Memory T cells
A review of some recent medical studies shows that memory T cells may play a role in a phenomenon known as cross-reactivity, which researchers found may perhaps give certain people, in some cases, some level of immunity to the virus that causes COVID-19 even without them having been exposed to or infected with the virus, or having received a vaccine. There are certain structural and clinical similarities between the virus that causes COVID-19 and the other coronaviruses that are related to it, which cause SARS and MERS.
Sources: MSN