Finally, scientists can estimate the duration of the immune response against the coronavirus. According to a new study published in Science, immunity to SARS-Cov-2 can last at least eight months.
One year after the emergence of Covid-19, scientists now have enough evidence to assess the duration of immunity induced by the viral infection that causes the disease. The latest study on the subject, published in Science in early January, was conducted using subjects mostly from New York and California. According to the study, immune memory persists for more than eight months after the onset of covid 19 symptoms.
Immunological memory is established after an initial encounter with a pathogen, either naturally or through a vaccine. One part of the immune system works to eliminate the pathogen, while another part remains behind to form a group of memory cells ready to attack more quickly and intensely in a subsequent infection with the same pathogen.
Memory immunity persists six months after infection
The evolution of this immunity was followed over eight months in 188 Covid-19 patients, 80 men, and 108 women. Participants presented with asymptomatic, moderate (the most common), and severe forms of the disease.
Initially, the researchers were interested in the circulation of anti-SARS-CoV-2 antibodies. Although heterogeneous among patients, the number of antibodies remained stable during the follow-up period, i.e., between 20 and 240 days after the onset of symptoms. After one month, 98% of patients were seropositive for antibodies against the S protein. After six months, the number of antibodies decreased but remained present in 90% of participants. Their half-life is estimated to be 103 days, just over three months.
Neutralizing antibodies, those that prevent coronavirus from entering cells, persist beyond six months; 90% of participants remain SARS-Cov-2 positive when tested between six and eight months.
The researchers also tested memory cells, B cells, CD8 T cells, and CD4 T cells specific for SARS-CoV-2. These cells appear long after the antibodies, about a month after the first symptoms appear (compared with about ten days for the antibodies). After six months, 70% of participants still have active CD8 T cells against coronavirus (estimated half-life 125 days) and 92% still have CD4 T cells (estimated half-life 94 days). Memory B cells appear even later and begin to proliferate about four to five months after the onset of symptoms before their numbers stabilize. Scientists have not measured the half-life of these cells, but it is likely that their presence lasts a long time.
Is that enough to protect against reinfection?
These results show that immunity to coronavirus is indeed present for at least eight months after the first symptoms. In this case, how can the cases of re-infection described in the scientific literature be explained? At this point, the authors of the study point out that they observed a large heterogeneity among the immune responses of their cohort. Therefore, the weaker responses would not be sufficient to protect against coronavirus re-infection.