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Once people recover from a virus infection, the immune system remembers it. The immune cells and proteins that circulate in the body can recognize and kill the pathogen if it is encountered again, thus protecting against disease and reducing the severity of the disease.
This long-term immune protection involves several components. Antibodies – proteins that circulate in the blood – recognize foreign substances like viruses and neutralize them. Different types of T cells help recognize and kill pathogens. B cells make new antibodies when the body needs them.
All of these immune system components have been found in people recovering from SARS-CoV-2, the virus that causes COVID-19. But the details of this immune response and its duration after infection are not clear. Scattered reports of reinfection with SARS-CoV-2 have raised concerns that the immune response to the virus is not sustainable.
To better understand the immune memory of SARS-CoV-2, researchers led by Drs. Daniela Weiskopf, Alessandro Sette and Shane Crotty of the La Jolla Institute of Immunology analyzed the immune cells and antibodies of nearly 200 people who had been exposed to SARS-CoV-2 and recovered.
The time since infection varied from six days after symptom onset to eight months later. More than 40 participants had been recovered for more than six months before the start of the study. About 50 people provided blood samples more than once after infection.
The research was funded in part by the NIH’s National Institute of Allergy and Infectious Diseases (NIAID) and the National Cancer Institute (NCI). The results were published on January 6, 2021 in Science.
Researchers found lasting immune responses in the majority of people studied. Antibodies to the SARS-CoV-2 spike protein, which the virus uses to get inside cells, were found in 98% of participants one month after symptom onset. As seen in previous studies, the number of antibodies varied widely between individuals. But, which is promising, their levels have remained fairly stable over time, dropping only slightly 6 to 8 months after infection.
The virus-specific B cells increased over time. People had more memory B cells six months after the onset of symptoms than a month after. Although the number of these cells appears to plateau after a few months, the levels did not decrease during the study period.
The levels of T cells for the virus also remained high after infection. Six months after the onset of symptoms, 92% of participants had CD4 + T cells that recognized the virus. These cells help coordinate the immune response. About half of the participants had CD8 + T cells, which kill cells infected with the virus.
As with antibodies, the number of different types of immune cells varied widely between individuals. Neither sex nor differences in disease severity can explain this variability. However, 95% of people had at least 3 out of 5 immune system components that could recognize SARS-CoV-2 for up to 8 months after infection.
“Several months ago our studies showed that natural infection elicited a strong response, and this study now shows that responses last,” Weiskopf says. “We hope that a similar pattern of long-lasting responses will emerge for vaccine-induced responses as well.”
This research report was first published on the National Institutes of Health website on January 26, 2021.
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