For some time I and others have noted that large numbers of people who are or must have been exposed to the current coronavirus strain don’t get infected. Why? It could be a dose issue; people almost certainly vary in the amount of virus they need to be exposed to in order to become infected. But even more likely is that at least some people have a pre-existing immune defense that acts against this strain. That would be very possible because we all are constantly exposed to coronavirus. A new paper published in the journal Cell supports that notion by finding cross-reactive T-cells in a very significant portion of the population. (Cell Paper) T-cells come in several varieties. They are called T-cells because they develop in the thymus. Two of the most important types are helper T-cells, which aid B cells in getting antibody production going, and killer T-cells, which destroy cells with foreign substances, like virus, in them.
The researchers looked at development of both of these types of T-cells in patients with the disease and those who had been unexposed. They used 20 recently recovered patients and tested blood samples collected in 2015-18, before this epidemic began. In regard to helper T-cells, all ten patients who had the disease showed T-cell activity against both the spike protein and other portions of the virus. Killer T-cell responses were found in 70% of the patients. Antibody response was correlated with the level of T-cell response.
But of most relevance was that 50% of blood donors had detectable T-cell response to the non-spike portions of the current strain, as well as a lesser response to the spike area. The helper T-cell cross-reactivity was more common than killer T-cell cross-reactivity. All the donors showed some immune reactivity to other common coronavirus strains. The authors concluded that around 40% to 60% of unexposed individuals may have existing immune defenses that function against COVID 19. These findings are consistent with the experience in influenza, where substantial cross-reactivity exists among strains, and are correlated with less severe disease.
This is a small study, but there is no reason not to suspect that the results would hold and they are consistent with other research.