The exact mechanisms of immune protection to SARS-CoV-2 are still unclear and researchers around the world continue to attempt to address this critical question. In an article now published in The Journal of Immunology, Dr. Tania Watts from the University of Toronto, unravels the characteristics and extent of B cell and T cell immune responses after recovery from COVID-19, finding that there was evidence of protective immunity persisting nine months following the onset of symptoms. The profile of these COVID-19 immune responses are also compared to long-term immunity from a common respiratory viral infection: influenza A.

The current study examined the characteristics of T cell responses in 24 individuals who had recovered from COVID-19 by taking blood samples at two time points over a 9-month period.

Key points:

  • SARS-CoV-2 induced robust T cell and antibody responses in most recovered individuals.
  • SARS-CoV-2 induces anti-viral responses with some features that are distinct from those seen in long-term immunity to influenza A.

The researchers used peripheral blood mononuclear cells from 24 COVID-19-recovered individuals to study T cell responses against SARS-CoV-2 and compare them with those mounted against the influenza A virus. While all subjects responded to at least one SARS-CoV-2 antigen, there were distinct differences in T cell responses between SARS-CoV-2 and influenza. For instance, SARS-CoV-2 induced a lower CD8:CD4 T cell ratio and a lower ratio of IFN-γ:IL-2-producing CD4+ T cells compared to influenza. Recovery from influenza also induced more multi-functional T cells, which are generally associated with a more effective antiviral response. Likewise, when looking at the cytokine response to SARS-CoV-2, they detected a lower ratio of IFN-γ:IL-6 and an altered profile of cytotoxic molecules such as granzyme or perforin when cells were stimulated with SARS-COV-2 peptides, which might indicate an altered anti-viral mechanism.

Finally, when analyzing antibody responses, they found that nearly 95% of recovered subjects mounted an IgG response against the SARS-CoV-2 spike protein, with only 79% and 53% mounting responses against the virus’ receptor binding domain (RBD) and nucleocapsid, respectively. In general, antibody responses to spike lasted longer than those against RBD and the nucleocapsid.

This study compared differences in immune responses between a new infection with SARS-CoV-2 and long-term immunity to influenza A virus. Their findings demonstrated discernable differences in the immune response against each virus. Researchers observed the continued presence of immune activity against SARS-CoV-2 9-months after symptom onset among individuals who have had COVID-19. Further monitoring is necessary to better understand the long-term immune response against SARS-COV-2 infection and how it changes with vaccination.

Law JC, Girard M, Chao GYC, Ward LA, Isho B, Rathod B, Colwill K, Li Z, Rini JM, Yue FY, Mubareka S, McGeer AJ, Ostrowski MA, Gommerman JL, Gingras A-C, Watts TH. Persistence of T Cell and Antibody Responses to SARS-CoV-2 Up to 9 Months after Symptom Onset. J Immunol. 2021 Dec 13. doi: 10.4049/jimmunol.2100727.