Since the coronavirus disease 2019 (COVID-19) pandemic began, scientists have been keen to know if primary infection in recovered individuals yields long-lasting immunity against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19.
A team of researchers at the University of Washington School of Medicine has found that people who had mild COVID-19 developed a multi-layered immune response against SARS-Cov-2. The researchers detected neutralizing antibodies against the virus for at least three months after recovering from the infection.
Specifically, the team found virus-specific memory B and T cells displaying anti-viral immunity. Memory B cells (MBCs) increase in number and express antibodies capable of neutralizing SARS-CoV-2.
In the study, published in the journal Cell, the researchers recruited 15 individuals, with an average age of 47 that developed mild symptoms lasting for about 13 days. The participants entered the study when the inflammation that accompanied acute infection was resolved. A control group has been included, consisting of healthy people who had no evidence of COVID-19 infection.
The researchers assessed SARS-CoV-2-specific immune responses at one and three months after the onset of symptoms in people with mild COVID-19.
They collected plasma and peripheral blood mononuclear cells from the participants. The first blood sample was obtained at least 20 days after a positive COVID-19 test. The team collected the second blood sample about 86 days post-symptoms onset to determine the number of long-lived memory cells.
The study revealed that in the participants who had mild symptoms, coronavirus-neutralizing antibodies persisted for at least three months after recovery. These neutralizing antibodies bind to the virus and block infection.
In particular, the study participants generated virus-specific memory B and T cells, which recognize a pathogen when it tries to infect the body again.
Furthermore, persistent memory lymphocytes display hallmarks of protective antiviral immunity, including a numerically increased population of virus-specific memory B cells capable of expressing SARS-CoV-2 neutralizing antibodies,” the researchers explained.
An antibody, also known as an immunoglobulin, is a protective protein the immune system produces in response to a foreign agent, called an antigen. Antibodies recognize and bind with the antigen to remove them from the body.
After infection, most infection-fighting cells die, but the memory B cells increase in number. These cells stay dormant in the body but are positioned to reactivate immediately when re-infection occurs. This way, they can rapidly disarm the virus to prevent infection and spread.
In the coronavirus pandemic, though a vaccine is required to attain herd immunity, understanding if natural infection induces immunological memory can help control the virus spread
Hence, understanding how the immune system attacks the pathogen can help scientists determine if the developed vaccines will work.
It is critically important to understand if SARS-CoV-2 infected individuals who recover from mild disease develop functional immune memory cells capable of protecting them from subsequent SARS-CoV-2 infections, thereby reducing transmission and future COVID-19 disease,” the researchers said.
They also added that one of the study limitations is the cohort size is small, making it hard to correlate patient characteristics with sustained immune memory. Hence, they suggested that there is a need for further investigation. They plan to continue the study to determine how the immune memory responses may change over time.
The vaccine race
Many countries have already rolled out their vaccination plans. The United States and the United Kingdom have begun vaccinating healthcare providers, front-liners, and high-risk groups.
The World Health Organization reports that there are 235 candidate vaccines against COVID-19. Of these, 63 are undergoing clinical evaluation, while 172 are in pre-clinical development.
- Rodda, L., Netland, J., Shehata, L., Campbell, D., Rawlings, D., Pepper, M., et al. (2021). Functional SARS-CoV-2-Specific Immune Memory Persists after Mild COVID-19. Cell. https://doi.org/10.1016/j.cell.2020.11.029, https://www.cell.com/cell/fulltext/S0092-8674(20)31565-8#%20