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Researchers think this may have to do with the immune system's memory of fighting other types of coronavirus than COVID-19.
COVID-19 is the seventh human coronavirus that has been discovered, and four of the human coronaviruses are what we call community-acquired coronaviruses.
Together, those four are responsible for 25% of our common colds.
Almost every person in the world has had some encounter with a coronavirus, and since they are all part of the same family, there is some cross reactive immunity that develops.