The goal of the COVID-19 vaccines being administered around the world is to stimulate our immune system to produce a protective response against the coronavirus, particularly by generating antibodies. These antibodies then circulate in our blood until they are necessary to attack and eliminate the coronavirus quickly from our bodies if we become infected. The speed with which the scientific and medical communities developed and tested these new vaccines has been extraordinary. However, due to this speed, questions are still pending. One of those key questions is how long the protection we receive from the vaccine or even from infection by the virus itself will last. We already know, for example, that antibody levels drop quite quickly after a covid-19 infection.

How The Immune System Remembers

The remarkable ability of our body to remember past encounters with infectious microorganisms and retain strong defenses against them is due to the phenomenon of immunological memory. This memory resides in the white blood cells known as lymphocytes, of which there are two main types T cells and B cells. When the body is faced with a new challenge, be it a new infection or a vaccine, specific T cells and B cells are recruited to meet it. The body maintains memory versions of these cells in case it encounters the same microorganism in the future. It is these B cells that are responsible for the release of antibodies into the blood. When an infection occurs or when we are vaccinated, some of those antibodies metamorphose into specialized antibody production factories, known as plasma cells.

Antibodies are proteins and, like any other protein, they will naturally break down and remove from the body in a few months at most. This is the reason why the protection of the antibodies that we receive passively, for example, from our mothers in the womb or through breast milk does not last long. For longer-term protection, we need to produce antibodies generated by ourselves. The ability of our body to maintain antibody levels after infection or vaccination is the result of two mechanisms. In the early stages, if memory B cells detect a persistent infection or a vaccine, some will continue to transform into new antibody-producing plasma cells. Once the infection or vaccine has been eliminated, memory B cells no longer replenish the plasma cell population, which decreases.

However, some can persist as long-lived plasma cells which can live for many years in our bone marrow, continually making and releasing large amounts of antibodies. Are not always generated after infection, but if they are, antibodies to a specific infection can be found in the blood long after the infection is gone. Although we still do not fully understand which immunization conditions are best for generating LLPC, the presence of these cells has been linked to certain sites in the body. For example, a group of researchers in the United States discovered that LLPCs appear to prefer the marrow of certain bones to others.

Ten years after tetanus vaccination, LLPCs were found in the bone marrow of the femur, humerus, and tibia much more frequently than in the ribs, radius, vertebrae, or iliac crest. It is not yet clear why LLPCs prefer marrow from these bones. An interesting possibility is that the answer lies in differences in the level of fat in the bone marrow.LLCs were found to be surrounded by a large number of fat or adipose cells in the bones that they prefer. This suggests that it may be the fat content of the bone marrow that affects the ability of LLPCs to move to and long-term reside in certain bones.

But even if LLPCs are not created, that does not mean that someone cannot generate more antibodies against a threat if they encounter it again in the future. As long as the person has generated memory B cells, they will recognize the threat and, once again, some will begin to transform into new plasma cells to begin the production of antibodies once again.

The Type Of Vaccine Also Affects The Durability

There are many reasons why vaccination or infection does not always provide long-lasting protection. This is due in part to individual variation in our response to a given vaccine. However, the characteristics of the vaccines also determine the nature of the response at the antibody level. One study found that although a higher proportion of people who received tetanus and diphtheria vaccines developed protective antibodies, these antibodies disappeared more quickly than those generated by vaccines against measles, mumps, or smallpox. The key difference between these inoculations is that tetanus and diphtheria vaccines contain only isolated proteins (modified versions of the toxins produced by tetanus and diphtheria bacteria), while the measles, mumps, and smallpox vaccines contain live and weakened versions of these viruses.

Some people may not produce good responses to live vaccines due to pre-existing immunity to the vaccine itself, generated because they have already had natural infection. However, those who respond well tend to hold onto their answers longer. This is partly due to the persistence of the live vaccine in the body, which stimulates short-term replacement of plasma cells. vaccines are also likely to be better at producing LLPC. We have already seen that the rate at which antibodies break down after a COVID-19 infection can differ, for example, between men and women.

Many of the new vaccines for COVID-19 are based on novel delivery methods, such as viral vectors or messenger RNA molecules. Clearly, these mechanisms are very effective in providing quick protection. But it remains to be seen how well memory B cells and LLPCs will activate to impart long-lasting immunity.