I am vaccinated against COVID-19 but I have tested negative for antibodies: why is it possible, but not worrisome

In current diagnostic tests, such as the antigen test or the PCR, false positives are not possible. However, in antibody tests or serological tests, these false positives can occur. A false positive result can give you the false sense of security that you are protected.

It can also happen that we test negative for antibodies even though we have passed COVID-19, either because the test does not detect them or because we were tested too close to the moment of infection, and the body has not had time to develop antibodies. .

However, for many it is more disturbing to have been vaccinated for COVID-19 and, even so, that when we perform an antibody test it can be negative.

How do you know if the vaccine has worked?

Vaccines like Pfizer are 94% effective. I mean, it still exists a small chance that the body will not react correctly and no antibodies against the coronavirus are developed once we have received the vaccine. Of course, it should be remembered that if the effectiveness of a coronavirus vaccine is 95%, it does not mean that 5% of those vaccinated are infected: things are a little more complicated, as we already explained.

Be that as it may, how can we know if we are immunized or not? How to know if the vaccine has worked and was not, for example, in bad condition?

After being infected with the coronavirus, the first antibodies that our immune system makes to fight the new infection are immunoglobulins M (IgM). Later the immune system will produce immunoglobulin G (IgG), which is the most abundant type of antibody. But if we do an antibody test against SARS-CoV-2 and neither of them appear, does not necessarily mean that we have not passed the disease. Nor does it mean that the vaccine did not work.

Antibody tests usually detect general antibodies, such as those mentioned, so not all of them detect antibodies. versus protein S or Spike protein, which is what we care about if we have been vaccinated.

The mechanism of action of any of the vaccines is the generation of a weak and specific immune response against the coronavirus. For it, use different formulations that express protein S, which is the protein of the outer covering of the virus that gives it its characteristic morphology and for which it has received the name of coronavirus. Like all other coronaviruses, the SARS-CoV-2 genome thus encodes the “spike” protein.

As the first antibody tests developed for this pandemic may not be able to determine the presence of antibodies known as anti-spike, whose presence is linked to the vaccine, test prototypes are being developed that do, such as that of a Spanish company called Ingenasa. This antibody test specifically targets protein S.

These tests are important for epidemiological studies, but they are not as relevant to our daily lives because the important thing is that all of us receive the vaccine: if someone is part of that small percentage of people who have not developed the antibodies known as anti-spike , then you will also be protected by group immunity.

In addition to all the above, keep in mind that the immune system is something very complex. Each person’s body is different and their reaction to immunization can generate very different immune responses. In addition, the immune response depends on several factors, such as age, nutritional status, and the presence of certain medications or infections that can suppress the immune system.

There is a possibility, for example, that someone has had the disease a long time ago and we no longer have detectable levels of immunoglobulins against SARS-CoV-2. But it could also happen that other types of antibodies have been generated.

In short, then, a negative antibody result does not necessarily mean that the person is not protected against the disease; Nor does the mere presence of antibodies ensure that the person cannot be infected, reinfected or develop mild COVID-19.