Several studies have shown that once our T cells have been exposed to a pathogen, they can respond to infections and strains that are closely linked to that original pathogen.
They can be called upon to battle different strains of viruses that cause the flu, for example. They can also be effective against viruses of the same family, like the dengue virus and Zika virus (Zika fever) which are from the same family.
This means if you've ever had dengue fever, then that exposure would have primed your T cells to battle Zika virus, too. (Of course, there is no guarantee that the defence that the T cells mount will be enough to protect you entirely, but will be good enough to give you a running start against the pathogen.)
The same is true for coronaviruses also.
A study published in the scientific journal Nature in July 2020 found that CD4 and CD8 T cells were not only present in the bodies of people who had had a cold because of a coronavirus, but their T cells were also able to identify the SARS-CoV-2 virus if it entered their body.
In fact, the study also found that patients who had recovered from severe acute respiratory disease (SARS)—which caused an outbreak in 2002-03—also had memory T cells for coronaviruses present in their bodies, 17 years on.
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However, the study went on to add that it wasn't only the patients who had either recovered from SARS or those who were infected with COVID-19 that had memory T cells present in their systems. Donors who were part of the study and had never contracted either infection, nor had they come into contact with any SARS or COVID-19 patient, also had SARS-CoV-2 specific T cells present in their systems.
Further, the uninfected donors in the study had a different immune response than COVID-19 patients—their T cells were identifying other, related coronaviruses that cause the common cold or beta-coronaviruses which cause SARS and MERS (and now COVID-19). While the researchers deduced that it is the presence of these specific T cells that can protect people against COVID-19, it is still unknown how these T cells help some people to avoid COVID-19 infection.
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Another study published in the journal Cell in June 2020 also detected the CD4 cells (CD4 test) in about 40% to 60% of people who had never been exposed to COVID-19, also suggesting that specific T cells are already present in response to common cold-causing coronaviruses.
Another study published in the journal Science this month also suggests that T cells that are routinely called upon by the body's immune system to protect against common cold-causing coronaviruses are also able to identify the SARS-CoV-2 virus through memory.
All the above-mentioned studies are in line with research that has been ongoing to find out about lasting immunity to COVID-19 around the world, but the findings remain hypothetical until concrete evidence linking T cell responses to older coronaviruses is firmly established.
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