Several recent articles report that the immune response to the coronavirus responsible for the Covid-19 is very good and that we can therefore be optimistic about the chances of developing resistance to the disease naturally or by vaccination.
The immune system plays a critical role in protecting us from the myriad pathogens that are present in the world in which we live. It is therefore not surprising that the vast majority of viral infections induce the development of protective immunity(1).
The effectiveness and durability of this immune response, however, can vary significantly from one virus to another, and to know the degree of protection against a new virus (as is the case with the coronavirus SARS-Cov-2), it is thus necessary to determine the extent to which the viral agent may enable different parameters of the immune response.
The first important parameter is of course the production of antibodies. These defense proteins produced by B lymphocytes are used to fight the infection, focussing specifically on certain regions (epitopes) of the virus to prevent it from entering cells and reproducing.
The data collected until now in patients who have survived the Covid-19 indicate that the infection has actually led to the production of neutralizing antibodies and that these prevent the entry of the virus by blocking its interaction with the receptor ACE2. The reactivity of some of these antibodies is excellent, and several teams are trying currently to produce in large quantities to treat the infected individuals(2). It also appears that these antibodies are produced even when the infection is mild and does not cause major symptoms : a study conducted by the Pasteur Institute with 160 people working in the hospital, and who showed mild symptoms of the Covid-19 showed the presence of antibodies in almost all (99 %) of these individuals(3). These antibodies are also able to neutralize the virus, suggesting that they may protect patients from another infection in the future if this immune response is sustained.
This duration of response is not yet known, but the results obtained in non-human primates (rhesus macaques) are promising. It has been observed that the infection of monkeys by the SARS-Cov-2 caused a strong immune response, characterized by the presence of several neutralizing antibodies. After to be able to eliminate the virus and cure of the disease (viral pneumonia), the monkeys reexposed to the virus a few weeks later showed a strong resistance to infection, indicating that antibodies generated during the initial infection were always effective(4).
T Lymphocytes in reinforcement
We talk a lot about antibodies, but another class of lymphocyte, the CD4 T-cells (auxiliary) and CD8 (killer), is also absolutely essential for the establishment of immune memory, long-term. These cells have several functions, one of them being to help the clones of B cells, producers of antibodies, to establish a sustainable way to be able to be quickly reactivated in case of future infection by the same infectious agent. The degree of activation of T lymphocytes, therefore, determines in large part if the immune response to a virus may maintain in the time.
The good news is that it seems that this is the case for the coronavirus current : researchers in the us have shown that people who had been affected by the Covid-19 produced more CD4 cells recognizing the protein in the peak external of the SARS-CoV-2, indicating an adequate immune response in the face of the virus(5). This T-cell activation bodes very well for the production of a vaccine against the Covid-19, since these cells accelerate strongly the production of antibodies essential to the effectiveness of the immunization.
The researchers also found that 30 % to 50 % of the population that has not been infected by the SARS-CoV-2 occur in spite of all of the T lymphocytes against the virus. This immune response cross would probably be due to previous infections by other coronaviruses, including those responsible for colds benin, given that these viruses have some similar proteins in coronavirus current. It would seem, therefore, that a significant portion of the population can fight against the SARS-CoV-2 thanks to this residual immunity from colds previous : even without antibodies, killer T-cells eliminate cells— are infected and can therefore control the infection.
Overall, it therefore appears that the immune response to the coronavirus that is fast, robust, and involves all the cellular systems needed to obtain optimal immunity. Therefore, it is appropriate to be optimistic on the development of natural immunity in the long term against this virus and, by extension, on the potential to develop a vaccine effective against the Covid-19.
- (1) Sallusto F et al. From vaccines to memory and back. Immunity 2010; 33: 451-463.
- (2) Wu Y et al. A noncompeting pair of human neutralizing antibodies block COVID-19 virus binding to its receptor ACE2. Science, published on may 13, 2020.
- (3) Fafi-Kremer S, et al. Serologic responses to SARS-CoV-2 infection among hospital staff with mild disease in eastern France. medRxiv, published on 22 may 2020.
- (4) Chandrashekar A and al. SARS-CoV-2 infection protects against rechallenge in rhesus macaques. Science, published on may 20, 2020.
- (5) Grifoni A et al. Targets of T cell responses to SARS-CoV-2 coronavirus in humans with COVID-19 disease and unexposed individuals. Cell, published may 20, 2020.