A study published in the journal Cell points to a vigorous and lasting immune response even after antibody levels to the virus fade.
Researchers examined blood samples from patients who recovered from COVID and found memory T cells in virtually all patients who had recovered from the infection, and many who were merely exposed or who had an asymptomatic infection. A full 100% of samples from those who had a severe infection showed such memory T cells, 87% of those with a mild case, and even 67% of those merely exposed to an ill family member. COVID specific memory T cells were present in 46% of healthy blood donors who had no recognized exposure to the virus.
Memory T cells form a part of the long term immune response that allows you to mount a vigorous response to a virus you may have previously had months or years prior.
Takeaway quote from the summary of the atudy:
“Importantly, SARS-CoV-2-specific T cells were detectable in antibody-seronegative exposed family members and convalescent individuals with a history of asymptomatic and mild COVID-19. Our collective dataset shows that SARS-CoV-2 elicits robust, broad and highly functional memory T cell responses, suggesting that natural exposure or infection may prevent recurrent episodes of severe COVID-19.”
And translating to more plain English…
Importantly, T cells specific to the Coronovirus were detectable in people who had been exposed to family members or sick people and who had even an asymptomatic and or mild case of COVID-19. Our data shows that the Coronavirus elicits robust, broad and highly functional memory T cell responses, suggesting that natural exposure or infection may prevent recurrent episodes of severe COVID-19.
Net result, recurrent severe infections seem less likely according to this study.
Will this be something that can be tested for? It seems like the antibody tests will give an incomplete picture of who has immunity, and also who has been exposed, since the antibodies disappear after a relatively short time. T-cells seem like they would give a much more complete picture.
The short answer is no, we will not have a high-throughput routine clinical test for cell-mediated immunity like the antibody test.
(Disclaimer - I’m not a expert in this specific field, but I have some knowledge of it.)
Antibodies recognize viral proteins in their natural state, and antibodies are small. So the antibody test immobilizes viral protein antigen on a surface, then any antibodies in the patient sample that are specific to the viral protein antigen will stick to that surface where they can be detected. (Google ELISA test for more detail)
Unlike antibodies, T-cell receptors recognize processed segments of viral protein in complex with MHC molecules, for interesting reasons that I talked about here:
So, there are two challenges. First, the thing that T-cell receptors recognize is significantly more complex than just a viral protein in its raw state, so you have to make that, and you also have to label it with a fluorescent tag because of the second problem… You’re looking for cells, which are much larger than antibodies and cannot be immobilized on a surface in the same way. I’m not aware of any T-cell assay that does not involve flow cytometry, a much more complex experimental technique.
The fact that these experimental techniques are more difficult is the reason that it’s taken time to produce this research on cell-mediated immunity.
T-cell-based immunity IS the way immunity-through-exposure generally works, isn’t it? As opposed to having active antibodies against hundreds of pathogens in our bloodstreams at all times?
Doubtful that one will be developed for this commercial purpose …
The study of the op has been in the news for a while from preprint. It implies that twice as many people may have had infections than antibody prevalence studies suggest, and that many have some level of pre-existing protection, but with small numbers to base it on. A big deal if true but needs replication(s) and further evaluation as to how strong the protection is in the wild.
Why do you think this would not be practical/useful for our virus? It obviously requires that cells remain viable in the sample to process and present antigen, so more care and complexity with sample processing…is it just a question of reliability? I’m wondering why it hasn’t popped up even in any research papers on cell-mediated immunity that I’ve seen.
I’m no expert either but I suspect getting that to work with any reasonable specificity in a not TB context is tricky and that it’s clinical utility long term will be limited.
So after this thing is endemic, we might see academic studies to determine the level of immunity, for herd immunity purposes, but I won’t be able to find out if the illness I had back in February was actually Covid-19?
The referenced study involved several researchers from various locations. Notably many are with the Karolinska Institutet in Sweden, a location often noted for their affiliation with awarding the Nobel Prizes.
The procedures they used in assessing T cell levels are not easily adapted to a mass testing program. It seems unlikely that such tests would be quickly rolled out on an individualized basis so you are unlikely to be able to be tested to see if you, as an individual, have developed responsive T cells.
But such tests of representative samples could be carried out on a more limited basis to provide insight into population level trends. Repeated over time such studies might show increasing levels of coronavirus reactive T cells in the population as a whole giving rise to an assumption of reaching herd immunity faster. But if such levels do not substantially increase over time it would raise many more questions about how our immune systems respond to this virus.
Even if the exact test was widely available it would only tell you so much. Some have specific T-cell protection from before the virus existed. Is someone positive from before or from infection? Some from infection without antibody positivity and some with. How much less risk are each at? Complete protection? Less severe infections likely? No one yet knows.