In a Boston homeless shelter, of 147 COVID-19 infections, 88% had no symptoms. In a Tyson Foods Poultry Plant – of 481 infections, 95% had no symptoms. Prisons in Arkansas, NC, Ohio, and Virginia: of 3,277 infections, 96% had no symptoms. The CDC estimates that 40% of all COVID-19 cases are asymptomatic.
Why are do many people with COVID-19 infections have no symptoms? It could be many things: genetics, location, strain of virus, pre-existing immunity. One hypothesis: memory T-cells that came from childhood vaccinations or exposure to common flu (which is a “less evil cousin of SARS-CoV-2”) are able to give people partial immunity.
There’s been a lot of focus on Ab testing to screen for COVID-19, however Abs only stick around for a couple of months, but T-cells (memory T-cells specifically), will stick around for years. Unfortunately, Memory T-cells are a lot harder to examine as we want to see how reactive memory-T cells are to virus.
Scientists at La Jolla Institute in San Diego tested blood samples containing T cells from 2015-2018 (before COVID-19 existed) to see how reactive the T-cells were to the virus. They found that 40-60% of T cells would respond to COVID-19.
1. Macrophage/Dendritic cell (professional APCs) eat the virus and chop it up and display parts of it on their class II MHC proteins to T cells.
2. T cells get activated, go find a B cell to activate and stimulate
3. Activated B cell becomes a Plasma cell: Plasma cells crank out antibodies
4. Antibodies exist in blood stream for a couple of months while the virus is fought off.
Interestingly, this 40% value matches up with the 40% figure from the CDC for the ratio of people who have a partial natural immunity to the virus.
Should we screen for T-cell response? We can, however it’s a lot more difficult than screening for Abs. Have to isolate T cells, see if they are expressing certain proteins on their surface or secreting proteins when they are in close proximity to the SARS-CoV-2 virus. Takes a while…
Do existing vaccines help prevent COVID-19?
People who got a pneumonia vaccine in the recent past appeared to have a 28 percent reduction in coronavirus risk. Those who got polio vaccines had a 43 percent reduction in risk.
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