Thread by @3DiMMUNE

#Antibody response usually protects against infection/re-infection but #Tcells could protect against clinical signs of disease. #COVID19

The paper from #1990 analyzed the antibody response and clinical symptoms of the common cold #coronavirus #229E. Neutralizing antibody titers declined within 1 year after 1st challenge, and 70% from the "immunized" cohort got re-infected but none showed clinical symptoms.

This most likely explanation is that clinical symptoms were controlled by virus-specific T cells. There is an assumption here that a protective level of antibody would have prevented re-infection in the first plays

of course it is possible that a low level of antibody found after 1 year is still high enough to control clinical symptoms but unlikely. presence of T cell memory is a more realistic scenario.

ncbi.nlm.nih.gov/pmc/articles/P…

So does it tell us? if we generalize this study to #COVID19 we could conclude that people positive for the virus but showing no/little clinical symptoms may harbor T cells "specific" for some antigens from #COVID19 but have no neutralizing antibodies.

since #COVID19 is a new virus people cannot have truly specific T cells to it but the presence of cross-reactive T cells could be the answer. These asymptomatic individuals most likely harbor cross-reactive memory T cells recognizing #COVID19 with sufficient accuracy

to prevent clinical signs of viral infection but still getting infected due to a lack of antibody titers against #COVID19. So, it also means that simply checking for antibody titers to determine who will be protected against #COVID19 would miss the larger population

naturally protected against #COVID19. And this is not just an idea. Here is another paper about the 2003 SARS coronavirus. They found that ~around 15% of healthy individuals never known to be exposed to #SARS harbored memory T cells against it.

jimmunol.org/content/175/1/…

Now, #COVID19 is less virulent than #SARS and available data already indicate that ~25% or even higher percentage of #COVID19 infected individuals show no/little symptoms but harbor virus. We could conclude that reason #COVID19 spread so fast compared to #SARS

is that a large part of the "naive" population (> 15%) already has memory (cross-reactive) T cells against #COVID19 that prevent clinical symptoms but they still carry a sufficient amount of infectious virus to spread it (until they too develop neutralizing antibodies).

of course, it will be really important to know what induces those cross-reactive T cells that can see #COVID19. It is another viral infection, pathobionts, or even commensals?