell response does not tend to fade as quickly and the internal replication machinery it targets is highly conserved in coronaviruses, meaning that a vaccine that also targets this region would likely protect against new strains, and possibly even against entirely new strains. pathogens.

“The insights from this study could be instrumental in designing a different type of vaccine,” said Andrew Freedman, infectious disease reader at Cardiff University School of Medicine. “A vaccine that primes T-cell immunity against different viral protein targets that are shared among many different coronaviruses would complement our peak vaccines that induce neutralizing antibodies. Because these are components within the virus, antibodies are less effective; instead, T cells come into play. “

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