What do we really know about immunity after SARS-CoV2 infection?

Are we immune after having Covid-19? How long does this immunity last? Will the vaccines soon available on the market induce sufficiently protective immunity? To shed some light on the subject, the High Authority for Health (HAS) publishes this Tuesday a summary of the scientific literature available on the subject. A document drafted by experts from the HAS Technical Commission for Vaccinations (CTV) that collects the knowledge unearthed by previous epidemics of other coronaviruses (such as MERS) and the Covid-19 pandemic.

The goal, a few weeks after the launch of a five-phase anti-Covid vaccination plan, is to provide elements for understanding and interpreting the results of clinical trials on candidate vaccines. What are the mechanisms of immunity developed after infection with this coronavirus? How are vaccines created to reproduce this immunity? And how could they be developed so quickly?

Antibody immunity proportional to the severity of the developed form

When you have been infected with the coronavirus, you can perform several tests. First the PCR test or antigen test, which looks for the presence of the virus in a person who develops symptoms of the disease or in a case of asymptomatic contact. The serological test is done later: when you are no longer sick. It makes it possible to identify the presence of neutralizing antibodies, which indicate – if the person had not been previously screened – that he was indeed infected, and also that he has developed immunity against the disease.

But the results of this antibody research show that patients are not equal in the face of the disease and in the face of the immunity it provides. The immunity developed after SARS-CoV2 infection is proportional to the severity of the developed form. Clearly, the sicker we are with Covid-19, the more immune we are. In contrast, the immunity of people who have developed an asymptomatic form or with few symptoms is weaker, the document recalls.

Furthermore, if these neutralizing antibodies are protective, the scientific literature shows that they are not present in the body for a long time. So if they passed a serological test today, many infected from the first wave would have a negative result: their blood would no longer contain neutralizing antibodies.

Immunity through cellular memory

In addition to the protection induced by antibodies, the body would develop another form of immunity. While an infected person’s antibodies are likely to disappear after a few months, an immunity found in the very heart of our cells could last for several years. In practice, after contracting the virus, infected people would retain some form of memory of the body’s activated immune response. A memory lodged in the heart of immune cells: the T lymphocytes, which destroy cells infected with SARS-CoV2.

This immune memory also affects asymptomatic or paucisymptomatic patients. Extensive tests show the presence of these T lymphocytes in patients with few or no symptoms who have not (or have little) developed protective antibodies against Covid-19. These cells could go into action to initiate a new immune response in case of re-exposure to Covid-19, and allow you not to contract the disease again or to develop a more benign form. The scientific literature has shown that with other coronaviruses, such as SARS-CoV1 (SARS) and MERS, “the duration of the persistence of the T cell response seems long”, greater than ten years, and “is important in healing and protection from infection” And according to HAS experts, “the anti-SARS-CoV2 T cell response also appears to be important in controlling infection.”

Knowledge crucial for inducing immunity through vaccines

What to do with this data in practice? They are crucial not only for a better understanding of the disease and its mechanisms, but also for interpreting the results of all clinical trials carried out on candidate vaccines. “This paper aims to provide immunological basis to explain how research has been able to develop vaccines so rapidly. And that’s because SARS-CoV2 is similar to SARS-CoV and MERS CoV, “says one on the side of the HAS, who insists on” the need [d’allier] a response of antibodies and T lymphocytes to guarantee protection ”against Covid-19.

Because we must first have deciphered the mechanisms of natural immunity to try to recreate immunity without being infected, induced by vaccines. In this summary, the experts of the Technical Committee on Vaccinations then return to the “different vaccination platforms” developed against Covid-19. Classically, researchers create an antigen which, in contact with the immune system, will prompt the latter to produce the famous antibodies. For this, the entire virus can be used, which is then inactivated or attenuated. Or just a “piece” of the virus. Or associate it with another known and controlled virus. The new method of the so-called messenger RNA vaccine, which uses only a fragment of the virus genome, does not rely on the same mechanisms. This involves injecting strands of genetic instructions called messenger RNA into the body, which will dictate instructions to the cells to fight the coronavirus. A technology never used before for human vaccination. And which was chosen by the Pfizer-BioNTech duo, which on Tuesday submitted an application for authorization in Europe. Ditto for Moderna, whose messenger RNA vaccine is currently under review by the FDA, the US pharmaceutical agency.

The advantage of these RNA vaccines is that they are very fast to produce, the paper points out, and cause the least number of side effects as they induce a more targeted immune response. With an announced efficacy of more than 95% – which must be confirmed by official publications – and declared minor side effects, these RNA vaccines exceed the expectations of the World Health Organization (WHO), which relied on at least one vaccine. 50% effective and free from serious negative effects to massively and sustainably contain the Covid-19 pandemic. For its part, HAS intends to continue and update the compilation of these scientific data. This will follow the evolution of the data that remain to be clarified, in particular those on the efficacy and safety of candidate vaccines on fragile and priority populations, on the most effective dosage or on the duration of protection they will offer. And remember that if the State foresees an imminent launch of its vaccination campaign – after the phase reserved for the most vulnerable groups, the one concerning the general public will take place “between April and June”, indicated the president -, Inserm will begin its clinical trials on a large scale on vaccines that have completed their phase 3 trials in the coming weeks.