At what level do the antibodies developed against Covid-19 protect us against the risk of infection or reinfection? More than 20 months after the arrival of the epidemic and at the dawn of a third injection in the elderly and frail, the question is not trivial.
Teams from the virology laboratory of the Toulouse University Hospital Center (CHU) provide an answer in a study published in the scientific journal “Journal of infection”.
This is the first article to find a link between the thresholds of antibodies directed against SARS-CoV2 and the levels of protection against the disease. This research work was carried out using data from the cohort of 8,758 staff from Toulouse hospitals monitored since June 2020, that is to say on leaving the first confinement. For this study, the follow-up stopped in April 2021, a period when the delta variant was not yet present in Haute-Garonne.
“We observed the distribution of antibodies at the different monitoring points of this cohort and we questioned the infection / reinfection rate over the duration of the monitoring. This allowed us to establish antibody thresholds associated with different risks of being infected with Covid-19 ”, explains Chloé Diméglio, doctor in applied mathematics, biostatistician in the virology laboratory of Professor Jacques Izopet at the Toulouse University Hospital. . Among 8,758 personnel monitored, more than half had received one or two doses of the Covid-19 vaccine between January and April 2021.

Vaccination protects

Two main pieces of information emerge from this: with less than 141 units / ml of total antibodies, the rate of protection against Covid-19 is only 12.4%; in contrast, beyond an antibody concentration greater than 1700 units / ml, no reinfection was observed. And, between 141 units / ml and 1700 / ml of total antibodies, the protection against infection or reinfection is 90%. It should be noted that in people who have been vaccinated and who do not have an immunity deficit, none has a total antibody concentration below the threshold of 141 units / ml in the month following the second dose. A good point for vaccination therefore.
In this sample of caregivers, the best protected are those who were infected for the first time by the virus and who received a dose of vaccine afterwards (2 months after the end of the symptoms according to the recommendations): “most of them have a total antibody level greater than 1700 units. Some can even go up to 147,000 units / ml, ”explains Chloé Diméglio. Among those infected subsequently unvaccinated, a large majority (79%) show an antibody level of less than 141 units / ml three months after infection. Here too, vaccination scores points.

“Thresholds allow strategies to be put in place”

“The interest of this study is to determine a threshold – here 141 units / ml – below considering the person is not protected and, for this person, it will be necessary to set up a strategy (for example an additional dose of vaccine , taking monoclonal antibodies for immunocompromised people such as transplant recipients). This also makes it possible to calibrate long-term monitoring and optimize vaccination strategies by estimating the duration of protection, ”adds the researcher.

The extension of this work is expected in the coming weeks. In particular, it will make it possible to tell how quickly the total antibodies, and therefore the protection, decline following vaccination. But it will also be necessary to protect oneself in the case of the more contagious variant and perhaps require the reassessment of the protection thresholds.