COVID Immunity: It’s All About Those T-Cells

One worry during the COVID-19 pandemic is whether those who had a mild of the virus would get it again, only worse. People also wondered whether having the virus could confer lasting immunity.

According to a study from the Karolinska Institutet, Stockholm, Sweden, and the Technical University of Denmark, Lyngby, T cells produced in response to the new coronavirus may give some immunity to recurring, severe reinfections with COVID-19. The study, in preproofs for the journal Cell, demonstrated that once a person has had COVID-19, their T-cells “remember” the virus.

The death rate for COVID-19 is estimated to be 0.5% to 3.5%. According to the study authors, most people are affected less severely, remaining either asymptomatic or developing only mild symptoms.

The researchers analyzed SARS-CoV-2-specific T-cell responses among (1) people who were never exposed to the virus, (2) exposed family members, and (3) those who had active COVID-19. They found T-cell activation to be a “hallmark of acute COVID-19.”

When people had active COVID-19, their T cells were mostly of a type specifically associated with being actively sick. In people who were recovering, most of the T cells were of a multifunctional type. These T cells had a “memory” of the virus. This kind of T cell was found even in exposed family members with no antibodies to the virus and in people recovering from the virus who had only mild COVID-19.

The study concluded that exposure to the new coronavirus elicited “robust, broad and highly functional memory T cell responses.” The T-cell responses suggest that either being exposed to the virus or being infected with it could prevent an individual from getting recurring, severe episodes of COVID-19.

Earlier studies in rhesus macaques found that being infected with SARS-CoV-2 generated “near-complete protection” against recurrence of the virus. Another study in humans didn’t find enough evidence to suggest that humans could be reinfected with the SARS-CoV-2 once they had it a first time.

The study authors were not willing to state definitively that the aggressive memory T-cell responses they saw would  completely protect an individual without detectable, circulating antibodies from reinfection with severe COVID-19. However, results of studies of the MERS and SARS-CoV-1 viruses (“cousins” to SARS-CoV-2) suggest this is probably the case. For both MERS and SARS-CoV-1, robust memory T-cell responses continued even as antibodies fell, a normal phenomenon after coronavirus infection. The fact that aggressive memory B cell and T cell responses form after infection with SARS-CoV-2 suggests that those who get the virus may have some immunity against severe reinfection.

None of the recovering individuals from the study were reinfected with COVID-19, even those who had a mild case. The fact that many study subjects with asymptomatic or mild COVID-19 generated robust memory T cell responses further indicates that exposure to or infection with the new coronavirus could prevent it from recurring severely.

This is good news both for herd immunity and with regard to lasting results from a vaccine, should one be approved and disseminated widely.

Disclaimer: This article does not provide medical advice. Do not take action based solely on this article and always consult with an appropriate healthcare professional. This article is purely for informational purposes.

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