As the COVID-19 vaccination process progresses, the question becomes increasingly pressing: How long will immunity last us?
When the immune system comes into contact with an antigen for the first time, it takes a few days for the components of the specific response to be fully activated. In addition, this primary response does not reach the full power of which the immune system could be capable, and that is why we sometimes succumb to infections.
However, as a result of this encounter, memory cells are generated, which have a long life and which store the information on how to destroy the antigen. If we meet him again, the secondary response will be much faster, more powerful and more efficient thanks to the activation of those memory cells.. That is why we vaccinate, to generate memory cells that are capable of controlling this pathogen if infection occurs through contagion.
Do coronaviruses generate memory?
We know yes, because there is four coronavirus causing about 20% of common colds, as well as two other serious illnesses: SARS (severe acute respiratory syndrome, released in 2003) and MERS (Middle East severe acute respiratory syndrome, released in 2012).
The memory against cold-causing coronaviruses is not very powerful, and that is why we get sick so frequently, in addition to the fact that there are other unrelated viruses that also produce it. And, as for SARS, we know that the antibodies in people who had the disease decreased rapidly and were barely detectable two years later, while the antibody-producing memory cells (B lymphocytes) disappeared before 6 years of age, so that from then on there would be a lack of protection.
However, recent studies have found neutralizing antibodies 17 years after infection. Therefore, fears that immunity to SARS-CoV-2 was also short-lived were justified.
Long-lived plasma cells
If we do a test, we probably still have antibodies against typical childhood diseases, such as measles or mumps, even though many years have passed since we suffered from that disease and we have not had contact with the antigen again. How is this possible, considering that the activation of memory cells requires a new encounter with the pathogen? How can antibodies last so long?
Well, because, in addition to memory cells, we have another important ally to protect ourselves. When the B lymphocyte is activated after recognizing the antigen, it becomes a cell, called a plasma cell, which is the one that actually produces antibodies.
Most of these cells die when the infection ends, and they are called short-lived plasma cells. But on certain occasions other very peculiar cells are generated that are found in some special niches in the bone marrow, and what are the calls plasma cells long life. Sometimes, of eternal life.
During all that time, they would be producing antibodies that would neutralize a new infection, such as occurs with rubella, infectious mononucleosis, mumps or measles. That is why we do not suffer from these diseases again.
Long-lived memory and plasma cells in COVID-19
Although logically we still do not know how long it will last exactly immunity against SARS-CoV-2, the outlook is now more promising than a few months ago, thanks to a host of findings.
On first place, It was found that anti-SARS-CoV-2 antibodies remained in the serum of patients who had suffered from the disease for at least 8 months, and that they were decreasing at a slower rate than initially feared.
On second Instead, the antibody-producing memory cells were very active and at very high levels throughout those 8 months, so it could be assumed that they would confer protection for a few years.
Very recent studies have raised this protection to at least 12 months with an apparent selection towards those more efficient memory cells. And what is more important: this protection was markedly increased in individuals who had passed the disease and who had subsequently received a dose of vaccine. Yet another reason to get vaccinated.
On third place, in those subjects who, due to having suffered a mild illness, did not find these memory B cells, they did present a very robust response in charge of memory T cells, responsible for cellular immunity. That is, not everything is the antibodies.
On fourth place, the response to vaccines induces a powerful formation of plasma cells in the so-called germ centers, a fundamental requirement for these memory B cells to be produced. So far, all good news.
But there is more. The researchers were surprised that the decrease in the concentration of antibodies after suffering the disease had two phases: a first, in which they decayed rapidly, and another after which they remained stable. This pattern suggested that long-lived plasma cells might be responsible for maintaining these antibodies.
The hypothesis was proven correct, since it was possible to isolate and purify these long-lived plasma cells, which had found their niche in the bone marrow, 11 months after suffering the disease. Great news. And it is because it tells us that, in addition to having a vigorous long-term response of memory T and B cells, we will also have plasma cells that will be producing antibodies against the virus for probably many years.
Dark clouds on the horizon: the new variants
Does this mean that we will never have to get vaccinated again? Probably not, although only time will tell. It is quite possible that booster doses will need to be given to boost immunity at some point if it is observed to decline. And of course, all this immunity is generated against the original virus, which is the content of the vaccines that we are administering.
We cannot exclude the emergence of new variants, sufficiently different from the original, so that they manage to escape our memory cells, which only remember what they have already seen. And, in this case, it will be necessary to administer vaccines directed against these new variants.
For this reason, and despite the current climate of greater optimism in the scientific community, we cannot lower our guard. We are going to live with the virus for many years, so we will have to monitor it closely. History cannot be repeated.
Originally published on The Conversation EN.Ignacio J. Molina Pineda de las Infantas, Professor of Immunology, Center for Biomedical Research, University of Granada.