After recovering from a virus infection, the immune system retains “memory” of that virus, that is, it has mechanisms and special cells that remember the initial contact with the invader.
These immune cells and antibodies (the specific proteins of the immune system that circulate in the blood and other body fluids) can identify and kill the pathogen if they come across it again. With this mechanism we are protected from many diseases or even the severity of the disease is reduced if absolute protection cannot be provided.
This long-term immune protection involves various parts of the immune system, which is very complex. Thus, it includes antibodies but also different types of T-lymphocytes that help identify and kill pathogens. On the other hand, B lymphocytes are the ones that create and produce new antibodies when the immune system needs them.
All of these cooperating parts of the immune system have been found to play an essential role in people recovering from SARS-CoV-2. However, the details of this immune response as well as how long the immunity lasts after the initial infection are rather unclear. In addition, there are widespread reports of re-infection with SARS-CoV-2 that have raised reasonable concerns that the immune response to the virus may not be long-lasting.
To better understand immune memory against SARS-CoV-2, researchers from La Jolla, USA, analyzed multiple strands of the immune system, including specific cells and antibodies in 254 samples from 188 cases (80 men, 108 women) of COVID- 19. Most had a mild illness, only 7% needed hospitalization. The samples included 43 samples taken at least ≥ 6 months after the initial infection with the virus. The time from infection ranged from six days after the onset of symptoms to eight months later. About 50 people donated blood samples more than once after the initial infection.
The researchers generally found that the immune responses in the majority of the subjects studied were resistant over time. Antibodies to the SARS-CoV-2 protein spike, which the virus uses to enter cells, were found in 98% of participants one month after the onset of symptoms. As observed in previous studies, the number of antibodies varied widely among individuals. However, it is very important that the levels of IgG immunoglobulin against the pin protein remained relatively stable for at least 6 months or more and began to show a relatively small decrease after 6 to 8 months of infection. An additional 88% of subjects had detectable antibody levels at 6-8 months.
Spike B protein lymphocytes (those that “remember” and take on immediate antibody production if they come in contact with the spike protein) increased over time, and most had more B memory lymphocytes six months after the onset of symptoms than that a month later. Although the number of these cells appeared to reach a plateau after a few months, their levels did not decrease during the period under study, nor did there appear to be any decreasing trend.
CD4 + T lymphocytes and CD8 + T lymphocytes specific for SARS-CoV-2 also remained high after infection although they gradually decreased with a half-life of approximately 3-5 months. Six months after the onset of symptoms, 92% of participants had CD4 + T lymphocytes that recognized the virus. These cells help coordinate the immune response. Also, about half of the participants had CD8 + T lymphocytes, which kill virus-infected cells. From the study of the kinetics of antibodies, memory B lymphocytes, CD4 + and CD8 + T-lymphocytes, the researchers observed that each strand of immune memory versus SARS-CoV-2 showed distinct kinetics. As with antibodies, the number of different types of immune cells varied considerably between individuals. Neither sex nor differences in the severity of the disease explain these differences between individuals in terms of antibody levels or specific cell numbers. However, in 95% of the subjects in the study at least 3 out of 5 key parts of the immune system could recognize SARS-CoV-2 up to 8 months after the initial infection. Thus, researchers are optimistic that a similar pattern of responses that will last over time may also occur after vaccination.
The research was partly funded by the National Institute of Allergy and Infectious Diseases of the NIH (NIAID) and the National Cancer Institute (NCI) and the results were published in January 2021 in the journal Science.
The data on the duration of immune memory against SARS-CoV-2 are summarized by the Professors of the Therapeutic Clinic of the Medical School of the National and Kapodistrian University of Athens, Evangelos Terpos, Efstathios Kastritis and Thanos Dimopoulos (Rector: EKPA m) .gr / covid19 /).
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