COVID-19 immunity

Protective immunity against COVID-19 shown to be robust in a recent study.

As new hospital admissions for people with COVID-19 continue to decrease (1), many locations are beginning to ease restrictions and return life to normal. Immunity to COVID-19 has increased due to vaccinations and natural immunity after infection. However, questions remain as to how long either immunity lasts (2).

In healthy adults, immunity is achieved no matter how an infection occurs – through vaccination or through naturally getting sick (3). Doctors classify immunity as either acquired or natural. Vaccination and antibody transfer result in acquired immunity, while infection /recovery and genetic transfer, such as through breastmilk, result in natural immunity (4).

Depending on the type and severity of infection, immunity can be strong or weak (4). Early in the COVID-19 pandemic, research suggested that natural immunity to COVID-19 lasted only two to three months (5). This caused concern that natural immunity for patients with mild COVID-19 might later be re-infected.

To determine whether a mild COVID-19 infection produced long-lived immunity, researchers from the Washington University School of Medicine in St. Louis conducted a study of the immune responses of 77 patients with mild COVID-19 and eleven people with no history of COVID-19 (6).

During an infection, the body attacks antigens through a variety of means. Initially the body’s innate immune response sends immune cells to surround and kill antigens no matter what type of infection.

Then the body’s adaptive immune response creates B-cells which make high levels of antibodies specially designed to fight that antigen, and T-cells to attack infected cells. B-cells also produce plasma cells that make antibodies.

Some of these B- and T-cells will become memory cells which will recognize that antigen (7). Long-lived plasma cells also remain in the body’s bone marrow secreting low levels of the antibody to guard against future infection (6).

There are different types of antibodies that serve different functions. One is immunoglobulin G (IgG), which coats microbes and speeds their uptake by the immune system (4). Another is Immunoglobulin A (IgA), which are concentrated in the mucosal membranes and guard the body against infection (4).

In the study, blood samples were collected at one, four, seven, and eleven months after COVID-19 symptoms began. Bone marrow samples were also taken from eighteen of the patients seven to eight and eleven months after infection.

The blood samples were analyzed for the presence of COVID-19 IgG, IgA, and bone marrow plasma cells (BMPC). In the samples taken during the first few months after infection, the IgG and IgA levels were high, as expected. In the later samples, the antibody levels dropped and leveled off in most patients.

Seventy-nine percent of the BMPC samples contained antibody producing cells, and levels of memory B cells were like those detected in flu patients. The study suggested that the immune response to COVID-19 infection follows the normal natural immunity pattern (6).

The study did have some limitations, however. Researchers did not detect BMPCs in four samples, suggesting the levels of those cells may be lower than the limit of detection. Also, most of the patients studied experienced mild cases of COVID-19. It is unknown if the response would be different for more severe COVID-19 infections.

Regarding severe infections, first author Dr. Jackson Turner said in a press release, “It could go either way. Inflammation plays a major role in severe COVID-19, and too much inflammation can lead to defective immune responses. But on the other hand, the reason why people get really sick is often because they have a lot of virus in their bodies, and having a lot of virus around can lead to a good immune response. So it’s not clear. We need to replicate the study in people with moderate to severe infections to understand whether they are likely to be protected from reinfection.”


  1. CDC. COVID Data Tracker. Centers for Disease Control and Prevention. Published March 28, 2020. Accessed May 26, 2021.
  2. Goldberg C, Pollak A. Bloomberg – Are you a robot? Published May 24, 2021.
  3. Dinerstein C. Is Natural Immunity or Vaccination Better? American Council on Science and Health. Published March 19, 2021. Accessed May 26, 2021.
  4. U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES, NATIONAL INSTITUTES OF HEALTH, National Institute of Allergy and Infectious Diseases, National Cancer Institute. Understanding the Immune System How It Works. NIH; 2003. Accessed May 26, 2021.
  5. Long Q-X, Tang X-J, Shi Q-L, et al. Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections. Nature Medicine. 2020;26. doi:10.1038/s41591-020-0965-6
  6. Turner JS, Kim W, Kalaidina E, et al. SARS-CoV-2 infection induces long-lived bone marrow plasma cells in humans. Nature. Published online May 24, 2021. doi:10.1038/s41586-021-03647-4
  7. Bucher K. The Immune Response. Accessed May 26, 2021.
  8. Image by Gerd Altmann from Pixabay 
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