a very good reason¶
There's a very good reason behind a lot of peculiarities of COVID-19: dysregulation of the innate immune system. This phenomenon can explain: 1) Pre-symptomatic transmission 2) Vaccines preventing serious illness much more effectively than transmission 3) Case fatality Your immune system has 2 main pathways: innate and adaptive immunity. Innate immunity is a generalized response that happens as soon as infection is detected to wipe it out quickly. Adaptive immunity is a more targeted response, once your immune system has identified the source. Part of innate immune response involves recruitment of macrophages, specialized cells that recognize pathogen-associated molecules and engulf them. This kick-starts pathways that encourage stronger immune response, including inflammation and priming the adaptive immune system. Inflammation and other immune responses are actually responsible for most of the symptoms you experience when you get sick. Uncontrolled inflammation is also a huge factor in the death rate of a lot of diseases, especially coronaviruses. During early infection, dendritic cells and macrophages present antigens from the virus to teach your adaptive immune system what to target with antibodies. Vaccines allow your immune system to skip to this step without needing to be infected. SARS-CoV-2 has the ability to infect macrophages and suppress expression of interferon, which is normally very important for restricting viral replication in the early stages of infection. This means that for the first little while, the virus spreads without you noticing. In addition to suppressed interferon production, SARS-CoV-2-infected macrophages seem to have uncontrolled production of pro-inflammation molecules, which is likely to contribute to the multisystem inflammatory syndrome that is responsible for a lot of the mortality in COVID-19. It is possible that this ability of SARS-CoV-2 to infect macrophages and suppress the recruitment of the adaptive immune system contributes to its ability to get up to a high enough viral load to infect others while you're still asymptomatic. It also lends insight to why immunity from vaccination and prior infection does not provide as robust protection against reinfection or transmission as we see with other diseases. The adaptive immune response is slowed, so the virus gets to spread for a while. Vaccination is still pretty effective at preventing serious illness, because that adaptive response will still come, and it will be better prepared to end the infection quickly compared to a naive immune system. Sources: https://t.co/40qrNBZgur