We know that the immune system is critical in the human body’s response and resolution to viral infections. It appears our long-term immunity to COVID-19 depends on the T cell. T cells are key part of the immune system (cellular immunity) that rejects anything that isn’t “self.” The CD4 cell (also known as the helper T cell) sits at the top of the immune hierarchy and directs the other members of the workforce. Each T cell is highly specific and can remain in the bloodstream for many years. If antibody levels fall over time, the T cells remember the “enemy” virus and can quickly produce antibodies if presented with the same virus once again. Researchers have discovered that some individuals have T cells capable of recognizing antigens from COVID-19 without having been infected nor ever having been exposed to either of the previous widespread coronaviruses SARS or MERS. Researchers were equally shocked to find T cells sensitized to COVID-19 surface proteins in blood specimens that had been drawn years before the onset of the pandemic. It appears some individuals may have had a degree of pre-existing resistance prior to the pandemic. Resistance was found to be surprisingly prevalent: some 40-60% of unexposed individuals had such T cells. The T cells present had a slightly different pattern of reactivity (responding to different surface proteins) in some cases compared to those individuals who recovered from a COVID-19 infection. The prevalence of sensitized T cells in a substantial part of the population could be explained by cross-reactivity with proteins from previous “common cold” coronaviruses. Another possible explanation is exposure to naturally occurring animal coronaviruses. The ability to retain T cell immunity could explain why falling antibody levels (often observed in convalescent patients) does not result in patients being re-infected with the virus. Although antibody levels are an important measure of “cure” it may not be the most important factor in permanent immunity.
As the work on a COVID-19 vaccine continues and intensifies, it is critical to produce a vaccine that induces rapid antibody production to protect us in the short term and provides long-lasting immunity. Recent research has shown that T cell activation and recruitment is the key to long-term immunity. Individuals who were infected by the SARS virus in 2003 were shown to have reactive T cells to the SARS CoV-2 virus (17 years later). With the addition of this recent knowledge more focus should be placed on T cell activation for long-term protection. As the vaccine candidates reach the final stages of development it will be interesting to see which vaccine or vaccines are mass produced and if patients will be allowed the option to choose based on an informed choice and not on a government mandate.