A peer-reviewed paper published in Nature today using subjects from Singapore provided three key findings:
- All of the 36 subjects who had recovered from mild or severe COVID-19 had virus-specific T-cells.
- All of the 23 subjects who had recovered from SARS 17 years ago still produced T cells specific to SARS, and in all cases they were cross-reactive for SARS-CoV-2, the coronavirus that causes COVID-19, and capable of expanding and activating in response to protein fragments from SARS-CoV-2.
- Half of the 37 subjects who were unexposed had existing T cell immunity to COVID-19, that could not be explained by exposure to coronaviruses that cause the common cold and might instead be explained by exposure to other coronaviruses harbored by animals.
The first two findings taken together provide hope that T cell immunity to COVID-19 could be long-lasting. This is particularly hopeful given that some reports have suggested antibodies can wane in many people after a couple of months. Indeed, if recovered SARS patients can maintain cross-reactive immunity to COVID-19 for 17 years, it is quite hopeful that recovered COVID-19 patients could maintain immunity for years.
I find the third finding particularly intriguing. 26 of these subjects had their blood drawn before July 2019, making them the least likely to have had any exposure to COVID-19, which is not known to have emerged until later in the year. The other 11 tested negative for COVID-19 antibodies and were not known to have been exposed. I feel less confident that they were truly unexposed, and I wish they reported data for these two groups separately. However, the number of people with T cell immunity to COVID-19 in this group was 19, meaning a minimum of 8 people sampled before July 2019 had the immunity. Furthermore, the specific protein fragments the T cells responded to were different in almost all of the immune people in the unexposed group when compared to those who had recovered from SARS or COVID-19. Therefore, it seems likely that almost everyone in the group had their immunity through some other source than COVID-19 exposure.
They investigated the T cell responses in more detail in a subset of the unexposed group. The primary protein fragments these T cells responded to were not derived from exposure to the common cold. The fragments had very low similarity to the corresponding protein fragments in any of the common cold coronaviruses, and exposure of the T cells to common cold protein fragments was unable to activate the T cells or expand their populations.
The authors noted, by contrast, that these protein fragments are very similar across multiple coronaviruses that infect animals rather than humans. They did not provide an analysis of which animal viruses were most likely the source of immunity, but they cited a paper comparing coronavirus similarity across animals such as camels, civets, horses, pigs, hamsters, ferrets, mice, and rats. The paper doesn't cover cats or dogs, but they appear to be able to get COVID-19.
Deliberate exposure to animals should not be used for COVID-19 prevention, given that this hypothesis is barely beyond speculation at this point and that animal viruses can cause illness. Some animals can even transmit COVID-19.
However, this raises the intriguing possibility that pet ownership or an agrarian lifestyle could provide immunity to COVID-19 through exposure to related coronaviruses in animals.
Regardless of the source of immunity, these findings suggest that half of us are already immune to COVID-19 without exposure. We shouldn't take this suggestion too far. Just because the T cells activate and multiply in response to COVID-19 doesn't mean a person can't get the illness, and we would need a much larger random sample across multiple populations to be certain of how many people have this T cell immunity. Still, it provides one reason to think that a large number of people would have existing immunity that might at least protect them from a severe illness and may even prevent them from getting sick upon exposure. The nature of this existing immunity deserves a lot more study.
Stay safe and healthy,
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* The term “preprint” is often used in these updates. Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.