A team of researchers sought to fill in the gaps in the understanding of immune memory after a SARS-CoV-2 infection by taking a deep dive into the immune response of patients recovered from COVID-19. To do this, they measured multiple components of the immune system, including circulating antibodies, memory B cells, and T cells specific for SARS-CoV-2, in patients with varying levels of disease, for up to eight months after infection. Their data suggests that each component of the immune system follows its own pattern post-infection.

The work is published in Science in the paper, “Immunological memory to SARS-CoV-2 assessed for up to eight months after infection.”

The collaborative team from La Jolla Institute for Immunology (LJI) and the department of microbiology, Icahn School of Medicine at Mount Sinai, studied antibody and immune cell responses in more than 180 men and women who had recovered from COVID-19. They reported that these patients’ immune memory to the virus—across all immune cell types studied—was measurable for up to eight months after symptoms appeared.

The results indicate “that durable immunity against secondary COVID-19 disease is a possibility in most individuals,” the authors said. Indeed, the authors wrote: “Our data show immune memory in at least three immunological compartments was measurable in ~95% of subjects five to eight months post-symptom onset.” As the number of daily COVID-19 cases worldwide continues to mount, whether an initial infection with SARS-CoV-2 leads to long-lasting protective immunity against COVID-19 remains a question.

Studying the nature of the humoral response to the virus, which includes an antibody response, and of the cellular immune response, which includes B cells and T cells, over periods of six months after symptoms start could help inform protective immunity’s duration.

To do this, Jennifer Dan, MD, PhD, clinical associate at LJI, and colleagues recruited more than 180 men and women from the United States who had recovered from the disease. The majority had had mild symptoms that did not require hospitalization, though 7% were hospitalized. Most subjects provided a blood sample at a single time point, between six days and eight months after symptoms took hold, though 43 samples were provided at six months or more following symptom onset.

In 254 total samples from 188 COVID-19 cases, Dan and colleagues tracked antibodies, B cells, and two types of T cells. IgG to the Spike protein was relatively stable over 6+ months and only exhibited modest declines at six to eight months after symptom onset. Spike-specific memory B cells were more abundant at six months than at one-month post symptom onset. T cells, meanwhile, showed only a slight decay in the body. More specifically, SARS-CoV-2-specific CD4+ T cells and CD8+ T cells declined with a half-life of 3–5 months.

While the authors caution that “direct conclusions about protective immunity cannot be made on the basis of [their findings] because mechanisms of protective immunity against SARS-CoV-2 or COVID-19 are not defined in humans,” they also say that several “reasonable interpretations” can be made from their study. These include support for resting immune memory compartments potentially contributing “in meaningful ways to protective immunity against pneumonia or severe secondary COVID-19,” the authors wrote.

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