Covid-19: the duration of the infection would be underestimated

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Although the Covid-19 pandemic peaked in March / April 2020 in many countries, the prevalence of the infection, the duration of the response and the protection it confers, however, are still poorly documented.

Last May, the Curie Institute launched in collaboration with the Pasteur Institute the "Curie-O-SA" serological study among all its personnel (hospital and non-hospital), that is, 3,500 potential volunteers with several specific objectives: to control the spread of the SARS-CoV-2 virus, better understand the immune response in a representative population of healthy active adults, and test the various serological techniques for antibody detection.

As a reminder, serological tests make it possible to determine whether a person has been exposed to SARS-CoV-2, identifying the presence in the body of the antibodies produced after infection. According to Inserm, "the most widely used serological tests detect the presence in the blood of virus-specific antibodies, however, without providing information on its neutralizing power to date. In the current state of scientific knowledge, the identification of antibodies (including neutralizing antibodies) by serological tests is not necessarily synonymous with protection, in fact, doubts remain about the immune response to the virus.

Do symptomatic cases involve antibodies?

In all, 1,847 people volunteered to participate in this study, having performed blood tests and agreed to complete a questionnaire about their symptoms, allowing for a precise correlation between immune response and symptoms. "We have developed sensitive, very high-throughput methods to quantify the response of antibodies and their neutralizing activity in people infected with SARS-CoV-2," explains Thierry Rose, a researcher at the Pasteur Institute. Taken between May and July, 11% of the 1,847 sera analyzed contained antibodies against the SARS-CoV-2 virus. Of these positive individuals, 21% were asymptomatic.

But in the 5% of the participants whose infection was proven by RT-PCR tests (swab inserted into the nostril), the tests did not reveal antibodies of the IgG type (immunoglobulin G) that represent 70 to 80% of the antibodies present. in the blood. Among all participants, loss of smell and loss of taste occurred in 52% of the IgG antibody-positive individuals and in 3% of the IgG-free individuals. By looking only at people who lost their taste and smell, the researchers found that 30% of them were HIV negative, that is, with no antibodies in their blood serum.

A gradual loss of antibodies in the body.

However, "other symptoms related to the infection had been presented at the peak of the epidemic", points out the scientific team, for whom this discovery published in the European Journal of Immunology suggests "that the real prevalence of the infection could have reached 16 , 6% ". In fact, with an infection rate for the SARS-CoV-2 virus of between 11% (serology only) and 16.6% (taking into account the loss of taste and smell without antibodies), these results reveal "A high prevalence of immunization and fairly brief immune responses."

Subsequently, the sera obtained 4 to 8 weeks after the initial sample were analyzed, always with the idea of looking for the presence of antibodies.

The researchers then found that the half-life (time for the antibody concentration to drop in half) of antibodies capable of neutralizing the SARS-Cov-2 virus was only four weeks after infection. An important finding because "during retrospective studies, this could lead to an underestimation of the true prevalence of infection," they add. However, the loss of circulating specific antibodies in the blood does not necessarily mean a loss of total protection because the study shows that other antibodies remain present: the "memory" immune cells (T and B lymphocytes) that prolong immunity.

The limits of serological tests for research

The study will last one year to follow the evolution of seroprevalence during and after the second epidemic wave of October / November. A second sample of more than 1,000 study participants has already been taken, which will include new volunteers from the Pasteur Institute. Therefore, more data are expected, but those revealed in this study indicate that, from an epidemiological point of view, the seroprevalence results may underestimate the actual number of people who have been infected with SARS-CoV-2 and this by two reasons:

The first is that the researchers observed what and there was a lack of systemic IgG response among positive individuals by RT-PCR test. The second reason is due to the fact of a progressive loss of these antibodies, and that this rather rapid decrease can "prevent any retrospective assessment of the true extent of the epidemic," the study estimates.

These two factors raise the still relevant question of a possible reinfection and persistence of the virus in a high-density population.

Therefore, the existence of safe and definitive protection after infection is not yet guaranteed. "Important parameters to guide future public health policies", concludes the scientific team.

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