A year after the start of the Covid-19 pandemic, the scientific world finally has a little perspective in order to estimate the duration of immunity induced by viral infection. A US study estimated that this would last at least 8 months. Is this good news?
In December 2020, a British study claimed that antibodies against coronavirus disappeared in just a few weeks. According to the results, 25% of people lose their antibodies within three months. Before that, a second US study suggested immunity for at least 6 months. Nevertheless, it was a pre-release to be considered with caution.
A study led by the La Jolla Institute of Immunology (USA) and published in the journal Science January 6, 2021 also evokes long-lasting immunity . However, this time, it is a question of an immune memory persisting more than 8 months after the appearance of the symptoms induced by SARS-CoV-2. Immune memory effectors were monitored in 188 patients , i.e. 80 men and 108 women. Participating Covid-19 patients were integrated into three groups, that of asymptomatic forms, that of moderate symptoms as well as that of severe forms of the disease.
Recall in passing that immune memory appears after a first encounter with a pathogen , natural or vaccine-induced. Therefore, certain effectors of immunity (neutralizing antibodies) try to eliminate the pathogen as soon as it arrives and others constitute a group of memory cells further back. The objective? Attacking faster and more efficiently in case of new infection.
Researchers observed circulating anti-SARS-CoV-2 antibodies , the quantity of which is heterogeneous between the participants. However, they discovered stability in the quantity of antibodies throughout the follow-up, i.e. between 20 and 240 days – depending on the patient – after the first symptoms. After one month, 98% of patients were HIV positive concerning the anti-protein S antibodies. After 6 months, the quantity of antibodies decreased, but they remained present in 90% of the volunteers. Thus, their half-life was evaluated at 103 days, or about three months. With regard to neutralizing antibodies, these are always present beyond 6 months . Between 6 and 8 months, researchers observed that 90% of patients still had them.
The study leaders also observed memory cells as well as B, CD8 T and CD4 T lymphocytes specific to the coronavirus. However, these cells appear after the antibodies, about a month after the onset of symptoms of the disease. After 6 months, 70% of participants still have CD8 T cells active against the coronavirus (half-life of 125 days) and 92% CD4 T cells with a half-life of 94 days. In addition, B lymphocytes appear even later, between 4 and 5 months after the onset of symptoms. According to the researchers, their half-life has not been evaluated, but their presence would be long-lasting.
Finally, how to explain the cases of reinfection after a few weeks or months if the immunity is at least 8 months? The scientists recalled the heterogeneity in the immune responses observed in the patients in the study. In other words, the weakest responses are not enough to constitute protection against the virus.
