Whether we catch the covid or get vaccinated, our body in both cases develops immunity against the coronavirus. But are the two situations equivalent?

The overall principle is the same: an antigen – the coronavirus, in whole or in part – enters the body. The immune system identifies it as an “enemy”. He will therefore seek to destroy it on the one hand, and to remember him on the other. In this way, he will be able to recognize this virus and destroy it quickly if he were to encounter it a second time. These mechanisms constitute the immune response and the establishment of immunity.

Overall, this immune reaction is based on two actions: production of antibodies specific to the antigen encountered, and activation of memory lymphocytes, white blood cells which have a very long lifespan.

Read also: Vaccines against covid: what we know, what remains to be discovered

Global natural immunity

But the immunity acquired by infection and that acquired by vaccination are not strictly identical.

During an infection, the virus multiplies in the body, infects different types of cells, and this for several days. By doing so, it presents itself to the immune system from every angle. The cells of the immune system learn to recognize all the antigens of the virus, so the immune response is comprehensive.

Vaccine immunity centered on the S protein

This is not the case with vaccines. By definition, they do not contain an active virus which would circulate and reproduce in the body, but only a part of the antigens of the virus. In the case of anti-covid vaccines, the immune response relies on the S protein, this surface protein of the virus that allows it to attach to and enter target cells.

For AstraZeneca and Johnson & Johnson, these are vectorized adenoviruses carrying the S protein of the coronavirus on their surface. A sort of empty shell that looks like SARS-CoV-2 on the surface.

For Pfizer and Moderna, it’s a piece of RNA, genetic information, which contains instructions for the human body to build the S protein on its own.

When variants are a problem

Knowing all this, theoretically, vaccine immunity could be less complete than natural immunity because the immune system has not met all facets of the virus.

In the case of covid, it has only encountered “only” the S protein. This is all the same enough to recognize the virus and neutralize it… unless it has mutated too much. The variants, for which the S protein is too different, could thus escape immunity. But this also applies to natural immunity: a variant that is too different risks escaping the vigilance of the immune system.

An infection does not dispense the vaccine

And when you’ve already had the covid, how does it work? In France, the Haute Autorité de Santé (HAS) recommends that people who have already had covid receive only one dose of vaccine. The disease would indeed be enough to trigger an immune memory and the first dose would then act as a reminder.

But the infection does not dispense with the vaccine, as natural immunity alone seems to last only a few months. At least two “exposures” to the virus would therefore be necessary for the immunity to last over time, whether the first exposure is via a dose of vaccine or via a natural infection.

Last thing to know: there is no risk in getting vaccinated after an infection. But there is also no point in doing it too early, when the body still has enough naturally acquired antibodies. This is why health authorities recommend that patients cured of covid wait three months before being vaccinated.