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The immune system can retain a memory of infection long after a person recovers from a viral illness. Immune cells and proteins that circulate in the body can identify and kill the virus if it is encountered again.
Long-term immunity depends upon several components:
Antibodies, circulating proteins in the blood, recognize viruses and destroy them.
Killer T cells, a type of lymphocyte, recognize and kill pathogens including viruses.
B cells make new antibodies when needed.
A recent study by Washington University researchers, published in the journal Nature, suggested that mild cases of Covid-19 leave lasting antibody protection and that repeated infections of Covid-19 are unlikely. B cells multiply rapidly during infection, but, as patients recover, most B cells die off and blood antibody levels drop. Some B cells, known as long-lived plasma cells, remain in the bone marrow, secreting antibodies, and are ready to respond against a new exposure. These plasma cells will likely confer lasting immunity.
In a recent study at the National Institutes of Health that analyzed Covid-19 cases of varying severity, 95% of participants had 3 or more of the 5 immune-system components studied when measured for up to 8 months after infection.
Based upon these results, it appears that Covid-19 infection does confer lasting immunity in most infected individuals. Both groups of researchers agree that it remains to be seen if vaccination also induces the immune responses needed to provide long-term immunity. Vaccine-related studies are ongoing.